ZOOLOGY Biology of Parasitism Morphology, Life Cycles, Mode Of

Paper No. : 08 Biology of Parasitism Module : 21 Morphology, Life cycles, Mode of entry of Schistosoma

Development Team

Principal Investigator: Prof. Neeta Sehgal Head, Department of Zoology, University of Delhi

Co-Principal Investigator: Prof. D.K. Singh Department of Zoology, University of Delhi

Paper Coordinator: Dr. Pawan Malhotra ICGEB, New Delhi

Content Writer: Dr. Haren Ram Chiary and Dr. Kapinder Kirori Mal College, University of Delhi

Content Reviewer: Prof. Rajgopal Raman Department of Zoology, University of Delhi

Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma

Description of Module

Subject Name ZOOLOGY

Paper Name Biology of Parasitism: Zool 008

Module Name/Title Protozoan

Module Id M21 Morphology, Life cycles, Mode of entry of Schistosoma

Keywords Schistosoma, cercaria, miracidium, sporocysts, gynaecophoric canal

Contents 1. Learning Objectives 2. Introduction 3. Schistosoma mansoni 4. Characteristic features 4.1 Habit and habitat 4.2 Morphological features 5. Modes of transmission 6. Life cycle and development 7. Other genus of Schistosoma 7.1 S. haematobium 7.2 S. japonicum 7.3 S. intercalatum 7.4 S. mekongi 8. Summary

Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma

1. Learning Outcomes

After studying this module, you shall be able to:  General aspects of Schistosoma  Recognise the digenetic nature of parasite and its habit, habitat and other characteristics.  Species affecting human population  Identify the morphology of different parasitic forms  Life cycle of Schistosoma and other species infecting human hosts.

2. Introduction

The parasites belong to class Trematoda of Phylum Platyhelminthes. The hierarchy of its systematic position is as underneath:

Phylum: Platyhelminthes Class: Trematoda Order: Prosostomata Family: Schistosomatidae Genus: Schistosoma

“Schistosome” stands for split body that defines the appearance of the couple. Schistosome are dioeciuos i.e. separate sexes, blood trematodes/flukes that belongs to Schistosomatidae family and Platyhelminthes Phylum. The species possess non-operculate egg and lack muscular pharynx. The species of the family mainly inhabit caval venous and mesenteric- portal blood vessels and within their life cycle lack encysted metacercarial stage. They develop into a fork-tailed cercariae possess organs of penetration in their pre-oral sucker for lytical and mechanical functioning. The species of the family are digenetic parasites that pass their life cycle in two distinct hosts, one intermediate and one definite host. Skin is the portal of entry into their definite host and vesical plexus of veins is the site of localization. Ranges of hosts are parasitized by this group of parasites including mammals, birds, turtles and fishes. Some species are potential parasites of man while others are agents of diseases in humans. S. haematibium, S. japonium, S. makongi, S. intercalatum and S. mansoni are the important agents of disease in man. Other less epidemiologically important species are S. bovis, S. malayani, S. curossoni and S. mattheei. The species cause Schistosomiasis in man. In the endemic infection areas it is a significant cause of disease affecting nearly 77 countries globally. Schistosoma species can infect cattle, sheep, horses and humans. Not all schistosoma infections cause diseases in humans. 3

Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma

3. Schistosomatidae species: S. mansoni

S. mansoni It is a species of Schistosomatidae family which enter their definite host i.e. man, hamsters and mice through skin and gets localized in the mesenteric veins. The developmental stage of S.mansoni called miracidium is an infectious larval form infecting an intermediate snail host, found in the freshwater bodies in shallow coastal regions. In the head foot of the snail the miracidium larvae transform into sporocyst and then migrate to the gonads or digestive glands. The sporocyst containing cercariae releases the parasitic form (cercariae) which moves through the snail tissues and ultimately released into the freshwater to further invade the definitive human host. Biomphalaria species is the molluscan host of this parasite. S. mansoni have synonyms like Afrobilharzia mansoni, Schistosomum americanum and Distoma haematobium. The parasite causes Manson’s intestinal schistosomiasis or Schistosomiasis mansoni, commonly called as bilharziasis. In the earliest times, the existence of schistosomiasis was reported by the studies of mummies of ancient Egypt. In the twentieth dysentery African Schistosomiasis was one of the earliest reported diseases noted by Theodor Bilharz.

