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2 Major Vector-Borne Diseases and Their Control 2 Major vector-borne diseases and their control 2.1 Introduction This section does not provide a comprehensive list of all the vectors that might be encountered in a refugee settlement. The intention is to describe the most important vectors that may occur, and to give particular attention to those that may be controlled successfully within a camp setting. Table 4 gives information on different vector groups that is likely to determine the type of control programme that may be undertaken. Insects which have a short flight range, localised breeding or resting sites (for example, mosquitoes which rest inside buildings) will be the easiest to control. Wherever possible, local advice should be sought on the particular vector species that may be found in the environment of the camp. The section is divided into sub-sections dealing with the diseases transmitted by specific vectors. 2.2 Mosquito-borne diseases Mosquitoes are vectors of malaria, filariasis, and some arboviral infections. Female mosquitoes may feed on a variety of mammals, birds, and reptiles, each species having its own preferences for a particular source of blood. Many species of mosquitoes bite people, but only some of them are vectors of disease. Although all mosquitoes lay their eggs either in water or on the moist surfaces at the water's edge, each species has a particular ecological niche, and their breeding sites can be very specific. Control measures must be targeted to the specific mosquito species that is causing the problem. 29 Table 4 Generalised biological information on insect vectors that affect the feasibility of control O §' Insect Vector Disease Breeding Resting Transmissioni Blood source Dispersal range Control vectors species sites sites source range feasibility 1 'ention mosquitoes Anopheles malaria; swamps, indoors/ night people and 2k can be very filariasis; still water, outdoors animals effective - through arbo viruses containers need specialist advice I 'ector Aedes encephalitis; containers, indoors/ day people and 0.1 -0.8km can be very filariasis small pools outdoors animals effective- , control of stagnant need water specialist advice Culex filariasis; organically indoors/ day and people and 0.1-0.8km can be very encephalitis polluted outdoors night animals effective - water need specialist advice lice Pediculus typhus; clothing on clothing on day and people only n/a very relapsing human body human body night effective , fever, trench or that has and simple fever been worn within the last two weeks. filth flies Musca eye disease; organic indoors/ day n/a 5km sanitation diarrhoeal matter, outdoors very disease faeces, food, important corpses tsetse-flies Glossina sleeping soil tree trunks day people and 2 - 4km need sickness animals specialist advice sandflies Phlebotomus sandfly mud cracks, indoors/ night people and 600m need Lutzomia fever; termite hills, outdoors animals specialist Ieisbmaniasis animal advice burrows, mud walls blackflies Simulium river fast-flowing outdoors day people and 10km need blindness rivers animals specialist advice ticks/mites Ixodidae typhus vegetation outdoors day people and n/a destroy Trom- animals vegetation buciliidae cone-nosed Reduviidae chagas cracks in indoors night people and 10 -20m insecticide *©• bugs disease walls and animals control ' < furniture simple o fleas Xenopsylla plague; rats with rats night and people and n/a treat rat i- m urine day animals burrows | typhus with «> insecticide «•; before § controlling | rats £ § Table 5 Mosquito groups and the diseases they transmit Mosquito Disease Distribution ANOPHELINE malaria endemic in most countries between latitude 30°N and the Tropic of Capricorn in the south with northerly extensions in Turkey, Syria, Anopheles Iraq, Iran, Afghanistan and China § (e.g A. gambiae the main Bancroftian filariasis endemic in tropical Africa, Middle East, South Asia, Far East and malaria vector in Africa) New Guinea, Tropical America 3 Brugian filariasis South-east Asia arboviral diseases Africa | CULICINE Bancroftian filariasis endemic in tropical Africa, Middle East, South Asia, Far East and New Guinea, Tropical America Culex (e.g. C.quinquefasciatus the Japanese encephalitis Siberia, Japan, Korea, China, Indonesia, Singapore, Malaysia, main vector of elephanti- Thailand, Vietnam, Burma, Nepal, India and Sri Lanka. Epidemics in asis in urban areas) Japan and Korea. Mansonia Brugian filariasis South-east Asia and South-west India major biting nuisance and occasional vector arboviral diseases Africa, Americas Aedes yellow fever endemic in West and Central Africa and in north and eastern parts of (e.g. A. aegypti, the main South America. Epidemics occur in central America. vector of urban yellow fever and dengue) arboviral