Freshwater Snails Identification, Distribution, Their Infection With

Freshwater Snails Identification, Distribution, their Infection with Cercariae and the Influence of the Associated Factors in Sememo Locality, Adi Quala Sub Zone, Debub Region, State of Eritrea (2018)

Asmerom ZerisenayGebrezgabherTeweldemedhn B.Sc (Honours) of Science in Public Health, Asmara College of Health Sciences (June 2015)

A Dissertation Submitted to the University of Gezira in Partial Fulfillment of the Requirements for the Degree of Master of Science

in Medical Entomology and Vector Control Blue Nile National Institute for Communicable Diseases University of Gezira

August,2018

Asmerom Zerisenay Gebrezgabher Teweldemedhn

Supervision Committee:

Name Position Signature

Prof. Bakri Yousif Mohamed Nour Main Supervisor ------

Dr. Lana Mohmmed Elamin Co-supervisor ------

Date: August, 2018

Asmerom Zerisenay Gebrezgabher Teweldemedhin

Examination Committee:

Name Position Signature

Prof. Bari Yousif Mohamed Nour Chairperson ------

Dr.Usama Abdalla Elsharief Ibrahim External Examiner ------

Dr.Albadwi Abdelbagi Talha Internal Examiner ------

Date of Examination: 12 / 8 /2018

DEDICATION

I have dedicated this dissertation paper;

To my families, parents, wife, kids & friends

To everyone who motivated and supported me

To everyone who was there when I was in need

To everyone who supported, helped and stood beside me

To all of you, my immense appreciation

ACKNOWLEDGMENT I would like to express my deep gratitude to my main supervisor Dr. Bakri nour and Co-supervisor Dr. Lana Mohmmed Elamin for their valuable advices and comments throughout this study.

My immense gratitude is also due to my lovely families, parents, friends and specially to my lovely wife, miss Saron Teklebrhan, for her continuous & endless motivation during conducting of this dissertation paper.

I would like to thank all the Blue Nile Institute staff especially the Laboratory head staz Arwa Osman for her continuous comments and corrections.

My thanks due to Mr.Eyasu Habte for his great contribution in statistical analysis and Dr. Araya Birhane; director of communicable diseases in the ministry of health, Dr. Eyob Azeria; head of department of Public health, for their valuable comments & corrections and sustainable motivation & guidance.

I would also like to thank to the ministry of health, South region branch, for their transportation, financial and material support during the data collection period of the study and for all the regional entomology staff for their great commitment in data collection & all the Laboratory works.

At last I want to thank, to everyone I might forgot who helped me and encouraged me, to all of you, I extend my deep gratitude.

Asmerom Zerisenay Gebrezgabher Teweldemedhin Abstract Fresh water snails are well-known intermediate hosts for the transmission of different types of Vector borne diseases. They harbor and transmit various types of trematode worms causing illness for mammals either man or animal, such as schistosomiasis, fascioliasis, paragonimiasis and clonorchiasis. Across sectional study was conducted in Sememo canal (Jan- June 2018) to explore the present snail species and their relation with trematode infection, with observation on environmental factors affecting them. Three surveys at different environmental conditions were conducted, first in January, the second in March and the third in June. A total of 4721 freshwater snails were collected from the study site with overall species prevalence Bulinus trancatus 80 (1.7%), Biomphalaria pfeifferi 3116 (66%) and Lymnaea natalensis 1524 (32.3%). The overall density in the three surveys was (9%), (59%), and (32%) respectively. Snails were screened for trematode cercariae under artificial light; two species were shed different trematode cercariae with infection rate in the three surveys 0 (0%), 43.77%, and 63.56% respectively. Three cercariae morphotypes were shed; Longi-furcate apharengeate monostome cercaria (LPM), undescribed furcocercous type-3 and virgulate xiphidio cercariae. A list of recommendations were made, since the planning of snail control reassures and evaluate their impact, knowledge of their ecology, population trends and dynamism are essential requirement towards understanding disease transmission and control

التوزيع والعوامل المؤثرة ألنواع القواقع الحلزونية في المياه العذبة وتحديد مدى إصابتها بالسركاريا في محلية سيميو،بمنطقة ديبوب،إريتريا)2018( أسميروم زريساني جيبريزجابهر ملخص الدراسة

Table of Contents

Content Pap Dedication………………………………………………………………………………… e iV

Acknowledgement ………………….…..……….………………………….. …………… V

Abstract (English) ………………………………………………………………………. Vi

Abstract (Arabic)…………...……………………………………………………………. Vii Table of content…………..……….……………………………………………………… Viii

List of tables………………..………….……………………………… X List of figures ……………………....………………………………...... Xi List of plates………………………………………………………………… Xii List of appendices……………………………….…………………………… Xiii Abbrevations………………………………………………………………… Xiv … CHAPTER - ONE ...... 1

INTRODUCTION ...... 1

1.1 BACK GROUND ...... ………………………………………………………….1

1.2 PROBLEM/TOPIC IDENTIFICATION:…………………………………………… ...... 3

1.2.1 Justification of the study………………………………………………………… ...... 3

1.2.2 Justification of the area……………………………………… ...... 3

1.3 RESEARCH OBJECTIVES; ...... ……………………………………………………4

1.3.1 General Objective ...... ……………………………………………………4

1.3.2 Specific Objectives: ……………………………………………………………. 4

CHAPTER –TWO……………………………………………………………………… 5

LITRATURE REVIEW………………………………………………………………...... 5

2.1 Global Distribution of scistosomiasis………...... ……………………………………..5

2.2 Regional distribution of schistosomiasis; ...... …………………………………6 2.3 Situation of schistosomiasisin Eritrea ...... ………………………………………….7 2.4 Major forms and distribution of schistosomes...... …………………………………9 2.5 Fresh water snails……………………………………………………………………...... 11 2.6 Some Common Fresh water snail species ...... …………………………………..14 2.7 Snails Vector Population and Infection Studies ...... ……………………………16 2.8 Environmental factors affecting snails' population: ...... ………………………….21 2.9 Snail control measures ...... ……………………………………………………………..25 CHAPTER –THREE……………………………………………………………………...... 29 RESEARCH METHODS AND MATERIALS USED………………………………………… 29 3.1: Study Area: ...... ………………………………………………………………….29 3.2 Study Population:...... ……………………………………………………………….31 3.3StudyDesign:...... …………………………………………………………………..31 3.4 Malacological Survey ...... …………………………………………………………….32 3.5 Laboratory work:…………………………………………………………………...... 35 3.6 Data Analysis ...... ……………………………………………………………………….36 CHAPTER-FOUR……………………………………………………………………...... 37 RESULTS AND DISCUSSION……………………………………………………………….. 37 4.1 RESULTS………………………………………………………………………………...... 37 4.1.1; Identification of Snails species ... ………………………………………………………….37 4.1.2; Prevalence Rate in all the Surveys ...... ………………………………………….37 4.1.3; Snails and their Infectivity Rate ...... …………………………………………42 4.1.4; Descriptive of environmental factors affecting snails’ population ...... 51 4.2 DISCUSSION………………………………………………………………………………. 62 CHAPTER-FIVE……………………………………………………………………………….. 68

CONCLUSION AND RECOMMENDATION………………………………………………… 68 5.1 CONCLUSION...... ……………………………………………………………………68 5.2 RECOMMENDATION ...... ……………………………………………………69 REFFERENCE ...... Error! Bookmark not defined.70 ANEXES ...... Error! Bookmark not defined.84 APPENDICES……………………………………………………………………………. 88

LIST OF TABLES Table number and type Page number Table4.1: Snails collected and examined at Sememo locality, Eritrea, Janurary-June 40 2018.

Table 4.2: Species examined, infected and their infectivity rate during the three 46 surveys, at Sememo locality, Eritrea, Janurary-May 2018.

Table 4.3: Comparison of infectivity rate of the different species within the same 47 surveyduring the study period Table4.4: Comparison of infectivity rate of the same species between the three 48 surveys during the study period Table4.5: Types of Cercariae found; human and non-human cercariae,in Sememo 49 canal during the study period Table4.6; Comparison on number of the different Species within the same 51 Surveyduring the study period Table 4.7: Potential of environmental and organic factors affecting snails’ population 55 in Jan.2018 Table 4.8:Potential of environmental and organic factors affecting snails’ population 56 in April 2018 Table 4.9:Potential of environmental and organic factors affecting snails’ population 57 in June 2018 Table 4.10; Difference in Ecological Factors across the three Surveys during the 58 study period Table4.11: Correlation of density and ecological factors during the study among the 59 snails population Table 4.12: Association of density with turbidity, water velocity, and vegetation 60 density (n=30)during the snail surveys Table4.13: Post hoc pair wise analysis on differences in density by turbidity and 61

vegetation density.

LISTOF FIGURES

Figure number and type Page number

Figure 4.1: Prevalence of Bulinus trancatus, Biomphlaria pfeifferi, and Lymnae 41 natenalis during the three surveys at Sememo canal

Figure 4.2: Prevalence rate of the three species by survey at Sememo locality during 42 the study

Figure 4.3 : Prevalence rate of the species by site at Sememo locality during the 43 study

Figure 4.4: Comparison on occurrence of Biomphlaria species among surveys 1, 2, 44 and 3 at Sememo locality during the study

Figure 4.5: Prevalence rate of each species by site (Out of the total species in each 45 site) at Sememo locality during the snail surveys Figure4.6; Types of cercariae present in Sememo canal during the study period, Jan- June 2018 49

Figure 4.7:Comparison on occurrence of Biomphlaria species among surveys 1, 2, and 3 at Sememo locality during the snail surveys 52 Figure 4.8:Comparison on occurrence of Biomphlaria species among surveys 1, 2, and 3 at Sememo locality during the snail surveys 52 Figure 4.9: Comparison on occurrence of Biomphlaria species among surveys 1, 2, and 3 at Sememo locality during the snail surveys 53 Figure 4.10: Comparison of the density of the three species of snails Surveys 1, 2, and 3 54 Figure 4.11: Two cluster formation according to the species density, turbidity, and vegetation density in Survey 1 (Red: Cluster 1, Green: Cluster 2). 62

Figure 12: Two clusters formation according to the species density, turbidity, and vegetation density in Survey 2 (Red: Cluster 1, Green: Cluster 2). 63

Figure 13: Two cluster formation according to the species density and water pH in Survey 3 (Red: Cluster 1, Green: Cluster 2). 64

LIST OF ANNEXES

Type of form Page number

Annex-1; snail collection form 87

Annex-2; Snail survey & incrimination form 88

Annex-3; snails collected in each site/ each scoopin each 89 survey

Annex-4; Influence of the ecological factors on snails 90 form

LIST OF APPENDICES

Types of plates Number of plate Plate-1: A researcher searching for fresh water snails using scooping 91 method Plate-2: Samples of fresh water snails collected during scooping 92 Plate-3: Artificial light used during determination of shedding 93 Cerceriae Plate-4: A researcher identifying the collected fresh water snails 94 through a compound microscope Plate-5: Sample of fresh water snails appeared in white dish ready for 95 identification Plate-6: fresh water snail, Biomphlaria pfeifferi: collected from 96 Sememo canal Plate-7, A fresh water snail, Lymnae natenalis, collected from Sememo 97 canal Plate-8; A fresh water snail, Bulinus trancatus, collected from 98 Sememo canal, Plate-9: Un-described furcocercous type-3 screened in regional 99 entomology La., Plate-10: Longifurcate-phatyngeate disome Cerceriae screened in 100 regional entomology La., Plate-11: Virgulate Xiphidio cerceriae screened in regional 101 entomology Lab, Eritrea, Plate-12; A rerional entomology Lab.which was used for snails 102 Identification, preservation and to estimate infection rates during the study period in Eritrea in June 2018

ABBREVIATIONS ANOVA: Analysis of Variable ACHS: Asmara College of Health Sciences BNNICD: The Blue Nile Naional Institute for Communicable Diseases Bio.pfeifferi Biomphalaria pfeifferi Bul. forskalii Bulinus forskalii Bul. Truncates Bulinus truncates BSc Bachler of Science CDCD Communicable diseases control division CI confidence Interval Cleo. Cleopatra snails HMIS Health Management Information System IQR Inter Quartile Range Lab Laboratory Mel. Melanoides snails S. Schistosoma SD Standard Devation Spp Species SPSS: Statistical Package for Social Science UNICEF Unated Natins Childrens Fund WHO: World Health Organisation

CHAPTER - ONE INTRODUCTION

1.1 BACK GROUND Snails are well-known intermediate hosts for the transmission of different types of Vector borne diseases. They harbor and transmit various types of trematode worms causing illness for mammals either man or animal, such as schistosomiasis, fascioliasis, paragonimiasis and clonorchiasis(Stricland G.T, 2000), also cercarial dermatitis, and Amphistomosis with great concern.One of these diseases transmitted by snails isschistosomiasis,which is mainly transmitted byBiomphalaria and Bulinus genera.They are found in almost all types of water bodies, ranging from small temporary ponds or streams to large lakes and rivers. Manmade lakes or water bodies, like irrigation canals and dams are in particular excellent habitats of snails(Yong et al., 2000).

Schistosomiasis is one of the common vector borne diseases which is transmitted by snails.It has a major public health and socioeconomic importance in the developing world. Even yet, direct mortality is relatively low, but the disease burden is high in terms of chronic pathology and disability. The distribution is particularly related to the large scale water development(Lotfyet al., 2010). Digenetic trematodes are common parasites of wild and domestic animals. These parasites all undergo obligatory larval development in snails that are common in the freshwater habitats. Fluke infections can kill or impair the health of their vertebrate hosts, including people, and some digenesis are either known to infect people, or are suspected of doing so (Yong et al., 2000). Furthermore, several studies suggest that the distribution and prevalence of trematodes of medical and veterinary significance will be altered and in some cases enhanced by global climate change (Mas-Coma et al., 2009).This potentially includes Fasciola spp., Schistosomaspp. and Paramphistomumspp. However, before we hope to control these trematodes, or to understand the impact of global climate change on their distribution or abundance, we must first have baseline information for what species are present, where and how they are transmitted, and the identification of their snail hosts. Some data are available from other studies of adult digeneans

from their definitive hosts and the Cerceriae in freshwater snails (Pandey 2001), but as noted by Devkota (2008), much remains to be learned. Human schistosomiases are water-based diseases because of their strong associations with domestic, occupational and recreational water-contact activities (Jurg et al., 2003).The diseases are caused by three main species of digenetic trematodes, namely Schistosomahaematobium, Schistosomajaponicum and Schistosomamansoni. All the species of the genus Schistosoma belong to the flatworms in the class trematoda and family Schistosomatidae. They are commonly called blood flukes, because the adult forms inhabit blood vessels of their definitive hosts (Cheesbrough, 1998).Moreover, there are various mammalian and avian schistosomes, such as Gigantobilharzia and Trichobilharzia of water birds that do not complete their life cycles in the human host but may cause cercarial dermatitis or”swimmers“itch, an allergic skin reaction caused by the cutaneous migration of schistosomula of these schistosomes in the human skin (Kalyanasundaram et al., 2003).

The causative agents are small parasitic flat worms, the schistosomes, which live inside the blood vessels of the gut, liver, or bladder. Over 300 million people in more than 76 countries throughout the developing world are sought to be infected. Even if only 10% of those infected with schistosomiasis have severe clinical diseases; there are still 20 million seriously ill people. The disease is recognized as one of ten tropical diseases of most concern to the world health organization.

1.2 PROBLEM/TOPIC IDENTIFICATION:

1.2.1 Justification of the study

Schistosomiasis is very common in the tropics and subtropics (Steinmann et al., 2006). It is a neglected tropical disease (Hotezet al., 2006) of considerable public health relevance.More number of populationsarevictims of the burden of schistosomiasis and other trematode diseases. The irrigation canals in the Sememo scheme provide flourish environment for water-borne diseases transmitted by fresh water snails, mainly schistosomiasis which is endemic in Debub region threatening both humans and animals. Other trematode diseases such as cercerial dermatitis, fascioliasis, and paramphistomiasis which have a huge economic effect on livestock and human.

1.2.2 Justification of the area; Sememo locality is found in AdiQuala sub zone in which the disease Schistosomiasis is very common and this locality is well known by this type of disease. This could be due to different reasons;

In the Sememo locality, there is a big dam called Sememo dam, from which there is a scheme canal which flows to wards west. Along the sides of the canal there are a lot of vegetations which are very suitable for the presence and breeding of the fresh water snails. In addition,the associated factors such as the climate, temperature, altitude, latitude, humidity, PH value, turbidity, velocity of the water and other factors of the area could be suitable for the presence and breeding of the fresh water snails.

1.3RESEARCH OBJECTIVES;

1.3.1 General Objective

To study the identification, distribution, Infection rate of freshwater snails with Cercariae and the influence of the associated factors in Sememo locality, Adi Quala sub zone, Debub region, State of Eritrea from Jan-June 2018. 1.3.2 Specific Objectives: 1. To identify species of snails in the selected area 2. To determine their distribution (prevalence) 3. To estimate the infection rate of snails by trematode cercariae 4. To observe the influences of the associated factors on snails population

CHAPTER -TWO LITRATURE REVIEW Intestinal parasites adversely affect the health of humans in many parts of the world. They continue to be global problem, particularly among children in developing nations (WHO, 1981). The most prevalent and important helminths in developing countries are the soil-transmitted group and schistosomes (WHO, 2004). For instance, of the global burden of schistosomiasis, an estimated 85% is found in sub- Saharan Africa (WHO, 2006). Schistosomiasis also known as bilharziasis is a parasitic disease caused by trematode fluke of the genus Schistosoma.

2.1 Global Distribution of scistosomiasis Schistosomiasis is mainly found in Asia, Africa and South America in areas where fresh water snails serve as the intermediate hosts (Gryseels et al. 2006). Schistosomiasis is endemic in 74 tropical countries worldwide, affecting over 200 million people while 500 to 600 million people are at risk of becoming infected (Rozendaal, 1997; Steinmann et al., 2006; Siddiqui, 2011). Schistosomiasis is a disease caused by one of six Schistosoma species, namely Schistosoma haematobium, Schistosomaguineensis, Schistosomaintercalatum, Schistosomamansoni, Schistosomajaponicum, and Schistosomamekongi (Davis, 2009). A number of animal species as Schistosomamargrebowiei or Schistosomabovis may also occasionally infect humans (McManus and Loukas, 2008).

School-age children who live in areas with poor sanitation are often most at risk because they tend to spend time swimming or bathing in water containing infectious cercariae (CDC, 2012).

As a mainly rural, often occupational disease, schistosomiasis principally affects people who are unable to avoid contact with water, either because of their profession (agriculture, ﬁshing) or because of a lack of a reliable source of safe water for drinking, washing and bathing(Gryseels B. 1989;pp134-142). As a result of a low level of resistance and intensive water contact when playing and swimming, children aged between 10 and 15 years are the most heavily infected. Increased population movements help to spread the disease, and schistosomiasis is now occurring increasingly in peri-urban areas. Although most people in areas of endemicity have light infections with no symptoms, the effects of schistosomiasis on a country’s health and economy are serious. In several areas (e.g., north-eastern Brazil, Egypt, Sudan) the working

ability of the rural inhabitants is severely reduced as a result of the weakness and lethargy caused by the disease (Gryseels B.1998).

2.1.1Schistosomiasis in Southeast Asia

S, japaponicum was discovered by the Japanese professor Katsurada in 1904 (Katsurada F 1904) and then found in China (Logan, OT 1905).The life cycle was established by Fujinami and Nakamura in 1909 (4). In the following year, human infection was found in the Philippines (1906) and later also in Indonesia (1937) and Lao PDR and Cambodia (1957). Schistosomiasis was thus established in six countries of the region. The transmission of S. japonicum has since been interrupted in Japan and, although animal reservoirs still remain there, no more new human cases have been discovered since 1977. In the other countries, a total of 860,000 annual human cases are currently reported though transmission intensity has been reduced significantly during the last 50 years. However, there are great differences in the distribution and impact as schistosomiasis affects large areas and large number of people in some of the countries, while it is limited to a few foci in others.