4. Characteristic features

4.1 Habit and habitat S. mansoni is a digenetic protozoan parasite. The life cycle completes in two hosts: the primary host is human and the secondary host (vector) is snail. Food Habits The host completely fills the dietary requirements of the parasite, Schistosoma mansoni. The blood feeding parasite as the name suggests gains nourishment from the host blood components, mainly monosachharides and proteins of the blood. Precisely, the parasite devours a lot of glucose to generate energy for its reproduction and other activities. It has been reported that the malnutrition status of host can negatively impact the parasite development. Habitat The parasite S. mansoni is commonly found in freshwater ecosystem. According to the time of the year and region, the distribution and location of parasite may vary. The reason that they are predominately found in the freshwater environment is that the environment remains comparatively stable in such type of ecosystem. Attributing to their parasitic nature, they can

Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma

infect other species present in the water body. Most commonly they infect intermediate snail hosts of the family Planorbidae, genus Biomphlaria. Other organism by ingesting the infected snails can infect themselves such as rats, monkeys and other rodents. On the flip side, the definite human host not becomes infected by consuming infected snails but through contact with contaminated water. Species used as Host 1. Humans 2. Rodents 3. Primates 4. Snails, Biomphlaria

Home Range The adult form of parasite survives within the host body. The other forms of worm such as cercariae and miracidia are free-swimming but limiting to range of areas with potential hosts.

Behavior S. mansoni has both a definitive as well as intermediate host. In most cases, the definitive host is a man and the intermediate host is a snail from the genus Biophlaria. The parasitic form commonly infect human host by invading the skin and flow into the circulation. The larval form of parasite called miracidium can swim and its motile nature provides them the ability to search for their intermediate host i.e. freshwater snail. Another form of parasite called ceracriae is motile and swims from intermediate host to definitive human host. Adult forms of parasite remain within the host forming a mating pair (male and female together) where male has residing female within its groove called gynecophoric canal.

4.2 Morphological forms Schistosoma have two separate sexes (male and female) and thus called as dioecious. The adult form is long, whitish and slim where females are longer than males whereas males are stouter than females. Living a life span of 20 to 30 years, these are long-lived worms. The adult worm of the three species: S. mansoni, S. haematobium and S. japonicum closely resemble each other. Males are flat and the lateral margins of the body are folded ventrally to form a female receiving gynecophoric canal. A syncytium of cells is contained in the tegument like surface of the worm. Many prominent tuberculations are present in the tegument of male whereas female worm surface is a tegument that is devoid of tegumentations. The worms maintain their attachment in the mesenteric blood vessels with the help of pediculated sucker that follows a second type of funnel-shaped sucker called oral sucker at the anterior end of the body. The buttons around the suckers as well as in the inner parts of the ventral and oral suckers, small thorn are present.

Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma

Males Males of S. mansoni species possess 8 to 9 testes arranged in a zig-zag row, situated dorsally with efferent ducts connected to form a single vas deferences reaching to seminal vesicles, near starting of gynacophoric canal. The cutica is grossly tuberculated. Adult male can reach the size of 1 cm long and 1mm wide. Just posterior to ventral sucker lie the genital pore that have a non-muscular cirrus tube opening of seminal vesicles, the vesicle formed by the swelling of vas deferences. The males have diploid number of chromosome (16). Behind the ventral sucker, the body of the adult male worm is flattened, characteristically forming gynaecophoric canal and within this grove the female worm is held (fig.1). The caecal region and large intestine is surrounded by the small inferior mesenteric blood vessels which are prominent sites of male parasitic worm localization. A short esophagus is followed by the intestine that bifurcates into two branches at the ventral sucker level. These two branches reunite to form a closed caecum at the posterior end. The gut of the adult parasites ends close to the posterior tip of the body. Numerous small tubercles cover the body wall of the worm. The membranocalyx is a bilayered external part of the tegument which is renewed and shed continuously. The bottom of the oral sucker is the anterior extremity of the body from where the digestive tube begins and shows a bifurcated esophagus that reunites to form a single blind caecum. The intestine end blindly.

