Aquaculture and schistosomiasis

1. INTRODUCTION

Schistosomiasis (Bilharzia) is caused by trematode flatworms of the genus Schistosoma. In Africa three species of schistosomes infect man — Schistosoma mansoni (Sambon, 1907) and S. haematobium (Weinland, 1858). S. intercalatum (Fisher, 1935) also infects man but is not as pathogenic to man as the other two schistosomes.

Some 200 million people in 76 countries are infected by schistosomiasis and another 500–600 million are considered as being at risk. The estimated mortality is approximately 200 000 people every year (Anon., 1990B).

The number of infected people in the world is probably expanding. This is partly because the number of young people in endemic areas — who generally have the highest prevalence of infection — is rapidly increasing; partly because some water-related projects make conditions ideal for the spread of schistosomiasis (Bruijning, 1982, Hunters et al, 1976).

One of the best-known examples of the effects of water development on the spread of schistosomiasis is the Aswan High Dam in Egypt. This much-acclaimed project made possible four crops per year by perennial irrigation; however, it has also created conditions vastly more favourable for the intermediate host snails of schistosomiasis. The prevalence of S. haematobium in the area between Cairo and Aswan has increased from 5% before the construction of the dam to an average of 35% after the construction (van den Schalie, 1969).

Aquaculture and small-scale fisheries are important components of national and international programs concerned with the development of animal protein resources in developing countries. However, these activities are also perceived as increasing the spread of breeding sites for vector snails in countries where schistosomiasis is endemic.

2. LIFE CYCLE

Adult schistosome worms are about 10 to 15 mm long (Figure 1, Table 1.). They live permanently twined together in mated female-male pairs in the human blood-vessels of the abdomen, where the female can produce hundreds of eggs a day. S. haematobium lives in the region of the bladders and kidneys while S. mansoni lives in the blood vessels supplying the bowels. The eggs are released into the blood stream and pass through the tissues. Eggs from S. haematobium pass out of the body in the urine, those of S. mansoni in the faeces. The spined eggs work their way from the blood vessels through the host's tissues to the bladder or colon. As the eggs move and lodge in the tissues, the hard coat or spines on the egg damage tissues and can give rise to infections causing most of the common symptoms of the disease, such as headaches, nausea, and blood in the urine or faeces. As many as 90% of the eggs can become trapped within the host's body, damaging the tissues.

Eggs hatch when they come into contact with fresh water, producing ciliated larvae known as miracidia. Miracidia are approximately 0.5 mm long and can live for 12 to 16 hours. During this time they must find and penetrate the correct species of water snail which acts as intermediate host. The miracidium can enter several additional species of snails, where the development is usually arrested. After penetrating the outer layer of the snail's skin the miracidium becomes a sporocyst, a thick-walled, rounded structure. The miracidium can multiply several hundred times in the snail. Two generations of daughter sporocysts are produced within the tissues of the snail, where the parasite damages the snail considerably, reducing its fecundity and life expectancy.

In S. haematobium, four to eight weeks elapse from the penetration of the miracidium to the liberation of cercariae; in S. mansoni under optimum conditions only four weeks are necessary. The length of the developmental period in the snail depends upon several environmental factors, of which temperature is one of the most important. Cercariae are about one mm long and have a characteristic forked tail. They generally emerge from an infected snail over several weeks, during which period thousands of cercariae may be released. Cercariae generally appear when the snails are in sunlight, especially around noon and in the early afternoon, a period which coincides with the peak time of schistosome egg output in the urine and the culmination of the human water-contact activity.