2.2 Regional distribution of schistosomiasis;

2.2.1 Schistosomiasis in Sub Saharan Africa

In sub-Saharan Africa, Nigeria carries the heaviest burden with an estimated 29 million cases of infection (P.J.Hotez, A.Kamath, 2009and A.F.Adenowo et al 2015; pp. 196– 205). Both urinary and intestinal schistosomiasis exist in Nigeria (Houmsou, R.S. et al, 2016;pp.477–484 and Ukpai O.M, 2015; pp.44–49) but urinary schistosomiasis is more widespread than intestinal schistosomiasis with varying prevalence across the country [Houmsou, R.S. et al, 2016;pp.477– 484, Houmsou, R.S. 2012;-Soniran, O.T et al, 2015;,pp.141– 144]. The disease is transmitted by the group of planorbid fresh water snails of the genus Bulinus found around sources of water such as streams, slow flowing rivers, ponds, and irrigation canals where rural inhabitants rely on for their recreational, occupational, domestic, and agricultural activities.

During the 65th World Health Assembly, it was advocated that Member States should intensify control intervention and initiation of elimination programs for schistosomiasis. These still remain a dream for several countries in subSaharan Africa particularly Nigeria where the coverage for

the preventive chemotherapy for schistosomiasisis 4.0% [WHO, 2013]. The non-implementation of the policies is impeded by the lack of political commitment, lack of public health infrastructures, and the necessary resources to initiate and sustain control programs across the country.

2.2.2 Endemicity of schistomiasis in Sudan:

Schistosomiasis has affected the people of Sudan for many centuries. Its spread could have been associated with the traders who frequented the Nile Valley in ancient times. In the eighth and seventh centuries B.C., southern Egypt and northern Sudan formed a single political entity. In 1918, Christopherson suggested that Schistosomiasis is endemic in all provinces, with the exception of the desert fringing the Red Sea (Ayad, 1956). Schistosomiasis is endemic in Sudan and cases can be detected in all states except the Red Sea State. It is a behavioral disease which always occurs where sanitary standards are low and man is the final sole host. Schoolchildren who live in such endemic areas are at risk of Schistosomiasis as they tend to swim and bath in water channels and get exposed to the infective cercariae (Mustafa, 2012).

2.3Situation of schistosomiasisin Eritrea

As a developing country and the existence of factors that help for the survival of the parasites, schistosomiasis is endemic in Eritrea. Schistosomiasis is known and suspected to be prevalent in Eritrea. However, the exact burden and distribution ofschistosomiasis is still unknown. Isolated surveys of schistosomiasis and intestinal parasites have been conducted by several individuals and institutions.Since, the existence of schistosomiasis is mainly based on routine and passive health facilities reports,a comprehensive mapping exercise of the whole country was needed to implement effective control/elimination of the disease.

Understanding the morbidity caused by the disease and its implication for the health of the people, Ministry of Health established the schistosomiasis Control unit under the communicable disease control division (CDCD) in 2012. The main goal of the schistosomiasis Control program is to reduce morbidity of Schistosomiasis to a level where it is no longer disease of public health importance. (Eritrean, MoH survey 2014 &2015)

2.3.1S. mansoni infection

The study findings showed that, with the exception of zobaDebubawiKeihBahri, S. mansoni infection is endemic in the five remaining zobas with relatively low level of prevalence. Overall the prevalence of infection was 2.8% (95%CI = 1.8 -4.5%).Ninety-one of the total surveyed schools (26.4%) were found to have at least one child infected with schistosomiasis but some of the schools had high prevalence.This was also true for zoba and sub-zobas. For example zobaDebub had the highest prevalence (8.68%) [5.9%, 12.6%], followed by zobasSemenawiKeihBahri, 1.19% [0.6%, 2.3%], and Anseba (1.17%) [0.3%, 4.4%].S. mansoniinfection was found in half of the sub-zobas surveyed(28 out of 58 sub-zobas). Sub- zobas with prevalence above 10% include: Dekemhare (15.35%) [5.2%, 37.4%], Maiani (15.22%) [6.8%, 30.7%], Adi Tekelezan (15.1%) [10.5%, 21.5%], Adi Quala (12.1%) [6.5%, 21.3], and Molki (10.2%) [2.6%, 32.5%]. Boyswere more likely to be infected with schistosomiasis than girls (4.3% [2.7%, 6.7% and 1.2% [0.7%, 2.4%], respectively). Likelihood of Schistosomiasis increases with an increasing age of children;-1.9% [1.1%, 3.2%] for children aged 10 years and 7.0% [3.2%, 14.4%] for childrenaged 14 years.(Eritrea survey 2014 &2015)

2.3.2 S. haematobium infection

S. Haematobiumstudy was conducted in only one sub-zoba where a clinical case ofhaematobiumwas reported from the nearby community hospital. S. haematobiumwas found in only two school boys aged 12 & 13 years out of 257 tested making the overall prevalence of Schistosomahematobiuminfection in the Goluj subzone of 0.78%. The two students are from the same school and they had also high intensity of infection. (Eritrea survey 2014 &2015)

2.3.3 Co-infection with Schistosomiasis and Soil Transmitted Helminthiasis

The study showed that co-infection with Schistosomiasis and Soil Transmitted Helminthiasis was not common across all of the zobas. Overall, only 11 of the surveyed children, 10 of them in Debub region and one in Anseba region, were found to have infections with both Schistosomiasis and Soil Transmitted Helminthiasis.(Eritrea survey 2014 &2015)

2.4Major forms and distribution of schistosomes

Five species of the trematode parasites are responsible for the major forms of human schistosomiasis. In 1996 intestinal schistosomiasis caused by Schistosomamansoni was reported from 52 countries in Africa, the eastern Mediterranean, the Caribbean and South America(Gryseels, B.1989). Oriental or Asiatic intestinal schistosomiasis, caused by S. japonicum or S. mekongi, was reported to be endemic in seven Asian countries. Another form of intestinal schistosomiasis caused by S. intercalatum was reported from 10 central African countries. Urinary (or vesical) schistosomiasis, caused by S. haematobium, was reported to be endemic in 54 countries in Africa and the eastern Mediterranean(Gryseels B.1989).

2.4.1Life cycle and mode of transmission

On reaching water, the eggs excreted by an infected person hatch to release a tiny parasite (a miracidium) that swims actively through the water by means of ﬁne hairs (cilia) covering its body (Gryseels B.1989).The miracidium survives for about 8–12 hours, during this time it must ﬁnd and penetrate the soft body of a suitable freshwater snail in order to develop further. Once inside the snail, the miracidium reproduces many times asexually until thousands of new forms (cercariae) break out of the snail into the water. Depending on the species of snail and parasite, and on environmental conditions, this phase of development may take 3 weeks in hot areas, and

4–7 weeks or longer elsewhere. The fork-tailed cercariae can live for up to 48 hours outside the snail. Within that time they must penetrate the skin of a human being in order to continue their life cycle(Gryseels B.1989).

As the cercaria penetrates the skin, it loses its tail. Within 48 hours it penetrates the skin completely to reach the blood vessels. This process sometimes causes itching, but most people do not notice it. Within seven weeks the young parasite matures into an adult male or female worm. Eggs are produced only by mated females. Male and female adult worms remain joined together for life; a period of less than ﬁve years on average but 20 years has been recorded (Gryseels B.1989).

The more slender female is held permanently in a groove in the front of the male’s body. Once eggs are produced, the cycle starts again. In intestinal schistosomiasis the worms attach themselves to the blood vessels that line the intestines; in urinary schistosomiasis, they live in the blood vessels ofthe bladder. Only about half of the eggs leave the body in the faeces (intestinal schistosomiasis) or urine (urinary schistosomiasis); the rest remain embedded in the body where they cause damage to organs (Gryseels B.1989).

2.4.2 Prevention and control of schistosomes

Individual protection from infection (e.g. in travellers) can in principle be achieved by avoiding contact with unsafe water. However, this requires an understanding of the risk of contact with water and knowledge of the sites where infected snails are likely to occur. For people living in areas of endemicity, contact is often unavoidable (farmers in irrigated agricultural areas, ﬁshermen) or difﬁcult to prevent (playing children). The control of the disease in known foci of transmission is possible by using one or a combination of the following measures: improved detection and treatment of sick people; improvement of sanitary facilities for safe and acceptable disposal of human excreta; provision of safe drinking-water; reduction of contact with contaminated water; and snail control (WHO,1993).

In areas with low to medium prevalence of schistosomiasis and good health services, improved case detection and treatment of reported cases of illness may be the most cost-effective approach to control;

(1) In areas where the disease is highly endemic, special schistosomiasis control campaigns, involving snail control measures, might be an additional cost-effective solution.

(2). Long-term sustainable improvements have to be based on safe water supply and improvements in sanitation and hygiene. Health education is essential for community understanding and participation in the proper use and continuous maintenance of sanitary and water supply facilities (WHO, 1993).

2.5Fresh water snails

Many species of freshwater snails belonging to the family Planorbidae are intermediate hosts of highly infective ﬂuke (trematode) larvae of the genus Schistosoma which cause schistosomiasis, also called bilharziasis, in Africa, Asia and South Americas(WHO, 1997). The infection is widespread, and although the mortality rate is relatively low, severe debilitating illness is caused in millions of people. It is prevalent in areas where the snail intermediate hosts breed in waters contaminated by feces or urine of infected persons (Gryseels B.1989).

People acquire schistosomiasis through repeated contact with fresh water during ﬁshing, farming, swimming, washing, bathing and recreational activities. Water resources development

schemes in certain areas, particularly irrigation schemes, can contribute to the introduction and spread of schistosomiasis.

The snails are considered to be intermediate hosts because humans harbor the sexual stages of the parasites and the snails harbor the asexual stages. People serve as vectors by contaminating the environment. Transmission of the infection requires no direct contact between snails and people. Freshwater snails are also intermediate hosts of foodborne ﬂuke infections affecting the liver, lungs and intestines of humans or animals (WHO, 1995).

2.5.1 Biology

Some 350 snail species are estimated to be of possible medical or veterinary importance. Most intermediate hosts of human Schistosoma parasites belong to three genera, Biomphalaria, Bulinusand Oncomelania (WHO, 1995). The species involved can be identiﬁed by the shape of the outer shell. Simple regional keys are available for the determination of most species. The snails can be divided into two main groups: aquatic snails that live under water and cannot usually survive elsewhere (Biomphalaria, Bulinus), and amphibious snails adapted for living in and out of water (Oncomelania) (WH0, 1993). In Africa and the Americas, snails of the genus Biomphalaria serve as intermediate hosts of S. mansoni. Snails of the genus Bulinus serve as the intermediate hosts of S. haematobium in Africa and the Eastern Mediterranean, as well as of S. intercalatum in Africa. In south-east Asia, Oncomelania serves as the intermediate host of S. japonicum, and Tricula as the intermediate host of S. mekongi. Among the snail intermediate hosts of trematodes, the species belonging to the genus Lymnaea are of importance in the transmission of liver ﬂukes. Lymnaea species may be either aquatic or amphibious (WHO,1993).

2.5.2Life cycle

All species of Biomphalaria and Bulinus are hermaphrodite, possessing both male and female organs and being capable of self or cross-fertilization. A singlespecimen can invade and populate a new habitat. The eggs are laid at intervals in batches of 5–40, each batch being enclosed in a mass of jelly-like material. The young snails hatch after 6–8 days and reach maturity in 4–7 weeks, depending on the species and environmental conditions. Temperature and food availability are among the most important limiting factors. A snail lays up to 1000 eggs during its life, which may last more than a year. The amphibious Oncomelania snails, which may live for

several years, have separate sexes. The female lays its eggs singly near the water margin (WHO, 1993).

2.5.3Ecology

Snail habitats include almost all types of freshwater bodies ranging from small temporary ponds and streams to large lakes and rivers. Within each habitat, snail distribution may be patchy and detection requires examination of different sites (WHO, 1997). Moreover, snail densities vary signiﬁcantly with the season. In general, the aquatic snail hosts of schistosomes occur in shallow water near the shores of lakes, ponds, marshes, streams and irrigation channels. They live on water plants and mud that is rich in decaying organic matter. They can also be found on rocks, stones or concrete covered with algae or on various types of debris. They are most common in waters where water plants are abundant and in water moderately polluted with organic matter, such as feces and urine, as is often the case near human habitations (WHO, 1993). Plants serve as substrates for feeding and ovi-position as well as providing protection from high water velocities and predators such as ﬁsh and birds. Most aquatic snail species die when stranded on dry land in the dry season. However, a proportion of some snail species are able to withstand desiccation for months while buried in the mud bottom by sealing their shell opening with a layer of mucus. Most species can survive outside water for short periods (WHO, 1995).

For reproduction, temperatures between 22 °C and 26 °C are usually optimal, but Bulinus snails in Ghana and other hot places have a wider temperature range. The snails can easily survive between 10°C and 35 °C. They are not found in salty or acidic water. In most areas, seasonal changes in rainfall, water level and temperature cause marked ﬂuctuations in snail population densities and transmission rates. Reservoirs that contain water for several months of the year in Sahelian Africa can be intensive transmission sites of urinary schistosomiasis during a very limited period, because surviving Bulinus species rapidly recolonize the reservoirs after the rains start. Oncomelania snails can survive periods of drought because they possess an operculum capable of closing the shell opening. In the temperate zone they can survive for 2–4 months, in the tropics much less. They live both in and out of water in humid areas such as poorly tilled rice ﬁelds, sluggish streams, secondary and tertiary canals of irrigation systems, swamps and roadside ditches. The vegetation in these sites is important in maintaining a suitable temperature and

humidity. Their food is similar to that of aquatic snails but they also feed on plant surfaces above water (Ling, B. et al, 1993).

2.6 Some Common Fresh water snail species

The most common snail species will be mentioned according to their relation with trematode worms:

2.6.1Biomphalaria species: Biomphalaria species is freshwater snails of the family planorbidae they are distributed all over the Sub-Saharan area, recently they found in Madagascar, probably due to human activity, spatial distribution, recruitment, and migration of Biomphalariapfeifferi is highly influenced by the rainy/dry periods of the season (Charbonell et al., 2002). It has a small migration distance and this through active dispersal, against or following water flow, long distance migration is possible through other animals, like birds, or through human activities (Charbonnel et al., 2002). All known African species are susceptible to S.mansoni, most of them have been found naturally infected in the field, they are classified in to four groups; Pfeifferi, Sudanica, Choanomphela and Alexandrina group.Infection of Biomphalaria pfeifferi with amphistome cercariae are new records for Zimbabwe (Chingwena,et al., 2002), Amphistome cercariae, furcocercous cercariae types, Echinostomes and Xiphidiocercariae were isolated from Biomphalaria species in Africa (Frandsen and Christensen, 1984).

2.6.2Bulinus species: The genus Bulinus is naturally divided into two different subgenera: Physopsis and Bulinus spp. which also includes the old subgenus Pyrgophysa (Mandhle-Barth, 1962). They divided to Africanus group, Reticulates group, Forsklii group, and Truncatus group which all contain the susceptible species to S.haematobium and intercalatum. Researchers reported various cercariae species in Bulinus species; the main intermediate host for Amphistomes was B. tropicus. Infections in B. globosus, B. forskalii with amphistome cercariae are new records for Zimbabwe (Chingwena,et al.,2002 ).in Spain Schistosoma bovis reported from Bulinus truncatus and B. wrighti (Mouahid and Théron, 1986). Echinstome, Xiphidiocercariae reported in Africa

(Frandsen and Christensen, 1984).In Sudan Ibrahim, (2007) reported Xiphidiocercariae, Echinstomecercariae and Amphistome types from Bulinus species. 2.6.3Lymnaea species: Family Lymnaeidae, are fresh water snails with a thin dextral right opening shell preferred to live in stagnant habitat in slow streams with heavy vegetation, they act as the first intermediate host of animal schistosomes throughout the world (Ferete et al., 2005), also the most important and widespread intermediate host of Fasciolahepatica in Africa, Asia, Europe and north America is Lymnaetrunculata (Soliman, 2008). Various studies indicated that Lymnaeidae were infected with larvae fasciolid species (De kock et al ,2003;Soliman,2008; Barraggan-Saenz et al ,2009) and trematodes such as Paramphistomum (Dryffuss et al.,2004) ,Holostom (Klockars et al.,1928) ,Trichobilharzia , Diplostomidae , Plagiorchiidae , schistosomatidae , Azygiidae, Notocotyidae and Strigeidae (Faltynkova and Hass,2006).C.T by 2.6.4Melanoides species: Belong to Order Sorbeococh, Family Thiaridae M. tuberculata (Muller, 1774), is now distributed in all Neotropical region. It was reported for the first time in Brazil in 1984 (Vaz et al., 1986),Introduced in some areas as a competitor of the native snail Biomphalaria glabrata, which is a host of Schistosoma mansoni (Pointier, 2001; Pointier,.et al 1993) and it reduces the number of (Schistosoma mansoni) snail hosts present in the community. M. tuberculata is known to be the intermediate host of several species of the eye fluke which has agymnocephlous type of cercariae. In Venezuela, Diaz et al. (2002, 2008) described a gymnocephalous cercaria of Philophthalmus gralli which utilizes M. tuberculata as intermediate host. M.tuberculata has been recorded for the first time as the first intermediate host of Echinochasmus japonicus and its natural infection rate was found to be 1.1% (Cheng and Fang, 1989), in Iran Farhanak et al., (2005) also reported Echinostomecercariae FromMelanoides tuberculata. These snails also collected from ponds around the Ninh Binh province, Vietnam settlements and were infected with cercariae of Clonorchis scinensis, the causative agent of human clonorchiasis, at a rate of 13.3% (Kino et al., 1998). Furthermore M.tuberculata-associated larval stages of Philophthalmus have been recorded from Jordan (Dimitrov et al., 2000). Xiphidiocercariae are described from M. tuberculata which collected from Giza and Qualiobyia (Wanas et al., 1993). M. tuberculata populations can harbor viable infections and infect birds by cercaria which could cause cercarial dermatitis in human (Barber and Caira, 1995).

2.6.5 Cleopatraspecies: Cleopatra species (Troshel, 1857) are a genus of freshwater snails with an operculum; belong to the super-family Cerithoidea, family Paludomidae within the subfamily Cleoptrinae. The habitat of species in this genus includes slow-running freshwater stream, Cleopatra Serve as an intermediate host for Prohemistomumvivax which infect dogs, cats and kittens in Egypt and occasionally infect man (Wykoff et al., 1965). 2.6.6 Lanistusspecies: They belong to order Architaenioglossa, family Ampullariidae, this species is known from Egypt, Ethiopia, Uganda and Kenya. They are probably found throughout the Nile, including Lake Nasser (Ibrahim et al., 1999).They occur in southern Sudan, southeast Ethiopia and Uebi and Giuba rivers in south Somalia (Brown, 1994).They live in standing and slow flowing fresh water with rich vegetation and have been incriminated for nematode rat- worms (Angiostrongylus cantonensis in Egypt (Yousif ; Ibrahim, 1978). In Kenya Lanistus purpureus was found infected with trematode cercariae (Kariuki, et al, 2004), and also there was a record of non-furcocercous cercariae of Lanistus carinatus. Lanistus carinatus is considered as a biological agent against Bilharzia hosts (Osman, A. 2012).

2.7Snails Vector Population and Infection Studies

For thepopulation density of a snaila combination of both abiotic and biotic factors exerts its influence on the fecundity in a given habitat (Betterton et al., 1988). According to Slootweg and Keyzer (1993), the principal reproduction period for snail vectors of human schistosomiasis in the Benue valley of Northern Cameroon is the cooler dry season (December –March) when the water temperatures are optimal for reproduction (between 20°C and 25°C). The second half of the rainy season (July –September) is a minor reproduction period for the snail vectors.

Similar observation was made in Nigeria by Etim et al. (1998) who reported that the snail population was highest in April just after the onset of the rains but dropped at the peak of the rainy season. Appleton (1978) found that snail growth requires between approximately 18°C and 32°C, as with the optimal conditions between 22°C and 26°C. Moreover, Idris and Ajanusi (2002) reported that in order for planorbide snails to maintain their number they need a warm

season (26.6°C to 31.4°C) of a few months. They also reported a high reproductive capacity of these species and that population may undergo marked seasonal fluctuation in density and infection rate with rainfall and temperature being the main determining factors.