Figure 1: Schistosoma male Females Females have cylindrical body held within the gynecophoric canal held within the gynecophoric canal of the male. The size of the female worm parasite can reach upto 1.4 cm in length and 0.25 mm in width, which can digest ten times more RBC’s in comparison to 6

Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma

male parasite. The reproductive system of the female parasites comprised of slightly lobulated, elongated ovary which is connected to a short oviduct located at the anterior half of the body (fig.2). The oviduct open in uterus connected ootype. The uterus is short and contains only one egg at a time. The genital pore opens ventrally whereas the oviduct’s posterior part acts as a receptaculum seminis. The shape of the ootype determines the shape of the eggs. Vitelogenic glands are present in the two-third posterior part of the worm body attached with a single median vitelline duct which unites with oviduct before reaching the ootype complex. A Laurer’s canal is absent.

Figure 2: Schistosoma female Mating and egg production The male worm form a characteristic groove called gynecophoric canal within which resides the adult female parasite, thus appearing as a characteristic couple. Although it is unusual to have separate sexes for trematodes but this can be explained by the fact that female worms lives a permanent relationship with male parasite. After mating, the coupled worm migrates against the flow of blood and finally reaches the small venules of intestine (mesenteric circulation). The egg of S. mansoni are light brownish brown, 175mm in length and 68 mm in width, elongated, without operculum and characterized by possessing large, backward pointing lateral spine, revealed under SEM and electron microscopy. The caecal region and large intestine region of the worm is surrounded by the small inferior mesenteric vessels which act as the final niche of the paired worms. The mature female of S. mansoni lays approx. 300 eggs per day, deposited on the venus capillary’s endothelial linings. From the endothelial lining the egg of S. mansoni moves into the lumen of intestine and excreted out into the environment with the host feces. As soon as the egg of S. mansoni is emitted into the water through fecal route, the egg hatches out to produce mature miracidium. Dilution of feces with water, light and temperature are the factors influencing the hatching of eggs. 7

Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma

Figure 2: (left) Schistosoma Couple and (right) High powered detailed micrograph of Schistosoma parasite eggs in human bladder tissue. Miracidium The matured miracidium moves through the water as the outer surface is covered with numerous cilia. The size reaches 150-180 micrometer in length and 70 to 80 micrometer in width. The outer covering has four layers of epidermal plates and at the apex of miracidium ciliated sensory organelles, apical papilla and number of gland cells are present. Terebratorium at its apex consists of number of membranous folds and helps in its instant attachment to its intermediate snail host prior to penetration. Miracidium has a variety of sensory organs connected to a nervous system. It has a pair of posterior flame cells and a pair of anterior flame cells constituting the excretory system. These are sexually differentiated forms of parasite. The energy required by the miracidium takes 4 to 6 hours to search for appropriate freshwater snail host. Biomphalaria straminea/ B. tenagophila/B.glabrata, the snails are the intermediate host for miracidium.

Figure 3: Different developmental stages of S. mansoni 8

Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma

Figure 4: Life cycle stages of a digenean human parasite, Schistosoma japonicum Cercaria An infective larval form of schistosoma characteristically has a bifurcated ended long tail (the furcal rami). The outer surface of the tegument is covered with glycocalyx, furnish with sensory papillae and spines. It is also covered with three layers of plasma membrane (trilaminate). Glycocalyx which covers the outer surface of cercaria is a sugar rich layer. The tegument of S.mansoni characteristically has prominent backwardly positioned spines. A number of ciliated sensory papillae are found on cercariae. The longitudinal muscles situated below the external layer of circular muscles that inter swerve with diagonal fibers and thus form the body wall musculature of cercaria. Nearly 2/3rd of the area of cercarial head is possessed by the five pairs of acetabular glands which are the prominent feature of larval anatomy and comprised of two-paires of pre-acetabular and three pairs of post-acetabular glands. The glands are unicellular and protrude through the cone at the anterior end. These unicellular glands has role in the host adhesion and penetration as suggested by the mucus like secretion from the post-acetabular and proteinase secretion from the pre-acetabular glands. Moreover, at the anterior end, they also possess a unicellular gland called “head glands”, which provide membrane material needed during the development of ceracarie to schistosomula form following host penetration. Cercaria characteristically possess an arteodorsal pair, a ventral and posterodorsal pair, a mid-ventral pair, and a pair of flame cells in the anterior tail, thus, total five pairs of flame cells. A tightly packed ciliary rotlets occupy a region at the base of the flame cells. The first tubule cell and the flame cells are interdigitated to form a basket or barrel like structure forming a ciliary tuft of the flame cells.