Cercariae live for up to 24 hours and can swim at about four metres per hour. Strong sunlight and lack of oxygen are rapidly lethal. On leaving the snail, the cercaria swims vigorously, at times sinking to the bottom, then swimming back to the surface film, where it may become attached temporarily. When people come into contact with cercariae infested water, the cercaria come into contact with the skin and, as the water evaporates, penetrate the skin. One cercaria only gives rise to one fluke, no multiplication takes place in the human host. In the bloodstream, the cercaria loses its tail and becomes a worm-like schistosomule, which initially settles in the lungs. After a few days the cercariae migrate to their final destination in the abdominal blood vessels. The flukes mature in two to three months and must then find a mate, at which point the cycle begins again (Beaver et al, 1984, Beck and Davies, 1976, Brown and Neva, 1983, Crompton, 1984, Crompton and Joyner, 1980, Hunters et al, 1976, Anon., 1990B, Tucker, 1983, Woolhouse, 1987).

3. INTERMEDIATE HOSTS

The snails that acts as vectors for schistosomiasis in Africa belong to two genuses — Biomphalaria (Preston, 1910) and Bulinus (Muller, 1781) (Brown, 1980).

The genus Biomphalaria is medium sized, reaching 22 mm in diameter and more than 3 mm in height when fully grown. The whorls of the snail's shell are evenly convex, angular or carinate (i.e. with a strong spiral ridge or keel); in some species the last whirl descends to produce a concave underside.

The genus Bulinus have a sinistral (the aperture is on the left when the shell is held with its aperture towards the observer) shell with a spire which is highly varied in its shape and height relative to the aperture. The whorls may be evenly curved, bluntly angular or rarely carinate.

In Tanzania and Malawi Biomphalaria sudanica is a frequent snail intermediate host for S. mansoni. B. pfeifferi is the common host for S. mansoni in Tanzania, Zimbabwe, Zambia, Mozambique and South Africa, while another vector in Tanzania is B. ruppellii (Figure 2).

The hosts of S. haematobium are Bulinus globosus in Angola, South Africa, Malawi, Mozambique, Zambia, Zimbabwe and Tanzania. B. africanus is the intermediate host in Tanzania, Zambia and Mozambique while B. nasustus is a host in Tanzania (Beaver et al, 1984, Brown, 1980, Taylor and Makura, 1985).

The snails that transmit schistosomiasis generally prefer stagnant or slow moving water, and show a high degree of tolerance to variation in the temperature of their habitat. In general, the optimum temperature lies between 22°C and 26°C, but they can withstand extreme temperatures for a considerable period of time.

When stranded on dry land by a falling water level the snails make little or no attempt to escape back to the water, thus being exposed to the risk of death by desiccation or predation. This causes a large proportion, although not all, of the snails to die.

The snails are generally found in shallow waters near the shore. Under natural conditions it is rare to find them in depths exceeding 1.5–2.0 metres. This is correlated with the availability of food and shelter for the snails, which are available only near the surface.

Water snails in general are able to subsist on a variety of different food materials and exercise little apparent choice. The young snails eat unicellular organisms or very small soft organic particles. Older snails subsist in part on vegetable matter and in part upon the microflora of their environment. It is rare that the snails attack the healthy tissues of living water plants, they seem to prefer partly decayed vegetable matter. There is a positive correlation between the occurrence of these snails and the presence of decaying organic matter of plant origin as well as pollution through animal and human excrement.

Water plants form a desirable but not an essential feature of the habitat of the snails. Plants apparently favourable to the aquatic snails include, among many others, Nymphaeceae, Potamogeton, Pistia and Myriophyllum. Some species of plants have been shown to be antagonistic to the snails, including Saponaria, Balanites, Eucalyptus, Tephrosia and Schwartzia.

Broad-leafed plants provide suitable surfaces for the deposition of eggs and for the growth of unicellular green algae which form a favourite food of the snails. The microflora and fauna on the leaves of these plants are more important to the snails as a food source than the plants themselves.

Water plants also provide the snails with shelter and protection from intense sunlight and from the mechanical effects of fast current.

The life-span of adult aquatic snail hosts does not normally exceed 12–15 months. Their reproductive cycles and population cycles depend on rainfall, water levels and temperature (Anon., 1957, Appleton, 1978).