Ndifon and Ukoli (1989) reported an increased snail density immediately after the rainy season and beginning of the dry season and peak densities of eggs and juveniles during hottest months of March, April and May and their low density during the rainy period. This may be due to the fact that snails require high temperature for eggs to be laid. In a similar vein, Ezeugwu and Mafe (1998) reported that snail intermediate hosts were more abundant during the hot dry season (March/ April) probably due to the fact that this coincided with the period when the aquatic habitats become stable in terms of water level and velocity. In a research conducted by Etim et al. (1998), the snail intermediate host populations were found to be widespread in the area of the prevalence study even though the snail densities were low and fluctuated with the seasons. They further observed that both densities and dynamics of snail populations and water contact pattern showed focal patterns in time and space. Consequently, the dynamics of transmission of schistosomiasis depend on a complex set of local and temporary conditions. Rainfall cycles are amongst the most important climatic factors that affect life history of snail intermediate hosts, but reproduction and population also depend on the temperature and various other factors. In areas where rainfall, water level and temperature are relatively constant, reproduction may take place throughout the year (Webbe, 1988). Temperature emerges as the abiotic factor of greatest importance in determining the distribution of host snails in lentic environments (i.e. standing water bodies). Water current velocity is the most important factor in lotic environment (Webbe, 1988). Snail vectors have been reported by Appleton (1978) to have a remarkably narrow tolerance to current velocity. For example, Bulinusand Biomphalaria species occur only in standing water habitats and in waters flowing at velocities of up to 0.3m/sec. This narrow tolerance range of 0.0 to 0.3m/sec restricts the longitudinal distribution of these snails in river systems and renders large parts of the water courses inhabitable despite water quality and temperature that is suitable. In addition, Lukaet al. (2005) reported a low density of snail vectors in fast flowing streams. However, these snails have been found in slow-flowing canals and in the back waters of main canals where the weirs provide suitable protection from the fast flowing water (Logan, 1983).

Freshwater snails constantly face desiccation, occasioned by drying up of surface water from small water bodies either regularly in a seasonal manner or occasionally due to unusual rainfall, among other factors. However, most of the freshwater snails have the ability to withstand considerable periods of desiccation (Webbe, 1982). During the periods of droughts, these snails have been found to burrow into the mud as a means of aestivation, around the periphery of these habitats (Oyeyi et al., 1988). The distribution of aestivating snails depend on the type of vegetation cover, relative humidity, temperature, size, genetic endowment and substratum where these snails aestivate, thus escaping drought periods hence prolonging their survival into the next favorable season. Other stimuli besides desiccation have been implicated in the commencement of aestivation. These include water level, aquatic flora and fauna, electric conductivity, and temperature. However, aestivation in bulinids is an active process which does not entirely depend on mere drying of a habitat (Oyeyi and Ndifon, 1988). Betterton et al. (1989) observed that B. rohlfsi did not aestivate during the early dry season, when the habitat was contracting through evaporation. When aestivation commenced, there was no abrupt change in any of the physical parameters monitored. However, there was a coincident decline in the population of unicellular algae with the onset of the process. In the case of B. Senegalensis aestivation occurs before the pools have severely contracted and that the species aestivates in response to a sudden fall in water temperature as reported by Betterton et al. (1988). Goll et al. (1984) found that only immature B. Senegalensis survived aestivation. In similar findings, Oyeyi (2000) reported that all the surviving snail intermediate hosts were about 3.0mm in shell length. Brown (1980) noted that the capacity for explosive population increase of bulinid and planorbid snails means that the few survivors will rapidly repopulate the habitat. Furthermore, the success of bulinid snails at colonizing water bodies was linked to its ability to aestivate (Appleton, 1978). It was also reported by Oyeyi and Ndifon (1988) that post-aestivators consumed food at a higher rate than the non-aestivators and that B. rohlfsi in particular, appear to maximize breeding and rapidly repopulate its seasonal habitat through large appetite after aestivation, implying that aestivation may involve the use of reserved foods by the snails in an economic manner, over a period of time covering the aestivation period when the reserves become depleted.

Snail intermediate host populations have been reported to be controlled by ecological factors such as physico-chemical factors of water habitat, vegetation, substratum and water current velocity which may act either singly or in combination thereby exerting their effects (Webbe,

1982). For example, salinity and turbidity influence the characteristics of the water. Salinity and oxygen tension decrease the survival of the snail vectors of schistosomiasis. Additionally, pollution decreases oxygen concentration in water. These factors also influence the development of the snail vectors. B. globosus was shown by Brown (1980) to be intolerant of high turbidities. It has been indicated that a turbidity of 360mg/l due to suspended minerals from granite erosion prevented development and hatching of B. pfeifferieggs (Webbe, 1982). On a general note, the water of most snail habitats is usually eutrophic, containing some dissolved organic materials and is not normally turbid (Webbe, 1982). Ezeugwu and Mafe (1998) found that total water hardness appeared to enhance snail abundance. Moreover, Meier-Brook et al. (1987) observed experimentally that B. truncatus is adapted to hard water, in contrast to Biomphalaria species. However, increase in chloride concentration was found to be apparently deleterious (Ezeugwu and Mafe, 1998). Generally, both low and high pH appeared harmful to snail vectors due to the possibility of denaturation of the mucus on the exposed skin surface (Webbe, 1982). On a general note, planorbid snails have been shown to adapt to a wide range of environmental conditions such as water bodies with moderate organic content, little turbidity and a substratum rich in organic matter and moderate light. Although these factors might vary from one species to another, optimum habitat conditions are usually similar for all of them (Webbe, 1982). Moreover, Luka et al. (2005) found an association of snail population with human activities such as organic pollution which all determine abundance and distribution of the snail vectors. Betterton et al. (1988) investigated 165 freshwater habitats throughout Kano State and revealed the presence of a number of potential snail intermediate host species, namely Bulinussenegalensis, B. forskalii, B. globosus, B. rohlfsiand Biomphalariapfeifferi. They also found that the most widespread species was B. senegalensis which inhabited shallow pools and excavations on a variety of substrata.

Yahaya (1988) conducted a research in the same state with a view to identifying larval trematodes naturally infecting snail vectors of Schistosoma recovered earlier by Betterton (in the same year), Save B. forskalii. He reported an overall field infection rate of 16.6%, a lower rate compared to the known prevalence of the trematode diseases in the area. However, Wright (1966) suggested that in Africa a higher infection rate for human schistosomes was preponderant in snails collected from ponds and pools than in those collected from open habitats such as streams and irrigation systems. Furthermore, Etim et al. (1998) recovered in Biase area of Cross

River State B. globosus, B. truncatus, B. forskaliiand Biomphalariapfeifferi with cercarial infection rates of 39.9% in the dry season and 35.9% in the rainy season. Similarly, Idris and Ajanusi (2002) observed that in Katsina State, a proportion of infected Bulinusspecies was highest in the months of May and June, during which infection rates were 21.05% and 58.33%, respectively. However, they recorded no infection in these snails between the months of July and December. In the case of Biomphalaria species, the highest rates of infection were seen in the months of February, March and April (25.0%, 55.56% and 61.11%) respectively. The least infection rates were observed between July and January (Idris and Ajanusi, 2002).

Gerard and Theron (1997) reported an alteration of the physiology and metabolism of freshwater snail hosts by larval trematodes (Cercariae) which, in turn, may have life history consequences such as effects on growth, fecundity and survival. Moreover, they observed an age-specific effect characterized by limitation of growth rate when the snails were infected as juveniles and reduction of reproductive effort when snails were infected as adults. Also there was a time- specific effect with early enhancement of growth rate and reproductive effort for infected juvenile and adult snails respectively during prepatency, before reduction and cessation during patency. It was however reported by Jozef et al. (2001) that the percentage of infected snails and the cercariometry at a given time point reflect both the level of transmission from the definitive host to snails and the vectorial capacity of the snails resulting from an accumulation of contacts with miracidia during the previous months.

2.8 Environmental factors affecting snails' population:

Limited ecological studies on actual or suspected vectors of bilharziasis have been undertaken by Cridland (1957, 1958) in Uganda, Webbe and Msangi (1958) and Webbe (1960) in Tanganyika. , McClelland (1956) and Teesdale and Nelson (1958) in Kenya, and Malek (1958) and Madsen (1985, 1988) inSudan. Environmental factors affecting snails include physical, chemical, and biological factors, each of these factors have been studied to evaluate their effect on snail population: 2.8.1 Physical factors:

2.8.1.1 Temperature:

All schistosome's intermediate hosts are tolerating themselves to the temperature variation of their habitat, the effect of the annual range of temperature upon the whole breeding season affect snail reproduction and their generation over the year. The optimum temperature for mollusks between 22°C and 26°C with an average 18°C per year and the snail ability for partly dehydration during hibernation or aestivation make them able to withstand extremes of temperature (WHO, 1956). In southern and central Sudan, the schistosomiasis hosts can withstand seasonal and diurnal temperature (Malek, 1958). Malek (1958) suggested that temperature was not a limiting factor in the distribution of bilharziasis vectors in the Sudan although he concluded that temperature does influence their rate of reproduction. Gaud (1958) found that temperature was important but not the only factor influencing the seasonal rhythms of snails. 2.8.1.2 Sunlight:

The light intensity has an indirect effect on snails by increasing the microflora (food source) and high dissolved-oxygen content, as it was proved by (Babiker, 1985), in Sudan when turbidity decreased seasonally, snail growth increased by the indirect effect of sun light which is flourished the aquatic plants. In Sememo irrigated area the majority of canals are almost without shade so snails are concentrating on the aquatic weeds and grass on edges, although they need light, some species like Bulinustruncatus can breed in complete darkness for five months under laboratory condition (Malek, 1958).

2.8.1.3 Watercurrentvelocity:

The water movement is important as it is motivating oxygenation of the habitat; schistosome intermediate hosts prefer stagnant or slow running water. Larger species and older snails are most affected by current than small species and younger snails. Some species do colonize in fast running water and that probably referred to the different geographical races show different resistance to the current, observations conclude that Biomphalariapfeifferri are less able to withstand rapid flowing water than Bulinustrancatus, that explain the only record of Biomphalaria of the Blue Nile was by Archibald (1933), and it hasn't been reported since then. Biomphalariaruppellii, Biomphalariasudanica and Bulinusugandae have been reported from slowly flowing White Nile upstream in Jebel Awllya dam (Malek, 1950). High water velocity after heavy rains may sweep away snails food-microflora- (Marti, 1986).

2.8.1.4 Turbidity:

The effect of turbidity is difficult to assess, short periods of high turbidity, such as it occurs during flooding, have no adverse effect upon snails, however, prolonged turbidity could probably affect the growth of algae on which snails are apparently dependent for their proper development (De Mellion et al, 1958). According to Welch, (1952) all natural waters show some degree of turbidity, resulting from rich plankton growth and organic matters suspension. According to Madsen (1990) turbidity alone doesn't affect snail population, although Myer- Lessen et al., (1994) proofed that clear water is essential for the presence of fresh water snails, where adult and juvenile are not affected by turbidity, the egg masses become unviable because of slit decomposition. 2.8.1.5 Waterdepth:

Schistosomiasis intermediate hosts are occurring in shallow water near shores, amphibious species occur on moist soil, water plants and surfaces near water (WHO, 1997). Under natural conditions, it is rare to find snails in depth exceeding 1.5-2 meters, which mainly associated with food and shelter availability near the surfaces. Experiments with Bulinustrancatus showed that they exist for periods of time at a depth of 10 meters, snail species which live near the bottom or in deeper water are of less importance for schistosomiasis transmission (WHO, 1956).

2.8.2 Chemical factors:

2.8.2.1 Salinity:

It refers to the total salt content in fresh waters and it can be measured by the sodium chloride proportion which is low in most in land waters, the total amount of dissolved solids or the total salinity of the water is less importance than the proportions of the constituent salts. Different snail species have different degree of salt tolerance thus Bulinustruncatus lives in less than 4000 parts per million of salt in the water. Calcium is required for the shell formation is taken mainly with food, since its concentration in water is very low many researchers found that the variation which occur in natural habitat without effect on snail development and life cycle although its affect algae composition (Madsen and Christensen, 1992). Copper, barium, nickel and zinc salts are toxic to snails causing distress or even death, but sudden rises in chloride content even if it happens for long intervals of time may be lethal for eggs or young snails (WHO, 1956). According to Levesque et al., (1978) the egg laying is decline when the concentration of NaCl, MgCl2, and KCl belw specific level.

2.8.2.2 Hydrogen-ion concentration (pH):

The intermediate schistosomiasis hosts are tolerant for wide range of pH between 4.8 to 9.8 within this variation there is no effect on snail’s density (Malek, 1958; Madsen, 1985). pH is affected by different environmental factors such as vegetation, chemical composition of water, sun light, carbon dioxide content and substratum matters (WHO, 1956).

2.8.2.3 Dissolved oxygen:

Pulmonates snail have lungs enable them to use atmospheric oxygen for respiration when they come to the surfaces, and they use their mantles for respiration when they are submerged. Snails depend more on oxygen dissolved in water than atmospheric oxygen but low oxygen concentration reduce movement, feeding and reproduction, in Sudan the dissolved oxygen averaged between 4.7 to 7 p.p.m (Malek, 1958).

2.8.2.4 Pollution:

Water contamination causes by decaying organic matters of animal or vegetation origin, urine and feces excreta, or even industrial wastes; such polluted habitat is favorable for intermediate hosts, decaying organic matters in canal waters have strong relation with snail occurrence, Chemical pollution from discharged industrial wastes has lethal effect on snail population (WHO, 1956). Pollution has an indirect effect on vegetation by reducing the oxygen supply which alters the nature of the bottom substratum. Although polluted water may harbor bilharziasis actual and potential hosts as it reported in the White Nile bank at kosti (Malek,1950), many perennial streams between Kalkuting and Niertiti, In Jabel Marra western Sudan Biomphalariapfeifferi and Bulinus spp. were found in heavily polluted water with human and animal excrement, but the water has well flow (Malek, 1950 ).

2.8.3 Biologicalfactors:

The effect of fauna and flora upon snail's population is very powerful, the relationships of snails with animals include commensalism, symbionts, predation and parasitism, while the associated plants are aquatic weeds and microflora. Many vertebrates and invertebrates attack snails; also ecto-parasites and endo-parasites, fungi, viruses, bacteria, digenic trematodes and leeches.

Cleoptera of the family Dytiscidae and the dragon fly nymphs prey on aquatic snail egg masses (Taylor, 1894; cooke, 1913), some snail species act as competitors or predators like Marisacornuariet , pila spp. ,Lanistus spp., also fishes like Gambusiaaffinis , Lebistesreteculatus can prey on snails just like some birds species such as open-bill stork (Anastomuslamelligerus) ,wood rail (Himantornishaematopus) ,the fin foot (Podicasenegalensis) and certain species of ducks (Pteronettahartlaubi) (WHO, 1956; Malek, 1958).

2.8.3.1 Aquatic weeds:

The aquatic fauna may be free-flouting, totally emerged or sub-merged; broad leaf plants provide suitable surface for egg deposition and protect snails of sun light. In canals of Sememo irrigated area Andrews in 1945 reported Spirogyradecina and S.maxima as flouting plants while the important plants near the banks were Cyperus, Vossia, Phragmites and Echinochloa. In the

Gezira irrigation scheme in Sudan Hilali et al. (1985) found that snails were often associated with aquatic macrophytes. A similar association was observed by Madsen et al. (1988).

Rain fall cycles affect snail life cycle by altering water level which has a reflection on population size, humidity, temperature and all of this elements are collaborated together leading in snail population fluctuation (WHO,1956). Canals of the Gezira scheme support snails where there is a diurnal rise or fall of 20-30 cm, and Malek (1958) points out that small gradual change of water level do not seem to disturb snails (Osman,A. 2012).

2.9 Snail control measures

Snails control is an important preventive measure in an integrated approach to control schistosomiasis transmission (Madsen and Christensen, 1992; Sturrock, 2001).With the introduction of new and safer drugs for the treatment of schistosomiasis, and, in many places, improvements in water supply and sanitation facilities, snail control is perhaps employed less often as a means of combating the disease (WH0, 1995). However, it remains an important and effective measure, especially when transmission occurs to a signiﬁcant extent through children playing in water. This type of water contact is not likely to be changed through health education and the provision of safe water supplies (Ling B et al, 1993). Prior to undertaking snail control measures, health authorities should screen water for the presence of snail intermediate hosts. Snails can be controlled indirectly by reducing their habitat or directly by removing them. Where these measures are not sufﬁcient to eliminate snail populations the use of chemicals that kill the snails (molluscicides) may be considered (Fenwick A., 1987).

The decision to do this, and the activity itself, must be the responsibility of technically qualiﬁed people. The use of molluscicides has been and still is the most important method for controlling snail hosts. It is most effective against aquatic species of the genera Bulinus and Biomphalaria(Klumpp RK, Chu KY,1987). Molluscicides are less effective against the amphibious Oncomelania species that transmit S. japonicum; environmental management measures are usually more cost-effective. Snail control may be carried out by special teams or by primary health care personnel with some training in the epidemiology and control of schistosomiasis. Where transmission sites are well known, small in number and easily accessible, the community may also play an active role in control activities.(Fenwick A., 1987).

2.9.1 Environmental management

The methods of environmental management include drainage, ﬁlling in, and the lining of canals with concrete. These methods are generally expensive but long-lasting(Ling B et al,1993).

2.9.1.2 Reduction of snail habitats

Snails need vegetation for food, shelter and a substrate for their eggs. The removal of vegetation in irrigation ditches and canals reduces the number of snails. However, to clear manually, someoneusually has to get into the water which is dangerous, while mechanical clearance is very expensive. The cleaning of canals may also help in the control of other diseases, including malaria, and may improve the effective use of irrigation water. A disadvantage of this method is the need for frequent repetition. Where sufﬁcient resources are available, canals can be lined with cement to prevent or reduce the growth of vegetation. People can also remove plants from places where children swim or where clothes or dishes are washed. Under certain conditions the plant-eating Chinese grass carp (Ctenopharyngodonidella) may be suitable for the biological control of aquatic plants (Ling, B. etal1995). 2.9.1. 3Alteration of water levels and ﬂow rates

Where water quantity is not a limiting factor, raising and lowering water levels and increasing ﬂow rates can disturb snail habitats and their food sources. Rapid complete drainage reduces the amount of vegetation and kills the snails by desiccation. This method may be of interest in areas with irrigated crops (WHO, 1997).

2.9.1.4Elimination of breeding sites

Borrow-pits, small pools and ponds serving no special purpose may be drained or ﬁlled if they are found to be important sites for the transmission of schistosomiasis (Ling B et al.1995)

2.9.1.5 Removal and destruction

Snails can be removed from canals and watercourses with dredges and crushed or left to die of desiccation. This happens in irrigated areas of Egypt and Sudan as a beneﬁcial side-effect of efforts to improve the ﬂow of water by removing mud from canal bottoms (Ling B. et al, 1993).

2.9.2Biologicalcontrol:

Friendly-environmental approach has to develop after the negative result of molluscicide by using predators and competitors organisms against snails.The use of Marisacornuarietis in Puerto Rico and Pomaceahaustrum in Brazil, also Barbosa (1973) reported that introduction of Biom. straminea into habitat of Biom. glabrata eliminate them after three years. Introduction of predators in snail habitat responsible for high mortality of young Lymnaeelodes (Eisenberg, 1970; Hopkins, 1973). Fishes also are used as bio-control agent in the countries of eastern mediterinain region, Somalia and Sudan. WHO (2003) lists 34 fish species, Tilapiamelanopleura, Astronotusocellatus (Feitosaand Milward, 1986), aquatic birds like ducks (Michelson, 1957), chelonian (Coelho et al., 1975) are also recorded as snail predators. Dragon fly nymphs, giant water beetles, are serving as snail predators, in laboratory observations leeches, ostracods crustacean found to be snail predator (Guimaret al., 1983). There are only few examples of successful biological control of schistosome intermediate hosts, and they have been applied only on limited scale (Pointier and Giboda 1999). The focus of biological control has been on the introduction of a non-susceptible snail species that may act as competitors of the intermediate hosts (Madsen, 1990, 1996b). One promising component of biological control is the introduction of parasite resistant snails into endemic areas to replace the resident susceptible snails and avoid the often destructive changes to the local ecosystem that accompany other methods of snail control (Sturrock, 2001). 2.9.3Chemical control

In the past, molluscicides were often applied on an area-wide basis. This costly and environmentally harmful method has been replaced by focal application (Fenwick A., 1987 and Klumpp RK, Chu KY, 1987). Studies are ﬁrst carried out to identify sites and seasons of transmission and only at such sites are chemicals applied periodically. Applications are usually restricted to places frequently used by the local population for swimming, washing, bathing and so on. Currently only one chemical molluscicide, niclosamide, is acceptable for operational use in snail control programmes (Fenwick A., 1987). Other molluscicides, including some of plant origin, are being evaluated. Because of its high cost, niclosamide is used only sparingly in a few local control programmes. At low concentrations it is highly toxic to snails and their egg masses. For practical use a concentration of 0.6–1 mg/l is recommended with an exposure time of eight hours. The compound is safe to handle and after dilution is non-toxic to water plants and crops; however, it is very toxic to ﬁsh. Fish killed by the molluscicide can be safely eaten. When used

focally and seasonally, molluscicide application should not cause any serious negative impact on the environment (Ling B et al, 1993).