5. Mode of Infection

The miracidium stage into snail host Once the miracidium gets attached to its intermediate (snail) host, it undergoes movements over the snail’s foot surface to reach its preferred sites of penetration. Exploratory 9

Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma

movements are followed by the boring movements of the miracidium and trigger the secretion of proteolytic enzymes from the gland cells to perform penetration. The muscle layers disappear and the ciliated epidermal plates are shed as the miracidium penetrates into the intermediate host and transform into primary sporocysts. Thus, miracidium develop into sporocyst inside the body of snail host. The mother and daughter sporocyst are the two generations of sporocysts. It’s the daughter sporocyst which produces the cercaria, an infective larval form which is released into water after 30 days from the snail host. There are no radial stages. When man came in contact with infected water carrying the infectious form of parasite (cercariae) of S. mansoni during bathing, wading and washing, the hosts may acquire the infection. The larval form of parasite first get attached to the skin surface of host with the help of their ventral suckers and as the time passes and water evaporates, the larva penetrates the skin and invade the periphery circulatory system. Infection rarely occurs on consuming infected water. In such cases, the larvae invade the buccal mucosa to reach the blood circulation. Penetration of the cercariae into man The penetration of infectious larval forms into their definitive hosts occurs in three phases: initial attachment, exploratory movements and ultimately penetration. Initially the larvae gets attached to the skin of the human host in contact (within water) followed by the exploratory movements of the cercaria that creep over the skin surface of the host for a suitable site of penetration such as hair follicle, further followed by the final penetration of cercaria into the skin epidermal layer using the proteolytic enzymes produced by the pre-acetabular as well as post-acetabular gland’s secretion. The cercaria takes nearly 30 minutes to reach the base of epidermis after penetration followed by the penetration it undergoes cercaria-schistosomula transition. After transformation into schistosomulum the larvae lost its tail and shed the tegument trilaminate plasma membrane along with the sugar-layer glycocalyx and substituted by the matured form of the tegument. The primitive gut also grows and the schistosomula invade the dermis. The parasitic form penetrates through the epidermal basement membrane to cross over in the dermis. Once the schistosomulae invades the dermis it takes about 10 hours to reach blood vessels where they get localized on penetration. Once they enter the mesenteric circulation through blood flow the parasitic form via pulmonary artery reaches the lungs.

The lung stage Schistosomulum After the eight day of penetration in lungs, the schistosomulum becomes much slender and longer attributing to a substantial increase in surface area without change in the body mass. It is suggested that during this lung stage the parasite starts feeding on plasma instead of blood cells of the definitive host. The parasite further migrates to the hepatic portal vein through blood vessels and finally penetrates into the liver. 10

Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma

The hepatic stage Schistosomulum The schistosomulum undergoes literal growth as it enters into the liver accompanying complete maturation of the gonads and development of the gut. The adult female and male parasitic worm pair up once they become mature and migrate against the blood flow through hepatic portal veins to ultimately reach their niche, mesenteric vessels surrounding the intestine. The mature form of parasite pair up and perform mating as they get localized in the mesenteric veins. The mature female parasite starts laying egg and it takes nearly 25 to 30 days to commence egg laying from the time of infection with cercaria.