diseases eEndemic throughout the tropics, particularly Asia, the Pacific and - dengue haemorrhagic Caribbean. While dengue occurs in Africa DHF has not been recorded fever (DHF) other arboviral disease highly endemic in Africa and Asia and also found in the Americas Bancroftian filariasis South Pacific, South Asia Major vector-borne diseases and their control Mosquito disease vectors are divided into to two groups, the anophelines and the culicines, which can be readily distinguished by characters that can be seen by the naked eye. Sanitation workers can be trained to recognise the different types of mosquitoes using the key in Appendix 3. Mosquito groups and the diseases they transmit are shown in Table 5, and Table 6 gives details of control methods which are available for different types of mosquito. 2.2.1 Malaria Malaria is the most important vector-borne disease worldwide in terms of morbidity and mortality. It is considered by the major relief organisations to be one of the five top causes of child mortality in the acute phase of a majority of refugee emergencies. Its severity is often dependent on the level of immunity built up in an individual from previous exposure to different malaria parasite species and strains. The movement of refugees from one area to another often brings them into contact with malaria forms to which they have no immunity, which can result in epidemics of severe malaria developing. This might necessitate mass drug treatment plus vector control measures. Where women have a low level of acquired immunity, malaria is a major cause of maternal death, spontaneous abortion, stillbirth, and premature delivery. This is particularly true during a first pregnancy. All malaria is transmitted by Anopheles mosquitoes. Anopheles mosquito: biological features • most breed in still water • breed in unpolluted water • females feed in the evening or at night • fly up to 2-3 km from breeding site • only older females (2 weeks or more) can transmit malaria • some insecticide resistance Distribution: tropical and subtropical regions where water is available. Characteristics: abundance can be highly seasonal and is linked to the timing and length of the rainy season. The population increases during the rainy season, when pools of water, representing potential breeding sites, are widespread. Disease transmission: transmits four species of malaria (ovale, malariae, vivax, and falciparum). Falciparum malaria is the most important as it is particularly dangerous to those without acquired immunity. In any one area of the world malaria is mainly transmitted by one or two Anopheles species. 33 Disease prevention through vector control 2.2.2 Control programmes Control programmes are highly specific to the species of mosquito involved in transmission. If a national malaria programme is not being undertaken then specialist advice from an experienced entomologist will be needed to ascertain the likely effectiveness of a control programme. Control options in an emergency situation will depend on the level of information and technical skill available. Assistance should be sought at once from national or international sources. Meanwhile, measures to reduce potential breeding sites should be started immediately. In Africa it can be assumed (until proved otherwise) that malaria vectors bite at night and rest indoors, and control measures can be taken accordingly (seeTable 6, p. 36). 2.2.3 Arboviral infections The term 'arbovirus' is used to describe any virus transmitted by an arthropod (insect, mite or tick). There are over 400 arboviruses but only a hundred of these have been shown to cause clinical symptoms in humans. Those which cause major diseases are well studied, at least in some areas. These include dengue and dengue haemorrhagic fever, yellow fever, and Japanese encephalitis. Arboviral diseases all occur naturally in non-human hosts, such as monkeys or pigs. Most are transmitted from their animal host by a range of mosquitoes or ticks. Dengue and yellow fever Dengue and yellow fever are usually transmitted by cuUcine mosquitoes belonging to the Aedes genus. Aedes: biological features • 'container breeders' • short flight range (30m) • eggs can survive drying out (unlike Anopheles) • most species bite and rest outdoors • some insecticide resistance • usually bite during the day. Distribution: Aedes aegypti (the most common urban vector of arboviral diseases) is found widely throughout the tropics. Characteristics: it breeds in water held in man-made containers, such as water storage jars, pots, tin cans, and old car tyres. It is nearly always associated with human habitats and mainly bites humans. Disease transmission: dengue is endemic throughout the tropics but tends to 34 Major vector-borne diseases and their control occur in periodic
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