2.9.4 Integrated snail control measures:

This approach is defined by the WHO as a rational decision-making process for the optimal use of resources for vector control, based on that the combination between environmental, bio-gents and chemical approaches has to give an exceptional result in snail elimination (McCullough and Mott, 1983) such progression was achieved in Philippines (Osman, A. 2012).

CHAPTER -THREE RESEARCH METHODS AND MATERIALS USED

3.1: Study Area:CountryProfile; Eritrea is located in the Horn of Africa, between latitudes 12 degrees 42’N and 18 degrees 2’N and longitudes 36 degrees 30’E to 43 degrees 20’E. It is bounded by the Sudan to the North and West, the Red Sea to the East, Ethiopia to the South and the Republic of Djibouti to the Southeast. The country has a surface area of about 124,000 square kilometres with four distinct topographic regions: central highlands (2000 meters above sea-level), western lowlands (1000 meters above sea level), eastern lowlands (500 meters above sea level) and coastal lands (500 meters above sea level).

Administratively the country is divided into six administrative zobas (see

Figure 1) known as Zobas, namely; GashBarka (GB), Anseba, Debub, Debubawi Keih Bahri (DKB), Maekel (Ma) and Semenawi Keih Bahri (SKB).The zobas are further divided into 58 sub-zobas, 699 local administrative areas (Memhdar Kebabies) and 2,564 villages. Although, no population census has been carried out to date, the estimated population of the country at the beginning of 2015 was 3,599,538.The population growth rate is 3.0% and total fertility rate was 4.8 children per woman in 2010 (NSO and Fafo, 2013). Life expectancy at birth is 62 years for both sexes (UNICEF, 2014).

At the end of 2013, 340 health facilities reported to the HMIS. An estimated 75% of the population haveaccess to health services at a radius of 10Kms (HMIS 2012). There are 870 Primary/elementary schools in Eritrea, distributed 6-30 per sub-Zoba and enrolment in 2011- 2012 was 482,609 in (Ministry of Education 2012).

Figure 1: Administrative map of Eritrea

(Figure 1, Eritrean, MoH, survey 2014 & 2015)

As described in the above, Southern region is one of the six zones of the country, bordered with Ethiopia to the south, with Gash Barka zone to the west, with the Southern red sea zone to the east and with Central zone to the north. Since the south region borders with Ethiopia and there is always cross bordering movements to and from the neibouring country Ethiopia. As a result, the disease transmission could also increase the number of cases of the disease.

Adi Quala sub-zone is one of the twelve sub zones of Southern region, located towards the south of the region. Bordered to south with Ethiopia, to west with Maimine, to north with Emnihaili

and to the east with Tserona sub zones with a total surface area of 860.73 square kms. Adi Quala sub zone has twenty two (22) local administratives, 130 villages and about 93,413 number of population and has 700mm average annual rain fall and is located with an altitude of 1500-2000 meters above the sea level(Eritrean, MoH, survey 2014 & 2015)

It includes one hospital, one community hospital and 02 health stations. And the study area, sememo locality, is one of the 22 localities of AdiQuala sub zone, which is well known in irrigational canals, schemes and agricultural activities. The study area is found inSememo locality,AdiQuala sub zone, in Debub region, Eritrea. AdiQuala sub-zone is one of the twelve sub zones of Debub region, located towards the south of the region. Borderes to south with Ethiopia, to west with Maimine, to north with Emnihailiand to the east with Tserona sub zones with a total surface area of 860.73 square kms.AdiQuala sub zone has twenty two (22) local administratives, 130 villages and about 93,413 number of population and has 700mm average annual rain fall and is located with an altitude of 1500-2000 meters above the sea level. It includes one hospital, one community hospital and 02 health stations.And the study area, sememo locality, is one of the 22 localities of AdiQuala sub zone, which is well known in irrigational canals, schemes and agricultural activities(Eritrean, MoH, survey 2014 & 2015)

.The selected breeding site The study is being conducted in the Sememo scheme canal. The selected canal is minor Canal and known site of infected communities with schistosomiasis.

The water contact of both human and animals is being observed during the study. One section with 200 meters length is divided into ten sites each of it is measured about 20 meters was selected to conduct the study. 3.2 Study Population: All species of adult snails found during the study. 3.3StudyDesign: Cross-sectional study was conducted in Sememo canal to collect medical and veterinary snail intermediate hosts. Six surveys have been done in Sememo canals, which is an endemic area.

Snails were collected using scooping method and were transported to the zonal entomology Lab. for species identification and screening for the presence of cercariae The snails were screened in the laboratory for cercariae and their prevalence of natural infection was estimated. Also the ecological parameters / the associated factors which affect the presence of Schistosome intermediate host and other snails which act as human and animal hosts in the canals like temperature, depth, pH of the water were measured &water velocity, water turbidity,vegetation type & density were observed. 3.4 Malacological Survey 3.4.1Sampling Techniques Three surveys were conducted from Jan-June (2018) in the selected section of the canal.Snail density was estimated by using scooping method. The snails collected were identified according to morphological characteristics, while the information on habitats and environmental factors were collected by specially designed format.

3.4.2 Collection method:

Snails were collected by scooping method, which is a flat wire-mesh of metal frame (40×30cm) supported a mesh of 1.5 micro-size attached to an iron handle of 1.5 meter long as described by (Amin, 1972), by taking 15 dips/site, start with the edge then scraped the bottom and vegetation. The collected samples were put in jars containing water of the canal and transported to zonalentomology laboratory for sorting out and identification.

Materials used for the collection of snails were; scoops, long forceps, plastic bottles, a bag to put all equipments, forms, a pencil, a note book, personal protection tools like; gloves, gown, mask, eye google, boots and straw hat.

3.4.3 Ecological factorsmonitoring: The main ecological factors influencing snail populations were monitored in each survey; measuring water temperature, water depth, water pH and observation of water turbidityand velocity. Also the presence of predators, vegetation density and types were recorded.

3.4.3.1Watertemperature: The temperature was measured per site using mercury thermometer (0-50°C) by immersing it for 15 minutes under the water surface. 3.4.3.2Waterdepth: A two meters wooden stick was immersed twice in the water to measure the depthand then the average was calculated.

3.4.3.3 Waterturbidity: It was observed at the canal and determined as clear, low, medium or high turbid water according to the colorof the water. 3.4.3.4 Watervelocity: It was observed and recorded as stagnant, slow, medium, and fast running water. 3.4.3.5WaterpH: The PH value of the water was measured using portablepH-meter by immersing it in the water of the canal for 30 minutes. 3.4.3.6 Vegetation density: The density of vegetationwas observed and reported as clear/ none, sparse where there is little amount; low, medium and thick, and then the vegetation samples were taken from the canal and transported to the MoA, Department of crop protection for species identification. 3.4.3.7 Presence of predators, animals and birds: Predators species presence was recorded, and the animals (cows, sheep, donkeys, goats, others) and birds in contact with the canal at the collection time were recorded. 3.4.4 Transportation: Dry shells and preserved specimens were sent to the zonal entomology Lab. in any suitable container. Each lot was properly labeled. Too frequently specimens were sent with illegible labels written in pencil on poor paper or with a reference number of no value to the recipient.The collected samples were put in jars containing water of the canal and thentransported to the laboratory for sorting out and identification.

Small snails were transported from large ones separately to avoid predators and the collected samples were labeled at least with the following information;

 Locations name  Type of snail species  Date of collection  Collectors group name  Time spent for collection i.e Starting & ending time

3.5Laboratory work: 3.5.1 Snails identification: All collected snails from the field were identified morphologically. The schistosomiasis hosts (Biomphalaria and Bulinus were screened first) and then were put in 96% Ethanol for one day. On the next day, the shells were left to dry out prepared for morphological identification. The dead animals were preserved in 80% Ethanol, The collected snails were identified to the genus and species level, guided by Mandle-Barth (1962) key to the identification of East and Central African Fresh water snail of medical and veterinary importance based on morphological features only. 3.5.2 Snails preservation The collected snails were put in 70% ethyl alcohol for preservation. For identification, this method is sufficient, if the volume of alcohol is at least twice the volume of snails. The snails were preserved in 70% ethanol or 5% formaldehyde for disinfection following the Danish bilharzia laboratory key developed by Frandsen and Christensen (1984). 3.5.3 Natural infection of the snails (Snail screening for cercariae):

The collected& identified snails were examined for trematode cercariae presence; 5-10 snails of the same species were put in 5ml of distilled water in glass bottles and then exposed to artificial light for 3-5 hours (Webbe and Sturrock, 1964). The presence of trematode cercariae was confirmed under dissecting microscope and some iodine drops were added to kill and stain cercariae for identification. The cercariae were preserved in 5% formaldhyde and 70% ethanol for identification following the Danish Bilharzia laboratory key developed by Frandsen and Christensen (1984).

Materials used for snails screeningwere;small glass bottles, long forceps, droppers,Artificial Lamp, distilled water, 70% & 80% alcohol, iodine, forms, a pencil, a note book and personal protection tools like, medical gowns, gloves, mask, eye google, and boots.

3.6 Data Analysis

Data was entered into SPSS (Version 22) after being collected. Data cleaning was done to control the possible data entry errors. Descriptive analysis was performed using frequency (percentage), mean (SD), and median (IQR) after checking for normality as appropriate. Taking the number of sites into consideration, non-parametric statistical techniques were mainly used in making comparisons across species and surveys. Comparisons of the infectivity rate across the pairs of species and surveys were conducted using difference between two-proportion approaches. Mann-Whitney U test was performed to look over the predominant species in each survey by making pair wise comparisons. Furthermore, in order to observe the possible changes in the average level of each species at survey 1, survey 2, and survey 3, Friedman two-way ANOVA was used. Similarly, Friedman two-way ANOVA was used to check the difference in the overall density of the three species at the three surveys. Spearman rank correlation coefficient was also used to assess the association of density of the species and the quantitative ecological factors. The difference in density of the species across categories of water turbidity, vegetation density, and water velocity were assessed using Kruskal-Wallis test, followed by Mann-Whitney post-hoc. Finally, the ten sites in each survey of the study were grouped into two, as recommended by non-hierarchical clustering, depending on the level of density and pH, using cluster analysis technique. The clustering was validated using Mann-Whitney U test. Bar graphs, line graphs, box-plots, dendrograms, and tables were used to present the data. P-values less than 0.05 were considered as significant throughout the study.

CHAPTER-FOUR RESULTS AND DICUSSION 4.1 RESULTS 4.1.1Identification of Snails Species; Three types of fresh water snail species (Biomphlaria pfeifferi, Lymnae natenalis and Bulinus trancatus) were found in the canal during the study period.In total, 4721 snails were collected during the three surveys (Table 1). Out of the total snails collected, 1427 snails were examined. The percent of snails examined during survey 1, survey 2, and survey 3 were 23.4%, 35.0%, and 42.6% respectively.

Table 4.1: Snails collected and examined at Sememo locality, Eritrea, Janurary-May 2018.

Snails Examined % Survey Collected Snails Examined Survey-1 2053 480 23.4% Survey-2 1559 546 35.0% Survey-3 1109 472 42.6%

4.1.2; Prevalence Rate in all the Surveys 4.1.2.1; Over all prevalence rate The overall prevalence rate in the ten sites during the three surveys of Bulinus trancatus, Biomphlaria pfeifferi and Lymnaenatenalis species were 1.7%, 66%, and 32.3% respectively. Comparison based on prevalence solely revealed that Biomphlaria (66%) were almost as twice prevalent as Lymnae (32.3%), however, very minor percent (1.7%) of bulinus trancatus occurred at the study region.

Figure4.1: Prevalence of Bulinus trancatus, Biomphlariapfeifferi, and Lymnaenatenalisduring the three surveys at Sememo, Eritrea, Janurary to June 2018.

4.1.2.2 Prevalence rate by Survey The trend of the three species at the three different periods of study is also another important issue needed to be explored. Prevalence of Lymnae was found to increase from 0.5% in survey 1 (January 2018) to 2.6% in survey 2 (April 2018) and finally to 2.7% in survey 3 (May 2018), in a trivial manner. Similarly, the prevalence rate of Biomphlaria was observed to increase from survey 1 (62.4%) to survey 2 (66.2%) and finally to 72.7% in survey 3. However, the prevalence rate of Bulinus trancatus species was observed to decrease from January (37.2%) to March (31.2%), which finally has reached 24.6% in May.

Figure 4.2: Prevalence rate of the species by survey at Sememo locality, Eritrea, Janurary- May 2018.

4.1.2.3; Prevalence rate by Site The prevalence rate of the three species at the ten different sites ranged from 4.7 to 17.0. Site 3 (4.7%) was found to have the least prevalence followed by site 8 (5.1%). Site 5 (17%) was the most prevalent followed by site 8 (15.1%). The prevalence rate by site is shown in Figure 4.3.

Figure4.3 : Prevalence rate of the species by site at Sememo locality, Eritrea, Janurary- May 2018. 4.1.2.4; Prevalence by Site and Species type Prevalence rate for each site was computed in two manners. In the first, the prevalence of each species out of the total of that species in all the sites was computed to make comparisons among the sites. In order to make comparisons among the species, another prevalence rate was computed out of the total of the three species in each site.

The Sites at which prevalence rate of Biomphlaria has exceeded 10% were 1, 4, 5, 9, and 10. Similarly, prevalence of Lymnae has exceeded 10% in Sites 2, 4, 5, 9, and 10. However, the prevalence of Bulinus trancatus exceeded 10% except in Sites 9, 5, and 2.

25.0%

20.0% 18.1%

20.5%

15.0% 16.5% 13.5% 12.6% 14.2% 15.3% 12.1% 10.0% 11.0% 10.4% 7.4% 8.8% 7.6% PrevalenceRate 5.1% 4.4% 5.0% 5.8% 5.3% 4.5% 4.8% 2.2% 0.0% 1 2 3 4 5 6 7 8 9 10 Site

Biomphlaria Lymnae Bulinus trancatus

Figure 4.4: Prevalence rate of the species by site (Out of the total of each species in all sites), at Sememo locality, Eritrea, Janurary-May 2018.

The prevalence rate of each species out of the total of each site has clearly revealed that Biomphlaria pfeifferi are the most predominant species in the region followed by Lymnae natenalis. Moreover, a trivial occurence of Bulinus trancatus was also realized. The lowest prevalence of the predominant species type, Biomphlaria, occurred at Site 10 accounting 55.0% and the highest prevalence occurred at Site 7 accounting 84.0%.

Relatively higher prevalence rate (43.7%) of Lymnae species was observed at Site 10, which actually complemented the prevalence of Bomphlaria. Lowest prevalence (12.3%) of Lymnae was observed at Site 7.

Prevalence of Bulinus trancatus was observed to be very small as compared with Biompharia and Lymnae in all the sites. The highest prevalence rate (4.2%) of Bulinus trancatus was observed at Site 8, followed by Site 7 (4.1%). On the other hand, the lowest prevalence rate (0.2%) was observed at Site 9.

90.0% 82.7% 84.1% 80.0% 70.2% 71.0% 66.9% 70.0% 62.7% 61.2% 58.7% 59.9% 60.0% 55.0%

50.0% 43.7% 40.0% 37.4% 39.1% 40.0% 33.2% 29.1% 28.9%

30.0% 26.3% PrevalenceRate 20.0% 15.7% 12.3%

10.0% 4.1% 2.7% 3.6% 4.2% 1.6% 1.3% 2.2% 0.7% 0.2% 1.3% 0.0% 1 2 3 4 5 6 7 8 9 10 Site

Biomphlaria Lymnae Bulinus trancatus

Figure 4.5: Prevalence rate of each species by site (Out of the total species in each site) at Sememo locality, Eritrea, Janurary-May 2018.

4.1.3; Snails and their Infectivity Rate From the total 1427 examined snails, 668 (44.59%, 95%CI: 42.06, 47.15) were found to be infected in the three surveys.

From a total of 127 Bulinus trancatus examined in the three surveys, not even one infected snail was obtained. However, higher infectivity rate was observed among Lymnae species, in which, 218 (63.56%, 95%CI: 58.22, 68.66) out of 343 were found infected in the three surveys. On the other hand,450 out of 1028 Biomphlariapfeifferiwere found to be infected in the three surveys making the infectivity rate 43.77% (95%CI: 40.71, 46.87).

There was no change in infectivity rate of Bulinus trancatus across the three surveys because no infected snail of this type was found. Analysis of Biomphlaria infectivity rate showed a decreasing trend from Survey 1 (52.69%), to survey 2 (42.94%), with further decrease in survey 3 (35.88%). However, infectivity rate of Lymnae showed increasing trend from survey 1 (57.45%), to survey 2 (62.25%), which finally has reached 71.43% in survey 3.

Table 4.2: Species examined, infected and their infectivity rate during the three surveys, at Sememo locality, Eritrea, Janurary-May 2018.

Infectivity 95% CI Survey No. Examined Infected Rate Infectivity Rate Bulinus trancatus Survey-1 52 0 0 0, 0 Survey-2 41 0 0 0, 0 Survey-3 34 0 0 0, 0 Total/ average 127 0 0 0, 0 Biomphlariapffeiferi Survey-1 334 176 52.69 47.19, 58.15 Survey-2 354 152 42.94 37.72, 48.28 Survey-3 340 122 35.88 30.78, 41.23 Total/ average 1028 450 43.77 40.71, 46.87 Lymnaenatanelis Survey-1 94 54 57.45 46.82, 67.59 Survey-2 151 94 62.25 54.01, 70.00 Survey-3 98 70 71.43 61.42, 80.10 Total/ average 343 218 63.56 58.22, 68.66

4.3.1; Comparison in Infectivity rate of the different Species within the same Survey Pair wise comparison of infectivity rate on the three types of species was done for the three surveys.

In survey 1, Biomphalria infectivity rate was significantly greater than Bulimus trancatus (p<0.001), Lymnae than Bulims trancatus (p<0.001). However, no significant difference in infectivity rate was observed between Lymnae and Biomphlaria in survey 1 (p<0.001).

In survey 2, significantly higher infectivity rate of Biomphlaria in comparison to Bulinus trancatus (p<0.001), Lymnae in comparison to Biomphlaria (p<0.001), and Lymnae in comparision to Bulinus trancatus (p<0.001) were observed.

Similarly, survey 3 results showed that infectivity rate of Biomphlaria was significantly higher than Bulinus trancatus (p<0.001). Moreover, infectivity rate of Lymnae was significantly higher than that of Biomphlaria (p<0.001) and Lymnae than Bulinus trancatus (p<0.001).

Table 4.3; Comparison of infectivity rate of the different species within the same survey, at Sememo locality, Eritrea, Janurary-May 2018.

Diff. Infectivity rate Survey Snail Species (95% CI) p-value survey 1 Biomphlaria Vs Bulinus trancatus 52.69 (47.34, 58.05) <0.001 Lymnae Vs Biomphlaria 4.75 (-6.59, 16.09) 0.411 Lymnae Vs Bulinus trancatus 57.45 (47.45, 67.44) <0.001 Survey 2 Biomphlaria Vs Bulinus trancatus 42.94 (37.78, 48.09) <0.001 Lymnae Vs Biomphlaria 19.31 (10.02, 28.61) <0.001 Lymnae Vs Bulinus trancatus 62.25 (54.52, 69.98) <0.001 Survey 3 Biomphlaria Vs Bulinus trancatus 35.88 (30.78, 40.98) <0.001 Lymnae Vs Biomphlaria 35.54 (25.25, 45.84) <0.001 Lymnae Vs Bulinus trancatus 71.43 (62.48, 80.37) <0.001

4.3.2; Comparison in Infectivity rate of the same species between the three surveys The three types of species that were found during the three surveys were compared to check on the difference of their infectivity rate at various points of time (months of collection).

Infectivity rate of Bulinus trancatus at survey 1, survey 2, and Survey 3, was not significantly different, for there was no infected snail of this type.