4. DISTRIBUTION

S. mansoni or intestinal schistosomiasis is endemic in the larger part of Africa. It is widespread in the Upper Sudan and Egypt, and occurs along the East African coast from Eritrea through Kenya, Tanzania and Malawi to the Zambezi River and Zimbabwe inland through Zambia and Tanzania to the Congo River, with some cases reported from South Africa (Figure 3.).

Figure 1a

THE LIFE CYCLE OF SCHISTOSOMIASIS HAEMATOBIUM, ALSO KNOWN AS URINARY BILHARZIA; THE EGGS PASS OUT WITH THE URINE.

Figure 1b

THE LIFE CYCLE OF SCHISTOSOMIASIS MANSONI, INTESTINAL BILHARZIA, IN WHICH THE EGGS PASS OUT WITH THE FAECES.

Table 1.

SCHISOSOMA PARASITES INFECTING MAN IN AFRICA
(Adapted from Brown, H.W. and F.A. Neva. 1983. pp. 8–17; Size of parasites from Crompton D.W.T. 1984. Beck, J.W. and J.E. Davies. 1976.)

Figure 2.

SNAILS THAT ACT AS INTERMEDIATE HOSTS FOR SCHISTOSOMIASIS.

a) Bulinus globosus (vector for S. haematobium)	b) Biomphalaria pfeifferi (vector for S. mansoni)

Figure 3.
DISTRIBUTION OF Schistosoma mansoni AND S. intercalatum IN AFRICA.

S. haematobium or urinary schistosomiasis is widespread in the greater part of Africa — the Sudan, Egypt, Ethiopia, the East Coast from Somaliland to the Cape; large areas in Central Africa; West Africa from Lake Chad and Nigeria as far south as Angola; North Africa from Egypt to Morocco (Beaver et al, 1984) (Figure 4.).

Figure 4.
DISTRIBUTION OF Schistosoma haematobium IN AFRICA.

The distribution of the different kinds of schistosomiasis corresponds with the distribution of vector snails — S. haematobium with the distribution of the genus Bulinus and S. mansoni with the genus Biomphalaria.

5. EPIDEMIOLOGY

Schistosomiasis is increasing in distribution as snails are carried from infected loci into new areas with water-related projects or as appropriate snails in previously uninfected areas become contaminated from eggs discharged into the water by infected individuals (Beaver et al, 1984).

Human infections are derived almost exclusively from human sources although monkeys, baboons and wild rodents in endemic areas have occasionally been found infected. In Tanzania a band of wild baboons is sustaining the life cycle of S. mansoni without human intervention (Beaver et al, 1984, Hunters et al, 1976).

Schistosomiasis seriously threatens the health and productive life of rural families, and (together with malaria) is held responsible for impeding the development of whole nations.

The severity of the disease is related to inflammatory and fibrotic response in the host to the eggs in the liver, lungs, intestine and urinary bladder. In urinary schistosomiasis (due to S. haematobium), damage to the urinary tract is revealed by blood in the urine. Urination becomes painful and there is progressive damage to the bladder and ureters and then to the kidneys. Bladder cancer is quite common in advanced cases. Intestinal schistosomiasis (due to S. mansoni) is slower to develop. There is progressive enlargement of the liver and spleen as well as damage to the intestine, due to fibrotic lesions around the schistosome eggs lodged in these tissues and hypertension of the abdominal blood vessels. Repeated bleeding from these vessels leads to blood in the stools, and can be fatal (Anon., 1990B).

6. PREVALENCE

Major factors affecting the distribution and prevalence of schistosomiasis are:

1. Surface water availability and permanence - important for the development of large snail populations.
2. Human water contact behaviour — related to the amount of contamination and the incidence of infection.
3. Environmental and climatic factors — including temperature and rainfall, which vary according to local geography and may produce conditions which adversely affect snail populations and the amount of human contact with water (Taylor and Makura, 1985)

Water contact is the most critical variable in the transmission of schistosomiasis. Each cercaria that enters the human body develops into only one worm, thus with augmented contact the infection will increase and the symptoms of the disease will be more severe.