Figure 5: Flow chart depicting the events and different stages of development and infection

6. Life Cycle of S. mansoni

Contaminated water is the source of infection in definitive human hosts. Snails are the intermediate host of parasite which produces water swimming larvae called cercariae that reaches their mammalian host in water. These free swimming larva forms penetrate the skin of human host and lose their characteristic tail after invading the body. At this point, the

Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma

cercarial body changes to schistosomulum, a migrating juveline worm. These migrating worms enter the blood circulation system of human body from where schistosomulum reaches lungs. The schistosomula of S. mansoni further migrate to the portal blood in liver and mature into adult forms. The mesenteric venules surrounding the gut are the final destination of these adult worms (male and female pair) which mature before homing there. Following skin penetration, the parasite takes about six weeks to reach mesenteric venules of bowel/rectum. These male and female pair feed on blood and can reside for many years inside the mesenteric veins and in the meantime lay 300 eggs per day that circulate to the liver and shed in the stools. The eggs of S. mansoni and S. japonicum are shed in feces, in part in urine whereas S. haematobium in urine and partly in feces. Nearly fifty percent of the total egg population circulates with the flow of blood and get imprisoned in the hepatic region and as a result of strong host immune response, granulomas are formed around eggs. After 7 days of egg laying by the matured adult female worm, the remaining eggs invade vessel walls and cross over the intestinal wall to exit the host. The eggs may take up six weeks in order to be secreted with the feces. Nevertheless, following oviposition the eggs die within 14 days, suggesting that not all eggs secreted with the stool are viable. Only when viable eggs hatches and released infectious miracidium larval form in water than only it can infect the aquatic intermediate snail host (Biomphalaria pfeifferi or B. glabrata). Within the intermediate host, parasite replicates and produces sporocysts. The snails initiate to shed cercariae after five weeks of infection and some of them will infect the definitive human hosts.

Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma

Figure 6: Life cycle of Schistosoma

7. Other species of Schistosoma

7.1. Schistosoma haematobium S. haematobium is commonly called as vesical blood fluke, producing urinary or vesical bilharziasis, schistosomiasis haematobin; schistosomal hematuria. Bilharz, 1852, provide important synonyms to the species: Bilharzia haematobia, Distoma haematobium. The vesical schistosomiasis is geographically distributed in several extensive endemic areas including various parts of Middle East and Africa. A few cases were also reported in India such as Ratnagiri in Maharashtra. Primarily the adult worms live in the pelvic plexus of venous-uterine, prostatic and vesical plexuses of veins. The adult worms are also localized in the portal blood stream of host. The males are stouter and shorter; measuring 1.5cm by 1mm. male worm is covered with finely tuberculated cuticula and has 4 to 5 testes arranged in groups. The females are slender and long, measuring 2cm in length and 0.25mm in breadth. The ovary of females lies behind the middle of the body. At one time nearly 20-30 eggs develop in the uterus of

Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma

female adult worm. The couple show a long reunited form of male and female worm where they reunite about the middle of the body. The eggs of S haematobium are 150micrometer in length and 50micrometer in breadth and also have characteristic terminal spine. There are total 5 pairs of cephalic glands in the cercariae form of parasite including 3 pairs of basophilic and two pairs of oxyphilic. Freshwater snails belong to genus Balinus (Balinus truncates) and other species such as Ferrissia tenuis (India) and Planorbarius metidjensis (Portugal) act as intermediate hosts for S. haematobium. Man is the definitive host for the parasite which is found in the prostatic and vesical plexus of veins. The life cycle of S. haematobium is same as that of S.mansoni.

7.2. Schistosoma japonicum (1904, Katsurada) S. japonicum also called as Oriental Blood Fluke causes Oriental or japonica schistosomiasis. In 1958, Le Roux provide a synonym Sinobilharzia japonica. The eggs of S. japonicum were first reported by Kasai (1903) in feces. Fujinami (1904) first detected the adult female in the portal vein. It was in 1904 only when the adult worm were first described by Katsurada from infected cats and dogs and assigned a new species name i.e. japonicum. The infection by S. japonicum is confined to areas of the Fast East being found in Japan, China, Philippines, Southern Formosa, Shan States of Burma and Celebes. The adult parasitic forms of S. japonicum are morphologically similar to species of S. mansoni with sizes of 1.2 to 2cm in length and 0.5mm in width. It has a non-tuberculated cuticula and 6 to 7 testes arranged in a single file. The ovary lies in the middle (post-equatorial plane) of the female body and uterus nearly contains 50 eggs. The couple (male and female) has a short reunited contact, reunting in the posterior fourth of the body. The eggs of S. japonicum are 100 micrometer by 65 micrometer in size and there are five pairs of cephalic glands. The parasite invades intermediate snail host belonging to Oncomelania genus. The definite hosts of S. japonicum are man and domestic animals such as dog, cat, cattle and pig. Field mice are the reservoir of infection. The parasites are localized in the mesenteric plexus of superior mesenteric veins and its radicles (ileocaecal area). The life cycle of S. japonicumis same as that of S. mansoni.