Infectivity rate of Biomphlaria at survey 1 was significantly higher than that of survey 2 (p=0.010). Similarly, significantly higher Biomphlaria infectivity rate was observed in survey 1 than survey 3 (p<0.001). However, there was no significant difference in infectivity rate between surveys 2 and 3 (p=0.057).

Infectivity rate of Lymnae at survey 1 was significantly higher than that of survey 3 (p=0.041). However, infectivity rate of surveys 2 and 3 (p=0.128) and surveys 1 and 2 (p=0.456) were not significantly different.

Table4.4: Comparison of infectivity rate of the same species between the three surveys, at Sememo locality, Eritrea, Janurary-May 2018.

Survey 1 Vs Diff. Infectivity rate Species type Survey 2 (95% CI) p-value Bulinus trancatus Survey 1 Vs Survey 2 0 (-) 1.000 Survey 2 Vs Survey 3 0 (-) 1.000 Survey 1 Vs Survey 3 0 (-) 1.000 Biomphlaria Survey 1 Vs Survey 2 9.76 (2.30, 17.19) 0.010 Survey 2 Vs Survey 3 7.06 (-0.20, 14.31) 0.057 Survey 1 Vs Survey 3 16.81 (9.42, 24.21) <0.001 Lymnae Survey 1 Vs Survey 2 -4.80 (-17.44, 7.83) 0.456 Survey 2 Vs Survey 3 -9.17 (-20.00, 2.65) 0.128 Survey 1 Vs Survey 3 -13.98 (-27.39, -0.57) 0.041

4.3.3; Types of Cercariae In this study, the investigated cercariae were classified into two; as human and non-human. Out of the total 928 cercariae, 490 (52.8%) were human and the remaining 438 (47.2%) were non- human. Viewed survey wise, 326 (35.1%) were found in survey one, 334 (36.0%) in survey 2, and 268 (28.9%) in survey 3. Moreover, less human cercariae (n=122, 37.4%) were observed than non-human cercariae (n=204, 62.6%) in survey 1. However, human cercariae in survey 2 (60.5%) and survey 3 (61.9%) were relatively higher than non-human cercariae.

Table4.5: Types of Cercariae found; human and non-human, at Sememo locality, Eritrea, Janurary-May 2018. Type of cercariae Human Non-human Survey n (%) n (%) Total Survey-1 122 (37.4) 204 (62.6) 326 Survey-2 202 (60.5) 132 (39.5) 334 Survey-3 `166 (61.9) 102 (38.1) 268 Total 324 (52.8) 438 (47.2) 928

Figure4.6; Types of cercariae present in Sememo canal during the study period, Jan-June 2018

4.3.4; Comparison on number of the different Species within the same Survey Comparison on number of species was performed to look over the predominant species at Sememo locality.

On the average, Biomphlaria existed significantly higher in number than Bulinus trancatus (p<0.001) in survey 1. In the same survey, Lymnae also existed in a significantly higher in number than Bulinus trancatus (p<0.001). However, Lymnae were not significantly different in number than Biomphlaria, (p=0.123).

In survey 2, the number of Biomphlaria species were significantly greater than Bulinus trancatus (p<0.001), Biomphlaria than Lymnae (p=0.019), and Lymnae than Bulinus (p=0.002). Hence, the predominant species found were Biomphlaria, followed by Lymnae.

In like manner, Biomphlaria species were significantly greter than Bulinus trancatus (p<0.001), Biomphlaria than Lymnae (p=0.005), and Lymnae than Bulinus trancatus (p<0.001) in survey 3.

Table4.6;Comparison on number of the different Species within the same Survey, Sememo canal, Eritrea, Jan-June 2018

Snail Type 1 Vs Survey Snail Type 2 Median (IQR) Mann-Whitney U (Z) p-value Survey 1 Biomphlaria 132.00 (131.50) 0.00 (-3.80) <0.001 Bulinus trancatus 1.00 (2.00) Lymnae 69.00 (107.50) 29.00 (-1.59) 0.123 Biomphlaria 132.00 (131.50) Lymnae 69.00 (107.50) 0.00 (-3.80) <0.001 Bulinus trancatus 1.00 (2.00) Survey 2 Biomphlaria 73.00 (77.25) 0.00 (-3.80) <0.001 Bulinus trancatus 4.500 (2.00) Lymnae 41.00 (58.00) 19.00 (-2.35) 0.019 Biomphlaria 73.00 (77.25) Lymnae 41.00 (58.00) 9.00 (-3.12) 0.002 Bulinus trancatus 4.500 (2.00) Survey 3 Biomphlaria 65.50 (66.50) 0.00 (-3.80) <0.001 Bulinus trancatus 3.00 (2.00) Lymnae 20.50 (30.50) 12.50 (-2.84) 0.005 Biomphlaria 65.50 (66.50) Lymnae 20.50 (30.50) 0.50 (-3.76) <0.001 Bulinus trancatus 3.00 (2.00)

4.3.4; Comparison of the same Species between the three Surveys The median (IQR) occurrence of Biomphlaria species during the three survey periods at Sememo locality was 73 (97.5). The median (IQR) frequency of occurrence of Biomphlaria species at surveys 1, 2, and 3 were 132 (132), 73 (77), and 66 (67) respectively. Friedman two way ANOVA has shown that, the average occurrence was not significantly different at the three time periods (Chi-square=1.80,df=2, p=0.407).

250

200

e u

q 150

e 132

a l

h 100

p 73

m o

i 65.5 B 50

0 1 2 3 Survey Number

Figure 4.7: Comparison on occurrence of Biomphlaria species among surveys 1, 2, and 3 at Sememo locality, Eritrea, Janurary-May 2018.

The overall median (IQR) number of Lymnae at all the surveys was 27 (IQR=59.75). The median (IQR) level of Lymnae in the three respective surveys (1, 2, and 3) were 69 (108), 41 (58), and 21 (31) respectively. No significant difference in the average level of the species was observed at the three surveys (Chi-square=5.40, df=2, p=0.067) using Friedman test.

200

150

q e

r 100

a 69

y L 50 41

20.5

1 2 3 Survey Number

Figure 4.8: Comparison on occurrence of Lymnae species among surveys 1, 2, and 3 Sememo locality, Eritrea, Janurary-May 2018..

On the other hand, the median (IQR) level of Bulinus species in all the three surveys was 3.0 (IQR=3). The median (IQR) across the three surveys were 1.0 (2.0), 4.50 (2.0), and 3.00 (2.0) at sureys 1, 2, and 3 respectively. There was significant difference in median level of Bulinus at the three surveys (Chi-square=14.49,df=2, p=0.001).

4.5 y

c 4

e u

q 3 e

r 3

i l

u 2 B 1 1

1 2 3 Survey Number

Figure 4.9: Comparison on occurrence of Bulinus species among surveys 1, 2, and 3 Sememo locality, Eritrea, Janurary-May 2018.

4.3.5;Comparison of Densities during the three Surveys The median (IQR) density of the three snail species during the three surveys at Sememo locality was 7.97 (11.05). The median (IQR) densities of the three species at survey 1, survey 2, and survey 3 were 14.27 (14.30), 7.97 (10.40), and 6.50 (5.90) respectively. Friedman two way ANOVA test has shown no significant difference in median level of species at the three time periods (Chi-square=3.23,df=2, p=0.199).

t i

s 15 14.27

e D

10 7.97 6.50 5

0 1 2 3 Survey Number

Figure 4.10: Comparison of the density of the three species of snails Surveys 1, 2, and 3 at Sememo locality, Eritrea, Janurary-May 2018.

4.1.4;Descriptive Analysis of environmental factorsand organic matters affecting snails’ population at the three different periods (Surveys) Minimum and maximum temperatures at Sememo locality in January 2018 (Survey 1) were 20.000C and 35.000C respectively, with median value of 25.500C. The median water depth and water pH were 27.50Cm and 7.50 respectively. Furthermore, the median number of cows, donkeys, sheep and goats, human beings, and birds were 17, 6, 25, 9, and 30 respectively.

Table4.7: Potential environmental and organic factors affecting snails’ population, Sememo locality, January 2018. Environmental and organic Factors Mean (SD) Median (IQR) Minimum Maximum Water Temperature (0C) 25.40 (4.27) 25.50 (5.50) 20.00 35.00 Water Depth (Cm) 27.00 (7.53) 27.50 (12.50) 20.00 35.00 Water pH 7.65 (0.31) 7.50 (0.56) 7.37 8.15 Number of Cows 17.10 (5.07) 17.00 (9.25) 9.00 25.00 Number of Donkeys 6.70 (3.09) 6.00 (3.25) 3.00 13.00 Number of Sheep and Goats 26.50 (5.46) 25.00 (11.25) 20.00 34.00 Number of Human beings 9.10 (2.64) 9.00 (5.25) 6.00 13.00 Number of Birds 28.70 (3.13) 30.00 (6.25) 24.00 32.00 Water Turbidity Clear (n=1, 10%), Less (n=5, 50%), Medium (n=4, 40%) Water Velocity Slow (n=8, 80%), Medium (n=2, 20%) Vegetation Density Clear (n=1, 10%), Medium (n=6, 60%), Thick (n=3, 30%)

The median temperature at survey 2 was 28.150C with a minimum and maximum values of 20.000C and 35.000C respectively. The median depth of the water, and pH, were 25.50Cm and 7.64 respectively. Furthermore, the median number of cows, donkeys, sheep and goats, human beings, and birds were 15, 8, 24, 9, and 28 respectively.

Table4.8: Potential environmental factors affecting snails’ population, Sememo locality, April 2018. Environmental Factors Mean (SD) Median (IQR) Minimum Maximum Water Temperature (0C) 28.53 (3.11) 28.15 (4.88) 23.00 32.80 Water Depth (Cm) 25.10 (5.28) 25.50 (10.00) 15.00 30.00 Water pH 7.74 (0.44) 7.64 (0.85) 7.15 8.40 Number of Cows 13.80 (4.05) 15.00 (5.75) 7.00 21.00 Number of Donkeys 8.70 (3.27) 8.0 (4.25) 5.00 15.00 Number of Sheep and Goats 23.30 (4.30) 24.00 (7.25) 15.00 28.00 Number of Human beings 10.70 (5.81) 9.00 (9.00) 5.00 23.00 Number of Birds 27.50 (4.45) 28.00 (5.50) 19.00 34.00 Water Turbidity Clear (n=3, 30%), Less (n=3, 30%), Medium (n=4, 40%) Water Velocity Stagnant (n=1, 10%), Slow (n=8, 80%), Medium (n=1, 10%) Vegetation Density Clear (n=1, 10%), Medium (n=6, 60%), Thick (n=3, 30%)

The median water temperature (0C), depth (Cm), and pH in survey 3 were 28.250C, 27.50Cm, and 7.53 respectively. Moreover, the median number of cows, donkeys, sheep and goats, human beings, and birds were 14, 12, 24, 9, and 24 respectively.

Table4.9: Potential environmental factors affecting snails’ population, Sememo locality, Eritrea, 2018. Environmental Factors Mean (SD) Median (IQR) Minimum Maximum Water Temperature (0C) 27.92 (2.73) 28.25 (5.38) 24.30 31.50 Water Depth (Cm) 27.00 (7.53) 27.50 (12.50) 15.00 35.00 Water Ph 7.55 (0.34) 7.53 (0.47) 7.02 8.17 Number of Cows 14.70 (3.47) 14.00 (5.00) 10.00 21.00 Number of Donkeys 10.50 (5.15) 12.00 (9.50) 3.00 18.00 Number of Sheep and Goats 23.40 (6.31) 24.00 (12.25) 13.00 32.00 Number of Human beings 8.70 (4.06) 9.00 (8.50) 3.00 14.00 Number of Birds 24.40 (4.17) 24.00 (7.75) 19.00 31.00 Water Turbidity Less (n=6, 60%), Medium (n=3, 30%), High (n=1, 10%) Water Velocity Stagnant (n=1, 10%), Slow (n=8, 80%), Medium (n=1, 10%) Vegetation Density Clear (n=1, 10%), Medium (n=5, 50%), Thick (n=4, 40%)

4.4.1;Difference in Ecological Factors across the time (three Surveys) The differences in ecological factors at survey 1, survey 2, and survey 3 were assessed using Friedman two way ANOVA. The median levels of water temperature at surveys 1, 2, and 3 were not significantly different from each other (p=0.741). Similarly, water depth (p=0.135), water pH (p=1.000), number of cows (p=0.407), number of donkeys (p=0.150), number of sheep and goats (p=0.202), and number of human beings (p=0.388) were not significantly different at the three surveys. However, number of birds at surveys 1, 2, and 3 were significantly different.

Table 4.10; Difference in Ecological Factors across the time (three Surveys),sememo locality,Eritrea,Jan-June 2018

Environmental Factors λ2 value(df) p-value Water Temperature 0.600 (2) 0.741 Water Depth 4.00 (2) 0.135 Water pH 0.00 (2) 1.000 Number of Cows 1.80 (2) 0.407 Number of Donkeys 3.80 (2) 0.150 Number of Sheep and Goats 3.20 (2) 0.202 Number of Human beings 1.99 (2) 0.388 Number of Birds 6.20 (2) 0.045

4.4.2; Ecological Factors affecting Density of species In order to assess the factors affecting density of species, all the data in the three surveys were utilized. This is because the potential difference in density due to the possible variation of environmental factors might not be revealed at surveys 1, 2, and 3 separately. However, the variation of environmental factors at surveys 1, 2, and 3 collectively could disclose the potential difference in densities of the species. Moreover, due to non-normality of density of the species, non-parametric tools (spearman rank correlation and Mann-Whitney/Kruskal Wallis tests) were utilized.

In order to find out the correlation of density of the species and water temperature, water depth, and water pH, spearman rank correlation coefficient was used. Water temperature (p=0.608), and water depth (p=0.691), number of cows (p=0.441), number of donkeys (p=0.970), number of sheep and goats (p=0.181), number of human beings (p=0.429), and number of birds (p=0.529) were not significantly correlated with density of the species. However, water pH was significantly correlated with density of the species (p=0.021).

Table4.11: Correlation of density and ecological factors, Sememo area, Eritrea, Jan-June 2018

Correlation, Variables r (p-value) Water temperature 0.097 (0.608) Water depth 0.076 (0.691) Water pH 0.421 (0.021) Number of Cows -0.146 (0.441) Number of Donkeys 0.007 (0.970) Number of Sheep and Goats 0.251 (0.181) Number of Human beings -0.150 (0.429) Number of Birds -0.120 (0.529)

In order to assess possible differences in density of the species in various levels of turbidity, water velocity and vegetation density, Kruskal-Wallis test was used (Table 4.12). Turbidity was measured at four levels, however, the fourth level ‘high’ has appeared only once and hence was discarded from the analysis. Density of the species was different in at least one of the three categories of turbidity (χ2=6.54, p=0.038). However, no significant differences in density of the species was found across the various water velocities (p=0.782). The data has generated remarkably higher differences in densities of the species across the three vegetation densities (p=0.003).

Table4.12: Association of density with turbidity, water velocity, and vegetation density (n=30)

Variables Median (IQR) Kruskal Walis λ2 (df) p-value Turbidity‡ 6.54 (2) 0.038 Clear 16.93 (15.67) Less 5.47 (4.73) Medium 12.40 (11.67) Water velocity 0.493 (2) 0.782 Stagnant 9.50 (*) Slow 8.70 (10.65) Medium 5.83 (10.40) Vegetation density 11.86 (2) 0.003 Clear 3.8 (2.14) Medium 7.20 (5.86) Thick 15.63 (7.05) * Only two sites were having stagnant water velocity and hence Q1 and Q3 could not be computed, which as a result IQR could not be computed. ‡ Only one value for high turbidity has occurred and hence was not included in Kruskall-Walis Analysis

The variables that were found to be significant using Kruskal-Wallis test were further analyzed using Mann-Whitney U test to make pair-wise comparisons. Post-hoc pair-wise comparisons using Mann-Whitney has revealed significant differences in density between ‘clear’ and ‘less’ levels of turbidity (Mann-Whitney U=6.00, p=0.018). However, density of the species were not significantly different between ‘clear’ and ‘medium’ (Mann-Whitney U=13.00, p=0.280) as well as ‘less’ and ‘medium’ (Mann-Whitney U=47.00, p=0.1107). Similarly, significant differences in density of the species was observed between ‘clear’ and ‘medium’ vegetation densities (Mann-Whitney U=2.00, p=0.007), ‘clear’ and ‘thick’ vegetation densities (Mann-Whitney U=0.00, p=0.007), as well as ‘medium’ and ‘thick’ (Mann-Whitney U=37.50, p=0.015).

Table4.13: Post hoc pair wise analysis on differences in density by turbidity and vegetation density in Sememo locality, Eritrea 2018 Variable Level 1 Level 2 Mann-Whitney (Z) p-value Turbidity Clear Less 6.00 (-2.34) 0.018 Clear Medium 13.00 (-1.18) 0.280 Less Medium 47.00 (-1.64) 0.107 Vegetation Density Clear Medium 2.00 (-2.49) 0.007 Clear Thick 0.00 (-2.54) 0.007 Medium Thick 37.50 (-2.39) 0.015

4.4.3; Grouping of the Sites by density Cluster analysis technique was used to classify the sites into groups, called clusters,which are relatively homogeneous within themselves and heterogeneous between each other, on the basis of a defined set of variables. In the analysis, any number of variables can be included, but since it is best to include variables that are meaningful to the research question, only density and water pH were used. Classification of the sites into clusters will help to make prioritized interventions through identification of sites having similar densities, and water pH.

In order to perform the cluster analysis, the four basic steps were followed: application of ward’s method on the principal component score, check the agglomeration schedule, decide the number of clusters, and apply the k-means cluster analysis. Non-hierarchical clustering technique using Ward’s method was first used to identify the number of clusters at each survey. Then, after fixing the number of clusters using the elbow rule, hierarchical clustering technique was used to construct the k-clusters of each survey.

Clusters in Survey 1 Non-hierarchical clustering using Ward’s method has revealed that 2 clusters need to be constructed in survey 1 based on their similarities. Subsequently, 2-cluster analysis has also revealed that sites 1, 2, 9, 10, 4 and 5 belong to cluster 1, and sites 3, 6, 8, and 7 belong to cluster 2. The dendrogram is showed in Figure 4.11.

Dendrogram Ward Linkage, Euclidean Distance

-123.15

y -48.77

i S

25.62

100.00 1 2 10 9 4 5 3 6 8 7 Site Number

Figure 4.11: Two cluster formation according to the species density, turbidity, and vegetation density in Survey 1 (Red: Cluster 1, Green: Cluster 2).

The median (IQR) density of cluster 1 and cluster 2 were 18.23 (9.17) and 5.33 (1.00) respectively. To assure the validity of the clustering, pair-wise comparison of the clusters was performed using Mann-whitney U test. The density of the species in survey 1 was different at the two formulated clusters (p=0.01). The results for surveys 1, 2, and 3 are shown in Table 4.14

Table 4.14; Validation of the clusters created at Surveys 1, 2, and 3.

Survey Cluster Number Median (IQR) Mann-Whitney (Z) p-value Survey 1 0.0 (-2.57) 0.01 Cluster 1 18.23 (9.17) Cluster 2 5.33 (1.00) Survey 2 0.0 (-2.56) 0.01 Cluster 1 16.53 (2.07) Cluster 2 6.77 (3.37) Survey 3 0.0 (-2.62) 0.008 Cluster 1 4.47 (2.57) Cluster 2 9.47 (4.57)

Clusters in Survey 2

Only two clusters were identified using the elbow method in survey 2. Cluster 1 consists of sites 1, 9, 10, 5 and cluster 2 consists of sites 2, 6, 3, 8,4, and 7. The dendrogram is depicted in Figure 6. The median (IQR) density of species at clusters 1 and 2 in survey 2 were 16.53 (2.07) and 6.77 (3.37) respectively, which are found to be statistically significantly different (p=0.01).

Dendrogram Ward Linkage, Euclidean Distance

-199.99

y -99.99

i S

0.00

100.00 1 9 10 5 2 6 3 8 4 7 Site Number

Figure4.12: Two clusters formation according to the species density, turbidity, and vegetation density in Survey 2 (Red: Cluster 1, Green: Cluster 2).

Clusters in Survey 3

Cluster 1 and Cluster 2, that were identified in Survey 3, consisted of sites 1, 2, 4, 3, 8 and sites 5, 6, 9, 7,10 respectively. The dendrogram is depicted in Figure 7. Validity of the classification was assured by confirming Cluster 1 (Md=4.47, IQR=2.57) had significantly higher density of species than cluster 2 (Md=9.47, IQR=4.57) (p=0.008).

Dendrogram Ward Linkage, Euclidean Distance

-85.94

y -23.96

i S

38.02

100.00 1 4 2 3 8 5 6 9 10 7 Site Number

Figure4.13: Two cluster formation according to the species density and water pH in Survey 3 (Red: Cluster 1, Green: Cluster 2).

4.2 DISCUSSION

Since there was no any entomological study conducted in Eritrea before, the neibouring couuntries studies was used as a baseline data.The agricultural sector development in different countries has a huge role in provision of ideal habitat for snail breeding and snail-water borne diseases progression. Water-associated diseases are debilitating and seriously reduce productivity of agricultural communities in the Sudan (El Tash, 2005). Studies on larval trematode infection in freshwater snails in Africa are modest in number to provide information about, snail-borne diseases, larval trematode fauna and the potential transmission sites. The international health community recapitulated that there is a serious gap of information in biomedical and epidemiological aspects of water borne diseases vectors information, causative agents and disease determinants, to evaluate the trematode effect on snail populations (Ibrahim, 2007).

Human changes to freshwater habitats, such as the construction of small impoundments, have been implicated in facilitating snail populations and creating environments conducive to trematode infection (Johnson, etal., 2002), andPopulations of freshwater snails are subjected to severe ecological stressors imposed by wide temporal fluctuations in their environment. Their success depends on their physiological capacity to tolerate these fluctuations (Russel-Hunter, 1961). Biotic constraints, including parasitism, predation and competition, are added to these ecological stressors. The present study proved that the canalization system in Sememo locality was found to be a good habitat for some of fresh water snails. However, the study found that, there were three different snails species collected from Sememo canal. A total number of 4721 snails were collected during the different three surveys. The overall prevalence of snail species was (1.7%) for Bulinus trancatus, (66%) for Biomphlaria pfeifferi and (32.3%) for Lymnae natanalis. Similar study conducted by Madsen e.tal. 1988 which aimed to investigate the distribution of aquatic macrophytes and molluscan intermediate hosts of schistosomes in the following irrigation systems in the Sudan: the Gezira-Managil Agricultural Scheme (GMAS), the Rahad Agricultural Scheme (RAS) and the New Halfa Agricultural Scheme (NHAS). According tothe study, the most snail species were recorded from GMAS, where Biomphalaria pfeifferi (Krauss) and Bulinus truncatus (Audouin) were very abundant and equally frequent. In RAS, B. pfeifferi was less common than B. truncatus; the opposite was found in NHAS. Density

of the intermediate hosts and of submerged plants was high, particularly in the terminal section of minor canals. Chemical and physical characteristics of the water showed remarkable variation among sites, which was related to the composition and density of the aquatic vegetation. In GMAS, positive and negative associations between snail and plant species were found. Contingency tests revealed no significant negative correlations between pairs of snail species. The low density (0.1%) of Bulinus triancutus might refer to aestivatation, although this species has widespread but sporadic distribution in every type of habitat. It depends on rainfall to increase in density, this finding are similar to those reported by Cridland (1957) in Uganda and Teesdale (1962) in Kenya when they reported the occurrences of B. triancatus after rain fall. The revelation conclusion by many researchers including (Brown, 1980) when they conducted behavioral studies on snails' population showed that they are capable of escaping un-favorable conditions during the day, so habitat measurements do not necessarily reflect the actual conditions experienced by the snails. Environmental factors effect on freshwater pulmonates have been studied by a few researchers in Africa, the chemical, physical, and biological factors determine the species diversity and snail density to be found in a particular water body at a given period of time, the ecological conditions for transmission of schistosomiasis in favorable snail habitats can vary considerably from site to site and area to area, even within short distances (Pesigan, 1958).Various species react similarly to the same environmental influence, their ecological requirements being qualitatively similar but quantitatively different, this applies to most African intermediate hosts of bilharziasis, and it is very difficult to identify and estimate the importance of an individual environmental factor whether physical, chemical or biological-when all may be mutually affecting one another and their combined effect influencing a certain species as recorded by (Webbe, 1962) in Tanganyika.

The present study only emphasized on part of the environmental factors such as water depth, water temperature, water pH, turbidity, velocity, vegetation density and the organic matters. The water temperature was varied among surveys with no correlation to snail density (r = 0.059 ,P>0.05), it had narrow fixed range between 25°C- 27°C in each survey/ site and that is the optimum temperature for mollusks which is between 25°C- 26°C, in such case the density couldn't get affected, that contract what reported by Malek (1958); Gaud (1958) and Teesdale (1962) which investigations shows that in small water bodies, a constant high temperature can

reduce the population, and this through the reduction of dissolved oxygen and volume of the water. Several studies have revealed facts about pH as an important and limiting factor of the freshwater pulmonate snails distribution (Madsen, 1985; Teesdale, 1962; Malek, 1958) according to them the pH itself is not a limiting factor, but its correlation with other more important factors, as the availability to substratum, sunlight, and carbon dioxide has more influence on the distribution (WHO, 1956), in South Africa De Mellion et al, (1958) reported PH ranged 7.0 - 8.2 which contracted this study, pH has a significant negative correlation with the snail density (r = -0.531, P>0.001), in all surveys the water was alkaline with PH range 7.3-8.2 and that may related to organic pollution, due to the increase in phosphate and ammonium salts as reported by (Feema, 1981). The water depth has no effect on snails except for those places where a sudden marked rise or fall resulted in the flushing out of the habitat, there appeared to be little correlation between water fluctuations and snail populations Teesdale (1962) and Malek (1958) points out that small gradual changes of water level do not seem to disturb snails in the canals of the Gezira scheme where there is a diurnal rise or fall of 20-30 cm, and that contract this study findings where there is anegative but significant correlation between water depth and snails density (r = -0.904, P>0.01). The study agreed with Nagla and Babikir, 2008, she seems that the environmental factors; such as water flow, water level and density of vegetation; have no effect in the distribution of the fresh water snails in the canalization system. While the study consistent to the conclusions of Madsen et al. (1988) to some extent who stated that the density of B. pfeifferi; B. truncatus; Cleopatra bulimondes and Lanestis carinatus were positively correlated with the densities of submerged plants in Gezira irrigated scheme. Out of the1427examined snails,668 snails were found naturally infected with trematode cercariae (46.8%) arranged asBulinustruncatus (0%), Lymnaeanatalensis (43%) andBiomphalariapfeifferi (30%) a high infectivity was observed in the third survey where 79% of the snails were infected and that could be refer to the high cercariae diversity as three morphotypes were isolated in this survey and that attracted livestock and birds, so the canal contaminated with fecal matters inducing the trematode transmission, also in the third survey each snail species shows its highest rate where the temperature was 27°C and probably the temperature is the reason, according to (Poulin, 2005) Cercarial output is directly influenced by the temperature due to both stimulating effect of temperature increasing the emergence from the

snail and the acceleration of cercarial production within the snail host, the combined effect of the two will increase the cercariae release in great numbers. in contrast the low infection rate in the first survey 15% contributed with snail density and cercariae diversity due to drought since there was no host contact, and the absence of final hosts led to the absence of trematodes, many studies showed consistent relationship between the density and heterogeneity of the snail populations with the trematode density and heterogeneity (Hechinger and Lafferty, 2005) which match the finding in this study both in density and heterogeneity of trematode. The effectiveness of exposure by a shedding snail depends upon numbers of factors, such as the amount of time spent in a given area, number of cercariae shed, water current, vegetation, and the number and distribution of suitable intermediate hosts (Campell, 1973 and Hyman, 1967). On the other hand the probability of a snail becoming infected increases with age and is dependent upon the time spent in the aquatic environment and the ability to inoculate by miracida, even though, snail populations grant higher trematode diversity, prevalence and intensity of infection are the major determinant factors of the rate of transmission of digenetic trematodes from the snail-intermediate host to the next host in their life cycle (Margolis et al., 1982; Anderson and May, 199). Univariate Analysis of Variance showed that there was significance difference between the three surveys conducted (F=29.352, DF=1, p=.032). The study reported that most of the vegetation found at the edge of fresh water canal which includes different species such as Lawsonia inermis, Prosopischilensis, Ricinus communis, Acacia nilotica, Chlorocy perus rotundus, Mangnifera indica, and Azadrachta indica. While the emerged one was Cynodon dactylon and the other floating was Ipomea. However, snail-plants association has been reported in many places of the world (Thomas and Tait, 1984). Aquatic plants are among the ecological factors affecting snail populations (Appleton, 1978; Hilali et al., 1985).

Frequently, very high densities of snails can be found at the starting point of low-order canals (Boelee et al.1997), this could be due to snails being washed into such canals and their subsequent upstream migration, in the study periodthe canal current was stagnant to slow run so the density wasn't affected. Often, there is a relationship between water flow velocities and the location of breeding sites in an irrigation system, more snails are found at sites with low-flow velocities (Boelee; Madsen, 2006).

Many workers including Archibald (1933), Barlow & Abdel Azim (1945), Gerber (1952), Olivier (1956a, 1956b) and Shiff (1960) have made observations on the reaction of molluscan hosts of bilharziasis to drought, the capacity of snails to survive out of water for weeks or even months has important consequences in relation to the epidemiology of bilharziasis and to control measures. The temperature may also be responsible for the non-infectivity of Bulinus because infection with schistosome renders snail less tolerant to desiccation hence, only those snails which have better adaptability are capable of reactivating from aestivation bearing schistosome infection (Marti, et al., 1985). Other reason for the non-infectivity of Bulinus species their infection with Echinostomecercariae, based on some researchers opinion in the Gezira irrigation system of Sudan, Echinostome cercariae have been noted to be abundant in local B. truncatus populations, a factor which may to some extent be responsible for the uneven distribution and low prevalence of S. haematobium in this region (Amin, personal communication cited by McCullough, 1981), since the Echinostome are more pathogenic for snails than other trematodes, it invade their reproductive organs and digestive glands causing castreation or even death (WHO, 1956), so some how they could of use as biocontrol agents against snails populations, it has to be noted that study of the trematode fauna in endemic areas may reveal the existence of certain species that could be manipulated to achieve biological control of snail-transmitted diseases (Loker et al., 1981), the same situation might applied on their infection with Xiphidiocercariae as what reported by Paperna (1967) who noted that if Bulinus truncatus rohlfsi previously infected with xiphidiocercariae were considerably less likely to develop S.haematobium infections. In addition, the prolonged drought may have extreme long-term impacts on snail populations and cercariae shedding in the study area, This in turn, may result in changes in human infection patterns and the age distribution of schistosomiasis (El Kholy et al., 1989), conversely Webbe and Msangi (1958) recorded that Bulinus nasutus shed mammalian-type cercariae only 21 days after a pond, which had been dry for five months, was refilled by rainfall; and they concluded that in view of the short period which elapsed between the refilling of the pool and the shedding of mature cercariae by the snails, the snail may possibly have contained immature infections, infected snails did not appear to survive for long periods so the survivors after a period of drought are not infected before their aestivation or they are in early ages to help cercariae maturation.

Clearly, the trematodes mainly schistosomes pose a real threat on humans and animals life, some of this families found in the current study and identified as furcocercous cercariae types, the need for understanding veterinary schistosome species, not only in relation to disease management in animals, but also to reduce transmission to humans from animal reservoirs, which carry the potential to create new hybrid species. Human Fasciolosis can no longer be considered as a secondary zoonotic disease but must be considered a major human parasitic disease (Sam-Abbenyi, 1985). Although during the study period Lymnaea nataleinsis shed only furcated cercariae type (LPM), and no fasciola cercariae (Amphistome) were librated even with animal contact (cows, goats and sheep), that donot mean there is no threat upon human since the fasciolosis high prevalence in humans are not necessarily related to areas where fascioliasis is a great veterinary problem and humans can contract fascioliasis when they ingest infective metacercariae by eating raw aquatic plants (and, in some cases, terrestrial plants, such as lettuce, irrigated with contaminated water), drinking contaminated water, using utensils washed in contaminated water or eating raw liver infected with immature flukes ( Mas-Coma, 2005). Finally, higher snail diversity leads to higher trematode diversity, apart from that, conditions influencing the proliferation of snail population will inevitably enhance the existence of trematode parasites in their intermediate host, and in some riverine habitats provide physicochemical conditions that favor the proliferation of freshwater snail fauna. For this reason, environmental conditions in different habitats, in different climatic zones may support the trematode life cycle differently (Upthalla et al., 2010). It is important to notice that spatial variation in infection can easily arise as a consequence of the distribution of the second intermediate and final hosts and / or habitat characteristics which affect the risk of infection (Curtis and Hurd, 1983; Frandsen and Esch, 1991; Sousa, 1994).

CHAPTER-FIVE CONCLUSION AND RECOMMENDATION 5.1 CONCLUSION The study concluded that; The canalization system, in Sememo canal, was found to be a good habitat for the three types (Biomphlaria pfeifferi, Lymnae natenalis and Bulinus trancatus) of identified fresh water snail species. A total number of 4721 snails were collected during the different three surveys. The overall prevalence of snail species was (1.7%) forBulinus trancatus, (66%) for Biomphlaria pfeifferi and (32.3%)Lymnae natanelis And a total of1427snails were examined,668 snails were found naturally infected with trematode cercariae (46.8%) arranged asBulinustruncatus (0%), lymnaeanatalensis (43%) andBiomphalariapfeifferi (30%) a high infectivity was observed in the third survey in which, 79% of the snails were infected. The investigated cercariae were classified as human and non-human. Out of the total 928 cercariae, 490 (52.8%) were human and the remaining 438 (47.2%) were non-human. Viewed survey wise, 326 (35.1%) were found in survey 1, 334 (36.0%) in survey 2, and 268 (28.9%) in survey 3. Moreover, less human cercariae (n=122, 37.4%) were observed than non-human cercariae (n=204, 62.6%) in survey 1. However, human cercariae in survey 2 (60.5%) and survey 3 (61.9%) were relatively higher than non-human cercariae.

5.2 RECOMMENDATIONS The study recommended that;  A continuous and sustainable community sensitization should be conducted so as to increase the communitys knowledge on the the prevention and control strategies of the disease and ellimination of the snail vectors.  Further studies are needed to be established to understand the environmental factors have to provide a base line data to maintain a rational, cost effectiveness and applicable snail control measures, in addition to expansion of study period to cover all seasons of the year.

 Chemical factors of water such as temperature, pH, conductivity and BOD5 should be studied in the further study to give a clear view of the effect of environmental factors on fresh water snails.  Well-designed and maintenance of irrigation canals and structures should be done to prevent snail-breeding sites.  Installation of appropriate sanitary facilities and educating the people on health risks should be done simultaneously in the study area.

5.3 REFFERENCE A.F.Adenowo,B.E.Oyinloye,B.I.Ogunyinka,andA.P.Kappo, “Impact of human schistosomiasis in sub-Saharan Africa,” Brazilian Journal of Infectious Diseases, vol. 19, no. 2, pp. 196– 205,2015.AdiAbaba, Ethiopia, November 10-14 2008. Charbonnel,N.; Angers,B.; Debain,C.; Rzatavonjiay,R.; Bremond,P.; and Jarne,P.(2002). Fine scale genetic and demographic studies Reveal complex metapopulation dynamics in Biomphalaria pfeifferi, , the intermediate host of Schistosoma mansoni. J.Evol. Biol.15:248– 261.

Chen, M.G., Mott, K.E. (1990). "Progress in assessment of morbidity due to Fasciola hepatica infection: a review of recent literature". Trop. Dis. Bull. 8,1-38.

Cheng, Y.Z., Fang, Y.Y. (1989). The discovery of Melanoides tuberculata as the first intermediate host of Echinochasmus japonicus (Abstract) 1: Zhongguo Ji Sheng Chong Xue Yu Ji Sheng Chong Bing Za Zhi 7:47-48. Chingwena,G. Mukaratirwa,S. Kristensen,T.K. and Chimbari, M. (2002).Larval trematode infections in freshwater snails from the highveld and lowveld areas of Zimbabwe. Journal of Helminthology.76: 283-293. Christensen,N.O., Furu,P. and Simonse,P.E. (1987).Aims and strategies in the control of human African Schistosomiasis.Notes of Danish Bilharziasis Laboratory, charlotte Enland, Denmark. Chu BK et al.The economic benefits result from the first 8 years of the Global Program to Eliminate Lymphatic Filariasis (2000-2007). Chu,K.Y., Vanderberg,J.A and Klumpp,R.K. (1981).Transmission dynamics of miracidia of Schistosoma haematobium in the Volta Lake.Bulletin of WHO59,555-560.

Coelho, P.M.Z; Boson, F.C.B.; Gerken, S.E. (1975). Potencialidade de predac o a Biomphalaria glabrata (Say 1818) por duas especies de quel“nios sul-americanos: Platemys spixii (Dumeril e Dibron, 1935). Ci Cult 27: 301-303.

Combes, C., Fournier, A., Mone, H., Theron, A. (1994). Behaviors in trematode cercariae that enhance parasite transmission: patterns and processes. Parasitol ,109:S3-S13. Control of foodborne trematode infections. Report of a WHO Study Group. Geneva, World Health Organization, 1995 (WHO Technical Report Series, No. 849). Cooke, A. H., (1913). Molluscs. In: The Cambridge Natural History, 2nd ed., Cambridge. Cridland C. C. (1958). Ecological factors affecting the numbers of snails in a permanent stream Journal of Tropical Medicine and Hygiene 61: 16-20.

Cridland, C. C. (1957) .Ecological factors affecting the numbers of snails in permanent bodies of water. Journal of Tropical Medicine and Hygiene 60: 6-250. Crompton DWT, Nesheim MC. Nutritional impact of intestinal helminthiasis during the Curtis, L.A. and Hurd, L.E. (1983). Age, sex, and parasites: spatial heterogeneity in a sandflat population of Hyanasaa obsolete. Ecology 64: 819-829. De Bont .J & Vercruysse .J. (1998).Schistosomiasis in cattle.Adv Parasitol1998; 41: 286–364. De Bont.J and Vercruysse.J.(1997).The epidemiology and control of cattle Schistosomiasis. Parasitol Today ; 13 : 255–262. De Kock, K.N; Wolmarous,C.T; and Bortman,M.(2003). Distribution and habitats of the snail Lymnaea trunculata, intermediate host of the liver fluke Fasciola hepatica, in south Africa .J.S.Afr.Vet.Asssoc.,74: 117-122. De Meillon, B.; Frank, g. H; Allanson, b. R. (1958).Some aspects of snail ecologyin south Africa, A Preliminary Report .Bull. WLd. hLth. org. 18: 771-783. De Silva NR, Chan MS, Bundy DA. Morbidity and mortality due to ascariasis: re- estimationand sensitivity analysis of global numbers at risk. Tropical Medicine and Internal Health, 1997, 2:519–528. Dechruksa, W., D. Krailas, S. Ukong, W. Inkapatanakul and T. Koonchornboon.(2007). Trematode infections of the freshwater snail family Thiaridae in the Khek River, Thailand.South- east Asian J Trop Med Public Health, 38: 1016-1028. Dennis E, Vorkpor P, Holzer B, Hanson A, Saladin B,Saladin K, Degrémont, A.(1983). Studies on the epidemiology of schistosomiasis in Liberia: the prevalence and intensity of schistosomal infections in Bong County and the bionomics of the snail intermediate hosts. Acta. Trop. 40: 205-229. Diaz, M.T., Hernandez, L.E. and Bishirullah, A.K. (2008). Studies on the life cycle of Hablorchis pumilio (Looss, 1896) (Trematoda: Heterophyidae) in Venzuela. Rev. Cient., 1: 35- 42. Diaz, M.T., Hernandez, L.E. and Bishirullah, A.K. (2002). Experimental life cycle of Philophthalmus gralli (Trematoda: Philophthalmidae) in Venezuela. Rev-Biol.Trop., 2: 629-641. Dimitrov, V., Kanev, I., Panaiotova, M., Radev, V. and Gold, D. (2000).Argentophilic structures of the miracidia and cercariae of Philophthalmus distomatosa n. comb.Israel Journal of Parasitology 86:1239-1243. Dinnik, J.A. (1964).Intestinal paramphistomiasis and Paramphistomum microbothrium Fischoeder in Africa.Bulletin of Epizootic Diseases in Africa, 12:439–454. Dinnik, J.A. and Dinnik, N.N. (1962).The growth of Paramphistomum microbothrium Fischoeder to maturity and its longevity in cattle.Bulletin of Epizootic Diseases in Africa, 10:27–31. Dinnik,J.A. and Dinnik, N.N.(1954).The life cycle of Paramphistomummicrobothrium Fischoeder, 1901. Parasitology, 44:225–299.

Dinnik,J.A.(1965). The snail hosts of certain Paraamphistomidae and Gastrothylacdiae (Trematode) discovered by the late Dr.P.L.LeRoux in Africa. Journal of Helminthology, 39: 141- 150. Dryffus,G.;Abrous,M.;Vignoles,P.;and Rondelaud,D.(2004). Fasciola hepatica and Paramphistomum doubry.vertical distribution of metacercariae on plants under natural conditions.Parasitol.Res.94:70-73. Eisenberg,R.M.(1970).The role of food in the regulation of the pond snail, Lymnaea eldoes. Ecology, 51:680-684. El Kholy, H.; Arap Siongok, T.K.; Koech, D.; Sturrock, R.F., Houser, H.; King, C.H, Mahmoud, A.A. (1989).Effects of borehole wells on water utilization in Schistosoma haematobium endemic communities in Coast Province, Kenya.Am J Trop Med Hyg 41:212–219. El Tash, L.A. (2005).Socioeconomic analysis of malaria burden in Gazira and Khartoum States.PhD Thesis in Economics, University of Khartoum, Sudan. El-Tash .L.A. (2000). Inter relationship of the socioeconomic status and Schistosomiasis infection in the Gunaid sugar cane scheme, Sudan .M.Sc.Thesis, Department of Economics, Faculty of Economics, University of Khartoum. Erasmus,D.A.(1972). The Biology of Trematodes, Arnold,London. Esch, G. W. & Fernandez, J. C. (1994). Snail–trematode interactions and parasite community dynamics in aquatic systems: a review.Am. Midl. Nat. 131, 209–237. Faltynkova,A. and Hass,W.(2006). Larval trematodes in freshwater Molluscs from Elbe to Danube rivers (Southeast Germany).Before and Today. Parasitol.Res.99:572-582. Farahnak,A.; Setodeh, S., Mobedi, I. (2005). A Faunistic Survey of Cercariae Isolated from Melanoides tuberculata and Their Role in Transmission Diseases. Arch.Razi institute 59. Feema,(1981).Métodos de Análises Físicoquímicas da Água,Cadernos Feema, Série Didática, 2nd ed., vol. 2, p. 14-79.

Feitosa, V.R. and Milward, de Andrade, R. (1986).Atividade predatoria de Astronotus ocellatus (Cichlidae) sobre Biomphalaria glabrata (Planorbidae). Rev Bras Malariol D Trop 38: 19-27.

Fenwick A, Savioli L, Engels D, Bergquist N. R, Todd M. H. (2003). Drugs for the Control of Parasitic Diseases: Current Status and Development in Schistosomiasis. Trends.Parasitol 19:509-15. Fenwick A. Experience in mollusciciding to control schistosomiasis. Parasitology today, 1987, 3: 70–73. Fernandez, J. and Esch G.W. (1991). Guild structure of the larval trematodes in the snail Helisoma anceps: patterns and processes at the individual host level. Journal of Parasitology 77: 528-539. Ferte,H.;De paquite,J.;Carre,S.;Villena,J.; and Leger,N.(2005). Presence of Trichobilharzia szidati in Lymnaea stagnalis and T.franki,Radix auricularis in Northern France: Molecular evidence, Parasitol. Res., 95:150-154. Fingerut, T.J, Zimmer, A.C, Zimmer, K.R.(2003).Patterns and processes of larval mergence in an Estuarine Parasite System.Biol Bull ,205:110-120.

Fisher, A.C.(1934).A study of the Schistomiasis of the Stanly Ville District of the Belgian Congo.Trans.Roy.Socio-Economic-Trop.Med.Hyg., 28,227-306. Frandsen, F. and Christensen, N.Q. (1984).An introductory guide to the identification of cercariae from Africa freshwater snails with special reference to cercarial of trematode species of medical and veterinary importance.Acta Tropica41: 181 – 202. Fredensborg, B. L., K. N. Mouritsen, and R. Poulin.(2005). Impact of trematodes on host survival and population density in the intertidal gastropod Zeacumantus subcarinatus. Marine Ecology Progress Series 290: 109–117. Fried, B., R. Laterra, and Y. Kim.(2002). Emergence of cercariae of Echinostoma caproni and Schistosoma mansoni from Biomphalaria glabrata under different laboratory conditions.Journalof Helminthology 76: 369–371. Gardner, M. and Campbell, L.C. (1992).Parasites as probes for biodiversity.Journal of Parasitology.78: 596-600. Gaud, J. (1958). Rythmes biologist des mollusca vecteurs des bilharzioses: facterus saisonniers et climatiques influncant le cycle de reproduction de Bulinus truncatus et de Planorbarius metidjensis en Africue du Nord, Bull. Wld Hlth Org., 18, 751. Gerber, J. H. (1952). Bilharzia in Boajibu; Part II. The human Population.J.trop. Med. Hyg., 55: 52, 79. glabrataSay, 1818 (Mollusca: Planorbidae), em laboratorio. Rev Saude Publ S o Paulo17: 481- 492. Global Programme to Eliminate Lymphatic Filariasis: progress report on mass drug administration.P.J.HotezandA.Kamath,“NeglectedtropicaldiseasesinsubSaharan Africa: review of their prevalence, distribution and diseaseburden,”PLoSNeglectedTropicalDiseases,vol.3,no.8, articlee412,2009. Goreish, I.A.and Musa, M.T. ( 2005). Prevalence of snail-borne diseases in irrigated areas of the Sudan. CGIAR Challenge Programe on Water and Food (2nd International Forum on Water and food, Addis Greany,W.H.(1952). Schistosomiasis in the irrigated area of Anglo-Egyptian Sudan.Public Health and field aspects.Annals of Tropical Medicine and Parasitology,46: 350-367. Grear,G.; OW, J.; yong, C.K; Indersinah,K;and Lim,H.K. (1980). Discovery of a side of transmission and hosts of Schistosoma japonicum 1,Ke Schistosoma peninsular Malysis, Transaction of the Royle Society of Tropical Medicine and Hygiene, 46:250-267. Gryseels B. Schistosomiasis. Infectious Disease Clinics of North America, 2012, 26:383–397. Gryseels B.The relevance of schistosomiasis for public health. Tropical and medical parasitology, 1989, 40: 134–142. Guimar, C.T.; Souza, C.P.; Consoli, RA.G.B.; Azevedo, M.L.L.(1983). Controle biologico: Helobdella triserialis lineata Blanchard 1849 (Hirudinea: Glossiphonidae) sobre Biomphalaria H. Diseases and development: evidence for hookworm eradication in the American

Hechinger, R.F. and Lafferty, K.D.(2005). Trematodes in snail intermediate hosts. Host diversity begets parasite diversity: bird final hosts and trematodes in snail intermediate hosts. Proceedings of the Royal Society.272: 1059-1066. Hilali, A. H. M.; Gindi, A. M. H.; Babiker, S. M. and Daffalla, A. A. (1995).Snails, Schistosomiasis and Expansion of irrigation around Khartoum state, Sudan.Proceedings of 'A Status of Research on Medical Malacology in Relation toSchistosomiasis in Africa', Zimbabwe, August 1995, p. 165-176. Hilali, A.H, Dessouqui, L.A, Wassila.M, Daffalla, A.A&Fenwic.(1985). Snails and aquatic vegetation in Gezira irrigation canals.Journal of Tropical Medicine and Hygiene88: 75–81. Hilali,A.H.(1992). Transmission of Schistosoma mansoni in the Managil area, Sudan.Ph.D Thesis, Department of Zoology, Faculty of Science, University of Khartoum. Hopkins,A.L.(1973).The turtle, Malayms subtrijuga, as apotential agent in the control of Schistosomiasis snail vectors. Trans.Roy.Soci.Trop.Med.Hyg.,67: 309-311. Horak, I.G.(1967).Host parasite relationships of Paramphistomum microbothrium in experimentally infested ruminants with particular reference to sheep.Onderstepoort Journal of Veterinary Research, 34:451–540. Horak. I.G.(1971). Paramphistomiasis of domestic ruminants.Advances in Parasitology, 9:33– 72. Horák.P and Kolár∨ová,L.(2001). Birdschistosomes: do they die in mammalian skin?Trends in ParasitologyVol.17 No.2. Hradkova, K. and Horak, P. (2002).Département de Biologie Animale, UA 698 CNRS, UniversitéNeurotropic behaviour of Trichobilharzia regenti in ducks and mice. Journal of Helminthology 76: 137-141. Human life cycle. Annual Review of Nutrition, 2002, 22:35–59.

Hyman,L.H. (1967). The Invertebrates: Mollusca I.Vol.6.McCrow-Hill, New York, 729pp.

Ibrahim, A.M.; Bishi, H.M., and Khalil, M.T.(1999). Freshwater molluscs of Egypt . Publication of National Biodiversity Unit No. 10, Agency of Egyptian Environmental Affairs, pp: 145. Ibrahim, A.N. (2007). Malacological observations related to Bilharzia Transmission with Special Emphasis on Parasite Aggregation and Dynamics in some water bodies in Khartoum state. MSc thesis, Department of Zoology, Faculty of Science, University of Khartoum. in an Area of Lake Province, Tanganyika. Bull. Wid Hlth Org.27, 59-85. Jarne, P. and Stadler, T. (1995).Population genetic structure and mating system evolution in freshwater pulmonates.Experientia51: 482-497. Johnson, P. T. J., K. B. Lunde, E. M. Thurman, E. G.Ritchie, S. N. Wray, D. R. Sutherland, J. M.Kapfer, T. J. Frest, J. Bowerman, and A. R.Blaustein. (2002). Parasite (Ribeiroia ondatrae) infection linked to amphibian malformations in the western United States. Ecological Monographs, 72: 151–168. Jordan,P. and Webbe,G.(1993). Epidemiology in human Schistosomiasis (Eds P Jordan, G. Webbe and R.F.Sturrock), CAB International ,Wallingford.pp.87-158.

Jurášek,V., Dubinský, P., (1993).Veterinárna parazitológia. Príroda a.s., Bratislava, 382 pp. Kariuki, H.C., J.A. Clennon, M.S. Brady, U. Kitron, R.F. Sturrock, J.H. Ouma, S.T. Ndzovu, P. Mungai, Ogun O. Hoffman, J. Hamburger, C. Pellegrini, E.M. Muchiri and King, C.H. (2004). Distribution patterns and cercarial shedding of Bulinus nasatus and other snails in the Msambweni area, Coast Province, Kenya.AmericanJournal of Tropical MedicineHygiene, 70: 449-456. Katsurada,F. (1904). Schitosoma japonicum, anew parasite of man by which an endemic disease in various areas of Japan is caused. Annotiones Zoologicae Japonenses, 5:146-160. Katz,N., Rocha,R.S., and Pereira,J.P.(1980).Schistosomiasis control in Peri-Peri (Minas Gerias, Brazil) by repeated chirid treatment and Molluscicide application.Reveista do instituto de Medicine Tropical de Sao Paulo, 22 (4):85-93. Khallaayoune, K.; Laamrani, H.; Madsen, H. (1998).Distribution of Bulinus truncatus, the intermediate host of Schistosoma haematobium in an irrigation system in Morocco.Journal of Freshwater Ecology 13 (1): 129-133. King, C.H.(2009).Toward the elimination of schistosomiasis. N EnglJ Med 2009; 360:106-109. Kino, H., Inaba, H., Van De, N., Van Chau, L., Son, D.T., Hao, H.T., Toan, N.D., Cong, L.D., Sano, M. (1998). Epidemiology of clonorchiasis in Ninh Binh Arch. Razi Ins. 59:113-119. Klockars, J.; Huffman,B.; Fried,W.W.;and Brooks,S.T.(1928). Studies on the Holostome cercaria from Douglas Lake,Michigan. Trans.Am.Microscopical Soc.,74: 75-80. Kloos, H. and Thompson.K (1979). Schistosomiasis in Africa: An ecological perspective. The Journal of Tropical Geography 48, 31-46. Kloos,H. and David,R.(2002). The Paleoepidemiology of Schistosomiasis in Ancient Egypt, Human Ecology Review, Vol. 9, No. 1. Klumpp RK, Chu KY. Focal mollusciciding: an effective way to augment chemotherapy of schistosomiasis. Parasitology today, 1987, 3: 74–76. Klumpp, R.K. and Chu, K.Y.(1987). Focal mollusciciding: An effective way to augment chemotherapy of schistosomiasis.Parasitology Today3: 74-76. Kurtis, A. M. & Lafferty, K. D. (1994). Community structure: larval trematodes in snail hosts. A. Rev. Ecol. Syst. 25, 189–217. Laamrani, H.; Khallaayoune, K.; Boelee, E.; Laghroubi, M.M.; Madsen, H. & Gryseels, B. (2000).Evaluation of environmental methods to control snails in an irrigation system in Central Morocco.Tropical Medicine and International Health5: 545–552. Lafferty, K.D.(1993).Effects of parasitic castration on growth, reproduction and population dynamics of the marine snail Ceritedia californica.Marine Ecology Preogress Series 96: 229- 237. Lafferty, K.D.; Sammoned, D.T.; and Kurtis,A.M. (1994). Analysis of larval trematode communities.Ecology, 75: 2275- 2285. Larsen, A.H., Bresciani, J. and Buchmann, K.(2004).Increasing frequency of cercarial dermatitis at higher latitudes.Acta Parasitologica.49: 217-221.

Levesque,C., Pointier,J.P., Toffart,J.L.(1978). Influence of some medium factors on the fecundity of Biomphalaria glabrata (say,1818) Molluscicide, Pulmonate in laboratory conditions. Ann.Parasitol.Hum.Comp, 53:393-402. Lie, K. J. & Ow-Yang, C. K. (1973). A field trial to control Trichobilharzia brevis by dispersing eggs of Echinostome audyi.SE AsianJ. Trop. Med. Public Hlth 4:208–217. Lie, K. J. Basch,P.F. and Umathevy,T. (1965). Antagonism between two species of trematodes in the same snail.Nature (Lond.), 206: 422-423. Lie, K. J. Bosch,P.F and Hoffman,M.A. (1967). Antagonism between Partphostomum segregatum and Echinostoma barbosai in the snail Biomphalaria straminea.J.Parasit., 53, 1205. Lie, K. J. Bosch,P.F and Umathevy,T. (1966). Studies on Echinostomatidae (Trematode) in Malaya XII.Antagonism between two species of Echinostome trematode in the same Lymnaeaid snail.J.Parasit., 52, 454. Ling B et al. Use of night soil in agriculture and ﬁsh farming. World health forum, 1993, 14: 67– 70. Loker, S; Moyoy, H. G. and Gardnerz. S.L. (1981).Trematode- Gastropod associations in Nine Non-Lacustrine Habitats in the Mwanza Region of Tanzania. Para-sitology ,83: 381-399. Madsen, H. (1992). Food selection by freshwater snails in the Gezira irrigation channels, Sudan. Hydrobiologia 228: 203- 217. Madsen, H. (1996 b).Methods for biological control of schistosome intermediate hosts, an update. In: Proceedings of A Status of Research on Medical Malacology in Relation to Schistosomiasis in Africa, Zimbabwe, August 1995. Madsen, H. and Christensen, N.Q.(1992). Intermediate hosts of schistosomes: Ecology and control. Bulletin of the Society for Vector Ecology17:2–9. Madsen, H.(1996). Control of fresh water snails, Charlottenlund-Demark:DanishBilharziasis Laboratory.

Nadamba J.,Chidimu, M.G.,Zimba, M.,Gomo,E. and Munjoma , M. (1994). An investigation of schitosomiasis transmission status in Harare. Central Africa Journal of mediciene, 40(12), 337-342.

Rozendaal J.A.(1997). Vector control: Methods for use by individuals & communities, WHO, Geneva; 337-339)

Ahmed, A.A. (2002). Micro epidemiological factors influencing transmission of schistosomiasis in Aba steen canal.

W0rld Health Organization, (2003).Action against worms. Report series 2003-Issu

Steinmann, P., Keiser, J., Bos, R, Tanner, M. and Utzinger, J. (2006). Schistosomiasis and water resources development: systematic review, meta-analysis, and estimates of people at risk. Lancet Infectious Diseases, 6: 411-425.

Frandsen, F and Christensen, N.Q(1984).an introductory guide to the identification cercariae from Africa freshwater snail with special reference to cercarial of termatode species of medical and veterinary importance . ActaTropica 41:181:202.Bleakley Madsen, H.(1996a). Ecological field studies on schistosome intermediate hosts: Relevance and methodological considerations. In: Proceedings of A Status of Research on Medical Malacology in Relation to Schistosomiasis in Africa, Zimbabwe, August 1995. Madsen,H.(1985).Ecology and control of African freshwater pulmonate snails.Notes of the Danish Bilharziasis Laboratory.Charlotte nlund, Denmark. Madsen,H.(1990).Biological methods for the control of freshwater snails.Parasitol.Today, 6:237-241. Madsen,H., Daffala,A.A., Karoum,K.O., and Frandsen,F.(1988). Distribution of freshwater snails in irrigated schemes in the Sudan. Journal of Applied Ecology, 25:853-866. Malek, E. A. (1958).Distribution of the intermediate hosts of bilharziasis in relation to hydrography.Bulletin of the World Health Organization 18: 691-734. Malek, E. A.(1950). Susceptibility of the Snail Biomphalaria Biossyi to Infection with Certain Strains of Schistosoma Mansoni.Amer.J. trop. Med. Hyg., 30, 887. Malek, E.A.(1985). Snail Hosts of Schistosomiasis and Other Snail-Transmitted Diseases in Tropical America: A Manual. Pan American Health Organization, Washington, DC, USA. Mandhle-Barth,G.(1962).Key to identification of East and Central African Freshwater Snails of Medical and Veterinary Importance.Bull.WHO. 27(1),135-150. Manjing,B.K.(1978).Snail population fluctuations in-relation to Schistosoma mansoni transmission in Gezira Irrigated Area.M.Sc.Thesis, Universty of Khartoum, Sudan. Margolis, L., Esch, G.W., Holmes, J.C., Kuris, A.M. and Schad, G.M. (1982).The use of ecological terms in parasitology (report of an adhoc committee of the American Society of Parasitologists).Journal of Parasitology 68:131–133. Marti, H.P.(1986). Field observations on the population dynamics of Bulinus globosus, the intermediate host of Schistosoma haematobium in the Ifakara area, Tanzania.Journal of Parasitology, 72 (1):119-124. Marti, H.P.; Tanner, M.; Degremont, A.A. and Freyvogel, T.A. (1985).Studies on the ecology of Bulinus globosus, the intermediate host of Schistosoma haematobium in the Ifakara area, Tanzania.Acta Tropica, 42: 171 – 187. Mas-Coma,M. S., Bargues, M.D., Valero, M.A, (2005). "Fascioliasis and other plant-borne trematode zoonoses".Int. J. Parasitol.35 (11-12): 1255–78. Mcclelland, W.F.(1956).Studies on snail vectors of schistosomiasis in Kenya.J. trop.med.hyg.59(10):229-42.

Mc.Cullough FS.The role of mollusciciding in schistosomiasis control. Geneva, World Health Organization, 1992 (unpublished document WHO/SCHIST/92.107; available on request from the Division of Control of Tropical Diseases, World Health Organization, 1211 Geneva 27, Switzerland). Mccullough, F. S. (1981). Biological control of the snail intermediate hosts of human Schistosoma spp.: a review of its present status and future prospects. Acta Tropica, 38: 5-14. Mccullough,F.S and Mott,K.E.(1983). IWA Publishing, London, on behalf of the World Health Organization, Geneva.The role of Molluscicide in Schistosomiasis control.WHO/Schisto/72-83. Meyer-Lassen.J, Daffala,A.A., and madsen,H. (1994). Evaluation of focal Mollusciciding in Rahad irrigation scheme, Sudan.Acta tropica, 58:229-241. Meyer-Lassen.J. (1992). Studies on snail populations in the Rahad agricultural scheme, the Sudan, with special emphasis on Bulinus truncatus (Audouin) and Biomphalaria pfeifferi (Kraus) (Gastropoda: Planorbidae), snail intermediate hosts for species of the genus Schistosoma. PhD thesis, University of Copenhagen, Denmark. Michelson, E.H. (1957). Studies on the biological control of schistosome-bearing snails. Predators and parasites of fresh-water mollusca: a review of the literature. Parasitology 47: 413-426 Mobarak,A.B.(1982).The Schistosomiasis control. In: the biology of Schistosomes from Genes to Latriens. Rollinson,D. and Simpon,A.J.G(editor). Academic Press Ltd. Morgan, J.A.T., DeJong, R.J., Snyder, S.D., Mkoji, G.M., Loker,E.S.(2001). Schistosoma mansoni and Biomphalaria: Past history and future trends. Parasitology 123 (Special supplement), S211–S228. Mouahidand Théron, A. (1986).Schistosoma bovis: Variability of cercarial production as related to the snailhosts: Bulinus truncatus, B. wrighti and Planorbarius metidjensis. Nguyen-Khoa, S.; Smith, L.; Lorenzen, K., (2005).Impacts of irrigation on inland fisheries: Appraisals in Laos and Sri Lanka. Comprehensive Assessment Research Report 7. Colombo, Sri Lanka: Comprehensive Assessment Secretariat. Niemann,G.M., and Lewis,F.A.(1990). Schistosoma mansoni: influence of Biomphalaria glabrata size on susceptibility to infection and resultant cercarial production. Experimental Parasitology 70:286-292. O. M. Ukpai and I. Ahia, “Intestinal schistosomiasis and contributory risk factors in Nsu, Ehime MbanoL GA, ImoState,” Nigerian Journalof Parasitology, vol.36, no.1, pp.44–49, 2015. O. T. Soniran, O. A. Idowu, E. M. Abe, and E. Ukor, “Urinary schistosomiasis among pupils in Afikpo District, Southeastern, Nigeria,”Nigerian Journal of Parasitology, vol.36, no.2, pp.141– 144,2015.

Olivier, L. (1956 b).Observations on vectors of schistosomiasis mansoni kept out of water in the laboratory. I. J. Parasit., 42(2):137-46.

Olivier, L. (1956a).The Location of the Schistosome Vectors, Australorbis glabratus and Tropicorbis centimetralis, on and in the Soil on Dry Natural Habitats. The Journal of Parasitology .42(1): 81-85. Olson, P.D., Cribb, T.H., Tkach, V.V., Bray, R.A., Littlewood, D.T.J. (2003). Phylogeny and classification of the Digenea (Platyhelminthes: Trematoda)1. Int. J. Parasitol. 22:733-755. Oomen, J.M.V., de Wolf, J. & Jobin, W.R. (1990).Health and irrigation.Incorporation of Disease Control Measures in Irrigation, a Multi- faceted Task in Design, Construction, Operation.International Institute for Land Reclamation and Improvement/ILRI Publication 45, Wageningen. Pages, J. R, Theron, A. (1990).Analysis and comparison of cercarialemergence rhythms of Schistosoma haematobium, Schistosoma intercalatum, Schistosoma bovis, and their hybrid progeny.Int J Parasitol20:193-198. Paperna, I. (1967).The effect of pre-existing trematode infection on the establishment of Schistosoma haematobium larvae in bulinid snails.Ghana Medical Journal, 6: 89-90. Pesigan TP, Hairston NG, Jauregui JJ, Garcia EG, Santos AT, Santos BC, Besa AA, (1958).Studies on Schistosoma japonicum infection in the Philippines. 2. The molluscan host. Bull WorldHealth Organ 18: 481–578. Pitchford,R.J., and Dutoit,j.F.,(1976).The shedding pattern of three little under outdoor conditions.Annals of Tropical Medicine and Parasitology,70,181-187. Pointier, J.P and Giboda, M. (1999). The case for biological control of snail intermediate hosts of Schistosoma mansoni.Parasitology Today. 15 (10): 395-397. Pointier, J.P. (2001).Invading freshwater snails and biological control in Martinique Island, French West Indies. Rio de Janeiro. 96:67-74. Pointier, J.P., Theron, A.and Borel, G. (1993). Ecology of the introduced snail Melanoides tuberculata (Gastropoda: Thiaridae) in relation to Biomphalaria glabrata in the marshy forest zone of Guadeloupe, French West Indies. Journal of Molluscan Studies. 59:421-428.

Poulin,R. (2005). Global warming and temperature mediated increases in cercarial emergence in trematode parasites. Parasitology 132: 143-151. R. S. Houmsou, E. U. Amuta, and T. T. Sar, “Profile of an epidemiological study of urinary schistosomiasis in two loal government areas of Benue State, Nigeria, ”InternationalJournal ofMedicine&BiomedicalResearch,vol.1,no.4,pp.39-48,2012. R. S. Houmsou, S. M. Panda, S. O. Elkanah et al., “Crosssectional study and spatial distribution of schistosomiasis amongchildreninNortheasternNigeria,”AsianPacificJournal ofTropicalBiomedicine,vol.6,no.6,pp.477–484,2016. R. S. Houmsou, S. M. Panda, S. O. Elkanah et al., “Crosssectional study and spatial distribution of schistosomiasis among children in NortheasternNigeria,”Asian Pacific Journal of Tropical Biomedicine, vol.6, no.6, pp.477–484, 2016.

Rolfe, P.F. and Boray, J.C. (1987).Chemotherapy of amphistomosis in cattle.Australia Veterinary Journal, 64:328–332. Rolfe, P.F., Boray, J.C., Nichols, P. and Collins, G.H., (1991).Epidemiology of amphistomosis in cattle.International JournalR of Parasitology, 21:813–819. Ross, A.; Bartley, P.B; Sleigh, A.C; Olds, G.R; Li, Y. and Williams, G.M.(2002). Schistosomiasis.N Engl J Med; 346:1212-1220. Sachs, R. and Cumberlidge N. (1989). Isolation of microcercous cercariae from snails caught in an endemic focus of Paragonimus uterobilateralis in Liberia, West Africa.Trop. Med. Parasitol. 40:69-72. Sam-Abbenyi A.(1985).Endemic pulmonary paragonimiasis in Lower Mundani (Fontem district of southwest Cameroon).Results of treatment with praziquantel.Bull Soc. Pathol.Exot.Filiales.78:334-341. Sambon,I.N.(1907).Remarks on Schistosoma mansoni, J.Trop.Med.Hyg.,10:303. Santos,A.T.Jr.(1984).The present status of Schistosomiasis in the Phillippiens.The South-East Asin Jurnal of Tropical medicine and Public Health, 15:439-445.

Shiff, C. J. (1960). Observations on the capability of freshwater vector snails to survive dry conditions.J.trop. Med. Hyg., 63, 89. Shostak, A.W. and Esch, G.W.(1990).Photocycle-dependent emergence by cercariae of Halipegus occidualis from Helisoma anceps, with special reference to cercarial emergence patterns as adaptations for transmission.Journal of Parasitology 76: 790-795. Skov, J., P.W. Kania, A. Dalsgaard, R.T. Jorgensen and K. Buchmann. (2009). Life cycle stages ofheterophyid trematodes in Vietnamese freshwater fishes traced by molecular and morphometric methods. Vet.Parasitol.,160: 66-75. Smyth, J.D. (1962).Introduction to Animal Parasitology, Cambridge University Press. Pp 196- 198. Soliman,M.F.M.(2008). Epidemiological review of human and animal fasciolosis in Egypt.J.Infect.Dev.Countries, 2:182-198. Sorensen, R. E. & Minchella, D. J. (2001). Snail-trematode life-history interactions: past trends and future directions. Parasitology 123:3–18. Soulsby,E.J.L.(1982).Helminth, Arthropods and Protozoan of domestic Animals (7th edition), Bailliere Tindall.London. Sousa, W.P. (1992).Inter-specific interaction of larval trematode parasites of freshwater and marine snails.American Zoologist, 32: 583- 592. Sousa, W.P. (1994). Pattern and processes in communities of helminth parasites. Trends in Ecology and Evolution 9: 4-57. south. Report of the Rockefeller Sanitary Commission. Quarterly Journal of Economics, 2007,122:73–117. Spithill, T.W; Smooker, P.M; Copeman, D.B. (1999). "Fasciola gigantica: epidemiology, control, immunology and molecular biology". In Dalton, JP.Fasciolosis. Wallingford, Oxon, UK: CABI Pub. pp. 465–525. Stephenson,R.W.(1947). Bilharziasis in the Gezira irrigated area of the Sudan.Trans.Roy.Soc.Trop.Hyg, 40:479-494.

Stricland G.T.(2000).Hunters tropical medicine and emerging infectious diseases. 8th ed. W. B. Saunders Company press. USA. Sturrock, R.F. (2001). Schistosomiasis epidemiology and control:how did we get here and where should we go? Memorias do Instituto Oswaldo Cruz 96(Suppl.), 17–27. Swart, P.J. & Reinecke, R.K. (1962a).Studies on paramphistomiasis. 1. The propagation of Bulinus tropicus Krauss 1848. Onderstepoort Journal of Veterinary Research, 29: 183–187. Swart, P.J. & Reinecke, R.K. (1962b).Studies on paramphistomiasis. 11. The mass production of metacercariae of Paramphistomum microbothrium Fischoeder 1901. Onderstepoort Journal of Veterinary Research, 29:189–195. Taylor, J. W. (1894-1900). Monograph of the land and fresh-water molluscs of the British Isles. Structural and general volume, Leeds. Taylor, M.G. (1987). Schistosomes of domestic Animals:Schistosomabovisand other animal forms. In Immunology, Immunoprophylaxisand Immunotherapy of Parasitic Infections, ed. Soulsby EJL. Boca Raton, Florida: CRC Press; 2: 49–90. Teesdale, C.(1962).Ecological observations on the mollusks of significance in the transmission of Bilharzia in Kenya.Bull. Wld.Hlth.Org. 27: 759-782. Teesdale, C. and Nelson, G. S., (1958).E. Afr. med. J., 35,433 The control of schistosomiasis.Second report of the WHO Expert Committee.Geneva, World Health Organization, 1993 (WHO Technical Report Series, No. 830). The use of essential drugs.Seventh report of the WHO Expert Committee.Geneva, World Health Organization, 1997 (WHO Technical Report Series, No. 867). Torgerson, P. and Claxton, J. (1999). "Epidemiology and control.".In Dalton, JP.Fasciolosis. Wallingford, Oxon, UK: CABI Pub. pp. 113–49. Toscano, C., Hai, Y.S, Nunn, P., Mott, K.E.(1995). Paragonimiasis and Tuberculosis, diagnostic confusion: a review of the literature.Trop. Dis. Bull.92:1-27. Ukong, S.; Krailas, D. ; Dangprasert, T. and Channgarm, P. (2007). Studies on the morphology of cercariae obtained from freshwater snails at Erwan Waterfall, Erawan National Park, Thailand.South-east Asian J Trop. Med. Public Health, 38: 302-312. Uthpala A. Jayawardena1,2, Rupika S. Rajakaruna1 and Priyanie H. Amerasinghe. (2010). Cercariae of trematodes in freshwater snails in three climatic zones in sri lanka.Cey. J. Sci. (Bio. Sci.)39 (2): 95-108. Van der Hoek, W.; Boelee, E.; Konradsen, F. (2002). Irrigation, domestic water supply and human health. In Water and Development: Some selected aspects, eds. C.M. Marquette; S. Pettersen. in Encyclopedia of Life Support Systems (EOLSS), Developed under the Auspices of the UNESCO. Oxford, U.K.: EOLSS Publishers. [http://www.eolss.net]. Van der Hoek, W.; Konradsen, F.; Ensink, J.H.J.; Mudasser, M.; Jensen, P.K.(2001). Irrigation water as a source of drinking water: Is safe use possible? Tropical Medicine and International Health 6: 46-55. Van der Hoek, W.; Konradsen, F.; Jehangir, W.A. (1999). Domestic use of irrigation water: Health hazard or opportunity? International Journal of Water Resources Development 15: 107- 119. Vaz, J.F, Teles, H.M.S; Correa, M.A, Leite, S.P.L. (1986). Ocorrência no Brasil de Thiara (Melanoides) tuberculata (O.F. Müller,1774) (Gastropoda, Prosobranchia), primeiro hospedeiro

intermediário de Clonorchis sinensis (Cobbold, 1875) (Trematoda, Plathyelmintes). Rev Saúde Públ São Paulo 20: 318-322. Velasquez, L.E., J.C. Bedoya, A. Areiza and I. Velez, (2006). Primer registro de Centrocestus formosanus (Digenea: Heterophyidae) Colombia. Rev. Mex. Biodiv., 77: 1-8. Verbrugge, L.M., Rainey, J.J., Reimink, R.L. and Blankespoor, H.D. (2004).swimmer's itch: Incidence and risk factors. American Journal of Public Health 94: 738-741. Voelker, J., Vogel, H.(1965). Zwei neue Paragonimus-Arten aus West-Afrika: Paragonimus africanus und Paragonimus uterobilateralis (Troglotrematidae; Trematoda).Z Tropenmed Parasitol. 16:125-148. Voge,M.;Bruker,D;and Bruce,J.I. (1978). Schistosoma mekongi species from man and animals compared with four geographical strains of Schistosoms japonicum.Journal of Parasitology, 64:684-577. Vogel, H., Crewe, W. (1965). Beobachtungen über die Lungenegel-Infektion in Kamerun (WestAfrika).Z Tropenmed Parasitol.16:109-125. Von Oefele, F. (1901). Studien über die Altägyptische Parasitologie. Archieve der Parasitologie 4 :481-530. Wanas, M.Q., Abou-Senna, F.M., el-Deen, A. and al-Shareef, M.F. (1993).Studies on larval digenetic trematodes of xiphidiocercariae from some Egyptian fresh water snails.Journal of Egypt Soc. Parasitology 23:829-850. Watson (1950).J.Roy.Fac.Med.Iraq, 14, 148. Webbe ,G.and Msangi,A.S.(1958). Observation on the East cost of Africa. Annalsof Tropical Medicine and parasitology ,52:302-314. Webbe, (1962). The Transmission of Schistosoma haematobium

Webbe, G. (1960). Observations on the seasonal fluctuation of snail-population densities in the northern province of Tanganyika.Ann.trop. Med. Parasit., 54, 54.

Webbe,G. and Sturrock, R.F. (1964).Laboratory test of some new Mollicuicide in Tanjanika .Ann.Trop.Med. Parasitol.,58:234-239. Welch, P. S. (1952). Limnology, 2nd ed, New York,Mcgrow-Hill Book Co.,pp 1-538. Williams, S. N. & Hunter, P. J. (1967).The Distribution of Bulinus and Biomphalaria in Khartoum and Blue Nile Provinces, Nature (Lond.),215, 1408. World Health Organization, (1956).Study group on the ecology of intermediate hosts of Bilharziasis, WHO Technical Report Series No.120. Geneva.WHO/Bil.Ecol./33,25. World Health Organization, (1991).Basic Laboratory methods in Medical Parasitology.Geneva;WHO. World Health Organization, (1993).The control of Schistosomiasis. Second Report of the WHO Expert Committee,WHO Tech.Rep.Set No.830,Geneva. World Health Organization, (1997). Vector control: methods for use by individuals and communities by Jan A.Rozendaal. WHO.Geneva.

World Health Organization, (1998).Report of the WHO informal consultation on monitoring of drug efficacy in the control of Schistosomiasis and intestinal nematodes,Geneva,8-10 July 1998, DocumentWHO/CDS/CPC/SIP/99.1. World Health Organization, (2003).Action against Worms,Report series2003-Issu 1. World Health Organization, (2006). Preventive chemotherapy in human helminthiasis: coordinated use of anthelminthic drugs in control interventions: A manual for health professionals and programme managers. Geneva. World Health Organization, Schistosomiasis. Progress Report 2001– 2011andStrategicPlan2012–2020, World Health Organization, Geneve, Switzerland, 2013. Wright, C. A. (1970).The ecology of African schistosomiasis.In R.W.J. Keay (ed.), Human Ecology in the Tropics, 67-80. New York:Pergamon. Wright, C. A. (1976).Schistosomiasis in the Nile basin. In J. Rzoska (ed.),The Nile, Biology of an Ancient River, 321-324. The Hague: Dr. W. Junk, B. V. Wykoff, D.E., Harinasuta, C., Juttijudata,P. and Winn, M.M. (1965). Opihorchis vivirrini, the life cycle and comparison with O. felineus. J. Parasit., 51: 207. Yoder, R. (1983). Non agricultural uses of irrigation systems: Past experience and implications for planning and design. ODI Network Paper No. 7, Overseas Development Institute, London, U.K.: Overseas Development Institute, London. Yousif, F. and Ibrahim, A.(1979).Four new xiphidiocercariae from fresh water prosobranchs in Egypt.J.Egypt. Soc. Parasitol., 2: 411-419. Yousif,F. and Ibrahim,A.(1978).The first record of Angiostrongylus cantonensis from Egypt. Z Parasitenkd 56: 73–80. Yousif,F.; Ibrahim,A.; El Bardicy,S.; Sleem, S. and Ayoub,M. (2010). Morphology of New Eleven Cercariae Procured from Melanoides Tuberculata Snails in Egypt. Australian Journal of Basic and Applied Sciences, 4(6): 1482-1494.

ANNEX-1:Snail collection form Site (……) Canal type………….Water temp. …….Velocity…………, Turbidity ……………., pH……………….Vegetation density……………

Canal Water Water Water Water Vegetation

Type Temp Velocity Turbidity PH value density

Survey-1

Survey-2

Survey-3

Average/Total

Rmark……………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… ……………

ANNEX-2; Snail survey form Snail incrimination

Total No. of No. of No. of

Number of examined infected cercariae Type of cercariae

Snails snails snails Found

collected Human Non-human

Survey -1

Survey-2

Survey-3

Average

Remark……………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………… …………………………………………………………………

ANNEX-3;Total no- of Fresh water snails collected in each site/each scoop in each survey

Site no-……….

No- of Survey1 Survey-2 Survey-3 scoops Bio Lym Bio lymn Bio Lym Scoop no -1 14 7 1 0 3 1

………………-2 16 12 2 1 9 0

……………-3 14 10 33 0 0 0 …………...4 20 12 23 0 7 2 ………… -5 1 0 47 0 0 0 …………..-6 1 1 27 0 7 4 …………..7 4 2 7 0 0 0 ------8 13 10 7 0 5 0 …………...9 7 4 23 0 5 0 ………….10 4 3 9 0 11 0 ………….11 10 7 33 0 10 0 ………….12 6 3 25 0 0 0 ………….13 2 1 11 0 4 0 ………….14 5 4 21 0 15 0 ………….15 12 8 11 0 17 0 Total 281 03 93 08 284 101

100

ANNEX-IV;Average summary of ecological factors FORM

Average summary of ecological factors

Canal Water Water Water Water Vegetat Water

type Temp in Velocity Turbidity PH value ion Depth in

Density cms

Survey-1

Survey-2

Survey-3

Average

101

APPENDEXES Plate-1: A researcher searching for fresh water snails using scooping method in Sememo canal, Eritrea, Jan-June 2018.

102

Plate-2: Samples of fresh water snails collected during scooping from Sememo canal, Eritrea, Jan-June 2018

103

Plate-3: Artificial light used during determination of shedding Cerceriae in Debub regional entomology laboratory during Jan-June 2018

104

Plate-4: A researcher identifying the collected fresh water snails through a compound microscopein the regional entomology Lab. Jan-June 2018.

105

Plate-5: Sample of fresh water snails appear in white dish ready for identification in the regional entomology Lab, Jan-June 2018

106

Plate-6: fresh water snail, Biomphlaria pfeifferi:collected from Sememo canal.Eritrea,

Jan-June 2018.

107

Plate-7, A fresh water snail, Lymnae natenalis,collected from Sememo canal,Jan- June 2018

108

Plate-8; A fresh water snail, Bulinus trancatus, collected from Sememo canal, Eritrea, Jan-June 2018.

109

Plate-9: Un-described furcocercous type-3 screened in regional entomology La., Eritrea, Jan-June 2018

110

Plate-10: Longifurcate-phatyngeate disome Cerceriae screened inregional entomology La., Eritrea, Jan-June 2018

111

Plate-11: Virgulate Xiphidiocerceriae screened in regional entomology Lab, Eritrea, Jan-June 2018

112

Plate-12; A rerional entomology Lab.which was used for snails Identification, preservation and to estimate infection rates during the study period in Eritrea in June 2017.

113