The prevalence of schistosomiasis varies widely in different areas and between age-groups, even in different localities within a highly endemic site (Beaver et al, 1984). In Zimbabwe the prevalence varies with the availability of surface water and thus with the population's contact with the water (Mason et al, 1986, Taylor and Makura, 1985, Woolhouse and Chandiwana, 1990).

In most endemic foci, children and adolescents are more exposed and infected than adults are (Beaver et al, 1984, Tucker, 1983). In the Middle Awash Valley in Ethiopia persons from five to 19 years of age had the highest rate of infection associated with water contact activities that increase from childhood through adolescence and decrease thereafter (Garfield, 1986). The peak prevalence and intensity of infection generally occur in children aged between 10 and 14, with a low prevalence and intensity of infection in the older age groups. In general, 60–70% of all infected persons are also in this age group. The prevalence of S. mansoni is generally the greatest in the 10–24 year age group with most of the heavily infected persons aged between 10 and 14. However, in contrast to the prevalence curve of S. haematobium, the prevalence of S. mansoni in older age groups tends to remain at high levels (Kumaresan, 1992). Since younger age groups make up 50% of the rural population in Africa, they are responsible for a very high proportion of the contamination of the environment with schistosome eggs, and hence the continued transmission of the disease.

The actual number of snails infected in nature is surprisingly low. A few scattered snails can provide enough cercariae to infect a large number of people, partly because the amount of contact the local people have with the infected water is so great (Tucker, 1983) and partly because of the large number of cercariae that each infected snail can release.

The prevalence of schistosomiasis varies with sex and age. These sex-related differences vary depending on the cultural habits of the community (Beaver et al, 1984, Garfield, 1986). Apart from water contact, the development of immunity may partially explain the age-related infection pattern (Garfield, 1986)

Schistosomiasis can be found in all the SADC countries The less severely infected nations are Botswana, Lesotho and Namibia (Table 2).

Table 2.

THE ENDEMICITY OF S. HAEMATOBIUM AND S. MANSONI IN THE SADC COUNTRIES
(From Iarotski, L.S. and A. Davis, 1981)

Note: No data concerning Lesotho

+ low endemity,
++ medium endemity,
+++ high endemity.

7. PREVENTION AND CONTROL

In recent years there has been a number of notable cases of detrimental changes in patterns of water-borne diseases resulting from development of water-related projects, mainly irrigation. Studies in Africa have claimed that fish farming is a significant factor in the spread of schistosomiasis (e.i. Berrie, undated, Lee et al, 1982).

Fish ponds can provide good habitats for snails. The water is stagnant; the water temperature is high; decaying organic matter is present in the form of a compost providing the snails with food; weeds give the snails a substrate on which to live. The habitat for the snail can be destroyed by pond management; cutting and removal of weeds; keeping the pond stocked at all times; and by feeding manually instead of using a compost.

It is thus imperative that sustainable water development projects should take into consideration the risk of spread of water-borne human diseases and should build into the process of project planning, design and operation the means to overcome these hazards.

Attempts to control schistosomiasis aim at breaking the transmission cycle at one of the four points: by preventing infective cercariae from reaching people; eliminating the parasites in the human host; stopping eggs from reaching the water; and controlling vector snails (Woolhouse, 1987). This can be done by preventing people from coming into contact with cercariae infected water; treatment of infected people; improving sanitation; and by eliminating snails by chemical, biological or environmental means.

It may not be possible to eradicate schistosomiasis completely, but the number of people infected with symptoms of the disease can be significantly reduced. Many projects are trying to introduce sanitation and safe water sources into affected areas as well as treating infected persons. The drugs used most often are praziquantel, oxamniquine and metrifonate (Anon. 1990C). Praziquantel is effective against all forms of schistosomiasis. Oxamniquine is used to treat intestinal schistosomiasis in Africa and South America, while metrifonate is used to treat urinary schistosomiasis. Vaccines are feasible, but many aspects still remain to be studied (Anon. 1991), so it will take time before a vaccine is developed.

In projects working with aquaculture and small-scale fisheries the aim is to break the transmission cycle mainly by eliminating the snails and by preventing water contact. It is important to note that even if fish ponds are not transmitting schistosomiasis people can still get infected from other sources. To fight schistosomiasis at a wider level, sanitation and health care must also be considered.

There are three main ways to control snails in water bodies:

1. The chemical method. This control method entails the use of a wide variety of poisons. It often has quick but temporary effect because of the resistance which vectors are capable of developing against most poisons. One way of controlling the number of snails is to use molluscicides. In aquaculture projects it is not feasible to use molluscicides to control the vector snails. This is because these chemicals are toxic to the environment, particularly fish. Also, the cost of such substances is rather high (Diamant, Undated, Tucker, 1983).

2. The biological method. This method is based on the use of biological enemies of the vector. The population of the vector decreases due to predation, competition or “accidental” physical disturbance or damage.

Biological control has the advantage over molluscicides in that the reduction in snail populations is of longer duration; it increases the area's ability to withstand reinfection; it is cheaper with respect to management and labour; and it eliminates any chance of toxicity to animal or plant life. The disadvantages are the relatively slow rate of action when compared with the molluscicides, and possible adverse effects of an exotic species on the local aquatic environment (Berg, 1973, Haddock, 1981, Lee et al, 1982).

It is possible that fish, for example some species of tilapia, can be a useful aid in the fight against diseases spread by snails. The wide range of natural material that can be part of certain species of tilapia's diet allows them to feed on and destroy the environment of the vectors as well as the vector itself.

Fish and aquatic disease vectors compete not only for food, but also for their habitat. Snails, for example, feed not only on plant material, but commonly use it as the substrate on which they live and lay their eggs. If the plants are removed, the habitat of the snail is made unfavourable (Anon., 1987).

By reducing aquatic vegetation, phytophagous fishes will keep down the level of snails and other vectors. Specific niches for these vectors will be reduced as will the amount of cover available to them. Reduction of cover can also make these vectors more available to any existing predators (Coates and Redding-Coates, 1981).

The transfer of fish of proven ability in vector control to a new environment may not give the expected results and perhaps cause the decline of other desired fish populations. It is thus preferable to use species that are already present in a locality even if their ability to control the vectors is not well documented. If there are no suitable local species, then the use of exotic species under strict control may be the alternative, but with full awareness of potential risks (Anon., 1987).

3. The environmental method. This method applies the modification of the environment to a point where it is no longer suitable for the breeding or development of the vector e.g. by fluctuations in the water level to kill snails that are stranded on land. The application of environmental control consequently requires prior information and knowledge on the habits and habitats of the vector.

8. RECOMMENDATIONS TO MINIMIZE THE HAZARDS OF SCHISOSTOMIASIS RESULTING FROM AQUACULTURE PONDS (adapted from Malek, undated):

1. Extensive aquaculture in ponds may give the vector snails favourable habitats for survival and reproduction. The sides of the ponds should be deepened to discourage snail breeding and propagation; aquatic vegetation should be removed whenever observed; the overhanging bank vegetation should be cut; and if vegetable food is provided for the fish, any surplus should be removed before it has time to decay.

2. Occasional drying of the ponds may have some effect in reducing the number of snails, but not in eliminating them completely. This is because several snail species have the ability to withstand drought by going through a dormancy period.

3. The pond's water supply is one of the sources of infestation of the ponds with snails. It is recommended that a small metal screen with 5 meshes per linear cm be installed across the inlet of each pond.

4. Biological control of snails by snail-eating (malacophagous) fish. Certain criteria should be followed in choosing the species of fish. For example: it should be obtained locally; it must be suitable for cultivation in fish ponds; it must be edible; it must survive in the same ponds with other species; and its diet must consist mainly of snails, but it should also eat other foodstuff in the absence of snails.

5. In addition to mechanical means of removing vegetation, consideration should be given to biological control of vegetation by introducing weed-eating (macrophytophagous) fish, which can also be cultured for consumption by the local people.

6. Snail control is possible in deep fish ponds by a periodic draw down of water level, stranding snails on the bank. This should take place during a period when the fish is not spawning.

7. Minimize all human contacts with water.

8. Quarantine regulations should be enforced whenever possible. All persons working with the ponds who are infected should be treated and should not be allowed to return to work unless treated.

9. Fish ponds should always be stocked with fish, since some fish species will consume a great proportion of the cercaria released by infected snails.

10. Water supplies for fish ponds should not originate or pass through inhabited areas, to avoid contamination with eggs from infected people who urinate or defecate in or near the water.

11. A programme of monitoring or surveillance and control concerning the number of snails should form an important activity of an aquaculture project.

9. CASE STUDY - ZIMBABWE

9.1 Intermediate hosts

The snail vectors in Zimbabwe are Bulinus globosus for S. haematobium and Biomphalaria pfeifferi for the species S. mansoni. The species B. globosus has a high intrinsic rate of natural increase and is able to survive and reproduce in both permanent and non-permanent water bodies. B. pfeifferi has a slow intrinsic rate of natural increase and is only found in permanent water bodies. B. globosus is found more widely and in larger numbers than B. pfeifferi, possibly because of the generally unstable nature of water bodies in Zimbabwe (Taylor and Makura, 1985).

B. pfeifferi is highly prevalent in the north-eastern part of the country, while a light distribution was noted in the south east where the prevalence is very low. B. globosus had high population densities in the north east and south east of the country, with patchy distribution in the south west, corresponding to the S. haematobium distribution (Taylor and Makura, 1985).

The distribution of B. globosus and B. pfeifferi correlates positively with the distribution of aquatic plants in the water, for example Typha latifolia and Nymphaea caerula (Woolhouse and Chandiwana, 1989).

In the Zimbabwean high-veld the recruitment of B. globosus and B. pfeifferi rises as the water temperature rises early in the hot-dry season and is somewhat patchy, being the highest in areas supporting suitable vegetation. The force of infection from man to snail is also the highest at this time, leading to peak prevalences of snail infection late in the hot-dry season. The prevalence varies between 2 and 19% with a peak in November for B. globosus, while for B. pfeifferi the prevalence varies between 0 and 10% with no patent infections in July-August (Woolhouse and Chandiwana, 1989). Unless the snail densities are very high they have little effect on the force of infection (Woolhouse and Chandiwana, 1990).

The distribution of snails supporting schistosome infections is related to spatial patterns in human contact with water, but the distribution and overall prevalence are also subject to seasonal variation (Woolhouse and Chandiwana, 1989).

9.2 Distribution and prevalence

A national survey was conducted by Taylor and Makura (1985). In the survey 14 614 school children from 157 schools in Zimbabwe were examined. Based on their results the country was divided into three sections according to the prevalence of S. haematobium (Figure 5.) and two sections for the prevalence of S. mansoni (Figure 6.):

Figure 5.

THE PREVALENCE OF S. haematobium AMONG EIGHT TO TEN YEAR OLD CHILDREN IN ZIMBABWE. From Taylor and Makura (1985)

Area a: high prevalence; Area b: medium prevalence; Area c: low prevalence.

Figure 6.

THE PREVALENCE OF S. mansoni AMONG EIGHT TO TEN YEAR OLD CHILDREN IN ZIMBABWE. FROM TAYLOR AND MAKURA (1985)

Area d: medium prevalence; Area e: low prevalence.

In north-eastern Zimbabwe the high rainfall and the soil type causes most streams and rivers to be perennial even in drought years, while in south-eastern Zimbabwe the lower rainfall causes only the major rivers to be perennial. In the western part of the country a combination of low rainfall in the south and sandy soil in the north makes even the major rivers dry up in most years, with some water remaining only in dams (Taylor and Makura, 1985).

The distribution of schistosomiasis in Zimbabwe thus reflects a close relationship with availability of surface water (Woolhouse and Chandiwana, 1990). In north-eastern Zimbabwe, which has the highest temperature and rainfall and where most streams have some water all the year round, the prevalence of S. mansoni and S. haematobium is at its highest. S. mansoni is also prevalent along the shores of Lake Kariba and in the lower reaches of the Sabi, Lundi and Nuanetsi rivers in the south-east, which are perennial (Taylor and Makura, 1985).

In recent years, over 10 000 small dams and reservoirs have been constructed throughout Zimbabwe. This does not appear to have resulted in high levels of prevalence of S. haematobium or S. mansoni on a national scale, although the prevalence of S. haematobium in medium and low prevalence areas may have gone up as a result of water conservation. There is no difference in prevalence of S. mansoni or S. haematobium in the western part of Zimbabwe (Taylor and Makura, 1985).

The higher prevalence of S. haematobium and S. mansoni in the commercial farming sector may be because of the greater water conservation (more dams) in these areas. Also the historical allocation of better farming land (with more water) to the commercial farmers (Taylor and Makura, 1985). Differences in the prevalence of S. haematobium and S. mansoni also indicate a variation in frequency of water contact, being the highest in communal areas with no piped water and the lowest in urban areas (Mason et al, 1986).

The prevalence of schistosomiasis and the force of infection in the high-veld for S. haematobium are at their highest for the age group 7–20 years, while for S. mansoni the peak prevalence is in the 16–30 year age group. The decline in infection and prevalence seems to reflect not only different water contact patterns, but also the development of an acquired immunity. Also, the infection in males generally exceeds that among females (Chandiwana and Christensen, 1988).

Over most of Zimbabwe the transmission is highly seasonal, with the exception of the Zambezi Valley and the south-eastern low-veld where because of the warmer climate and the presence of perennial water, the seasonality may be less marked (Shiff et al, 1979, Taylor and Makura, 1985).

Schistosomiasis control will be most effective when the rate of increase of the heavily infected snail population is low, e.g. between June and August in Zimbabwe. The low temperature during this period results in low recruitment rates and a long prepatent period (period between the snail's infection and its infection of others) (Woolhouse and Chandiwana, 1990).

10. ROLE OF ALCOM

Do ALCOM's activities affect the distribution and prevalence of schistosomiasis? If that is so what can be done?

Schistosomiasis is spread widely throughout the entire SADC region, affecting mainly the health and well-being of the rural population. In the areas where schistosomiasis is already established, the introduction of aquaculture will not increase the spread of infection. This is because rural populations already have a high frequency of water contact in their daily activities; the additional contact caused by aquaculture will not increase the infection.

Aquaculture can actually increase the prevalence of infection if ponds are introduced into areas with very little surface water, or where schistosomiasis did not exist previously. This situation is not very likely since aquaculture, by necessity, is feasible only in areas with sufficient surface water. In these areas it is also likely that schistosomiasis is already established.

It has been questioned whether cichlids or other snail-eating fish can actually control schistosomiasis in a pond (Pullin, undated). The basis of this argument is that each infected snail sheds such large numbers of cercariae that even if all but one of the infected snails are removed, this single individual can still infect people that come into contact with the water. Nevertheless a well-managed pond stocked with fish not only reduces the number of snails; the fish eats the cercariae, thus diminishing the infection risk of people.

Fish ponds can provide a good habitat for vector snails. With management, the environment for the snails will be destroyed, since intensification means less live and decaying herbaceous matter for substrate and food, and better water management. Consequently, increased production of fish goes hand-in-hand with the mitigation of schistosomiasis.

To be able to completely eradicate schistosomiasis, the causal problem of poverty, resulting in poor sanitation and health care, must be solved, mainly through the process of rural development. ALCOM's work within this area consists of improving the lives of the rural people through the introduction of aquaculture, while the improvement of health care and sanitation lies outside the scope of the programme.

ALCOM's main contribution to the mitigation of schistosomiasis is consequently to disseminate information about pond management with respect to the snail's intermediate host.

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