7.3. Schistosoma mekongi First reported in 1978, the Schistosoma mekongi naturally infects humans and dogs as the definitive hosts. The infection is endemic to Khong Island. The eggs of Schistosoma mekongi closely resemble the eggs of Schistosoma mansoni, varying in size range of 30- 35 by 50-65 micrometer, being less elongated and smaller in size. Aquatic snail host for the parasite is Trienla aperta. Adult worm live in mesenteric veins.

7.4. Schistosoma intercalatum (Fischer, 1934) 14

Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma

Fischer first described the S. intercalatum occurring in Central and Western Africa. Man is the definitive host for the parasite. The eggs have characteristic terminal spine up to 20micrometer with a slight bend, measuring 175micrometer in length and 60micrometer in breadth. The adult worm lies in the intestinal plexus of veins but not in the vesical plexus of veins. The parasite lives in the intermediate snail hosts, Balinus globosus and B.africanus.

Figure 7: Different species of Schistosoma

8. Summary

Schistosome are dioeciuos i.e. separate sexes, blood trematodes/flukes that belongs to Schistosomatidae family and Platyhelminthes Phylum. The species of the family mainly inhabit caval venous and mesenteric-portal blood vessels. The species of the family are digenetic parasites that pass their life cycle in two distinct hosts, one intermediate and one definite host. Skin is the portal of entry into their definite host and vesical plexus of veins in the site of localization. Ranges of hosts are parasitized by this group of parasites including mammals, birds, turtles and fishes. S. haematibium, S. japonium, S. makongi, S. intercalatum and S. mansoni are the important agents of disease in man. The blood feeding parasite, S. mansoni, as the name suggests gains nourishment from the host blood components and commonly found in freshwater ecosystem. S. mansoni has both a definitive as well as intermediate host. In most cases, the definitive host is a man and the intermediate host is a snail from the genus Biophlaria. Once the miracidium gets attached to its intermediate (snail) host, it undergoes exploratory movements followed by the boring movements of the miracidium that trigger the secretion of proteolytic enzymes from the gland cells to perform penetration. The muscle layers disappear and the ciliated epidermal plates are shed as the miracidium penetrates into the intermediate host and transform into primary sporocysts. Thus, miracidium develop into sporocyst which produces the cercaria, an infective larval form which is released into water after 30 days from the snail host. There are

Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma

no radial stages. When man came in contact with infected water carrying the infectious form of parasite (cercariae) of S. mansoni during bathing, wading and washing, the hosts may acquire the infection. The larval form of parasite gets attached to the skin surface of host to penetrate the skin and further invades the periphery circulatory system. The penetration of infectious larval forms into their definitive hosts occurs in three phases: initial attachment, exploratory movements and ultimately penetration. The cercaria takes nearly 30 minutes to reach the base of epidermis after penetration followed by the penetration it undergoes cercaria-schistosomula transition. After transformation into schistosomulum the larvae lost its tail and shed the tegument trilaminate plasma membrane along with glycocalyx and substituted by the matured form of the tegument. The parasitic form penetrates through the epidermal basement membrane to cross over in the dermis. Once the schistosomulae invades the dermis it takes about 10 hours to reach blood vessels where they get localized on penetration. Once they enter the mesenteric circulation through blood flow the parasitic form via pulmonary artery reaches the lungs. The parasite further migrates to the hepatic portal vein through blood vessels and finally penetrates into the liver accompanying complete maturation of the gonads and development of the gut. The mature form of parasite pair up and perform mating and migrate through hepatic portal veins to get localized in the mesenteric veins. The mature female parasite starts laying egg and it takes nearly 25 to 30 days to commence egg laying from the time of infection with cercaria.

Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma