Focal mollusciciding: an effective way to augment chemotherapy of schistosomiasis

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Focal Mollusciciding: an Effective Way to Augment Chemotherapy of Schistosomiasis R.K. Klumpp* and K.Y. Chu** * 147 Laurel Street Atherton CA 94025, USA ** 4835 Dempsey Avenue, Encino CA 91436, USA

Fig. 3. Technician spraying niclosamide into a site of Schistosoma haematobium

transmission in the Volta Lake, Ghana

The demise of mollusciciding as the primary means of controlling schistosome transmission in endemic areas has been a consequence of the appearance of cheap, safe, and effective oral drugs, which have revolutionized control strategies during the last 15 years. The increase in cost of the molluscicide of choice, niclosamide, to over US$18 per kg (70% wettable powder) has also been a factor in the decline of chemical snail control. A third reason has been the failure of blanket or area-wide mollusciciding to control schistosome transmission in major control projects6,14,15. But the medical estabkishment may now be placing too much faith in the use of chemotherapy alone in control schemes and primary health care to diminish morbidity and control the transmission of schistosomiasis. This optimism has grown with the increasing availability of the 'new wonder drug', praziquantel, and its continuing reduction in price. Similarly, immunologists may be overly optimistic about the prospects for producing and delivering an effective anti-schistosome vaccine in the near future. A balanced approach to schistosomiasis control should not be overlooked. Properly designed preventive control measures such as improving water supply, sanitation, health education, and environmental management, though expensive in the short run, will reduce the long-term risks not only of schistosomiasis,

but of many other diseases as well. And despite its bad reputation of late, mollusciciding retains a useful role in schistosomiasis control. Snail control can be an effective supplement to chemotherapy programs, in augmenting community drug treatment in areas of high prevalence and intensity of infection, in preventing rapid reinfection after chemotherapy in areas of high migration and/or transmission potential, and in controlling transmission in areas of high population density per unit area of water, with or without chemotherapy. If chemical snail control is justified by local epidemiological and ecological conditions, the question is how to carry out the mollusciciding operations where and when they are needed. There are two basic approaches to chemical snail control: areawide and focal mollusciciding. The former approach stresses snail control itself, rather than transmission control. The molluscicide is dispensed throughout entire water courses (rivers, streams, and irrigation canals), or major stretches of them without much regard to where the transmission sites are located. The latter more recent approach is to target the chemical to those specific small areas or foci where transmission is known or suspected, and to time the operations so that infected snails do not have a chance to develop. From our field experience in Iran, Egypt, and Ghana, we have found that area-wide mollusciciding (Fig. 1 and 2) is expensive, wasteful, ecologically unsound, and generally ineffective. On the other hand, we have found that focal mollusciciding (Fig. 3 and 4) is an effective method in virtually all habitats.

Rationale for Focal Mollusciciding

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~1987,E;sevierPublications. Cambridge or 69 4758/87/$0200

The fundamental reason for focal mollusciciding is that prevalence and intensity of schistosomiasis ~md transmission are almost never randomly distributed in endemic areas. Thus, the strategy of focal mollusciciding is to target the moUuscicide, at the proper time, to those areas or loci where it is needed. These can range from large foci such as a stretch of canal or small dams near entire villages, to individual transmission points or sites. But in any area of widely

Parasitology Toddy, vol. 3, no. 3, 1987

scattered water contact points, the molluscicide need not be targeted to every known or suspected transmission point, but should be directed mainly to sites within villages, close to villages, or near clusters of households where human water contact is significant. A great advantage of focal mollusciciding is that it serves to augment chemotherapy in a structured project or primary health care scheme. It can be directed to those villages and foci where chemotherapy alone either would not achieve significant reductions in prevalence and intensity of infection (mainly in the attack phase of control), or could not prevent a rapid increase of infection (e.g. in the maintenance and consolidation phases). If chemotherapy is inadequately delivered, reduced, or abandoned, transmission can build up swiftly at foci of high transmission potential and cause a rapid rise in prevalence and egg output in villages16. By targeting molluscicides to these 'hot spots', the spread of infection and reinfection can be keot under control. Focal mollusciciding is ideally suited for small water bodies such as local reservoirs, ponds, and pools. It can also be cost-effective in rivers and stream,;, and even in large lakesl7,18. However, apart from a long-term effort in Zimbabwe n, :it has never been under serious consideration for snail control in the largest endemic habitats such as large perennial irrigation networks. We believe that in these irrigation canals, focal mollusciciding makes better sense than area-wide application.

Record of Area-Wide Molluscicidiag in Large Irrigation Canals Area-wide mollusciciding as a primary control measure failed to control schistosome transmission and human incidence rates in two well-documented projects in two large areas of pe~rennial irrigation: Egypt 49 in the Nile Delta 14, and the Gezira scheme in the Sudan 6. Niclosamide was applied chiefly by drip-feed application in Egypt 49 (Fig. 1). The rrmin problem there was poor timing of the chemical applications. In the Gezira, triphenmorph was applied by aerial spraying (Fig. 2). Incidence data suggests that the chemical failed to penetrate into weedy, smaller canals that contained some of the most important transmission sites. The one success story for area-wide mollusciciding has been in Fayoum, Egypt, where diligent and thorough application of niclosamide in the main Bahr Youssef canal between 1969 and 1972 virtually stopped

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Fig. 4. Focal application of niclosamide into sluicegate, Gezira scheme, Sudan

I

transmission of S. haematobium throughout the entire 400 000 feddan oasis (1 feddan = 1.04 acres). Even with patchy treatment with niridazole, it was claimed that the overall prevalence of infection in Fayoum dropped from 46% to 7% (Ref. 19). But the withdrawal of German technical support and the increased price of the chemical in the 1970s led to infrequent applications after 1973. Because of this and poor delivery of niridazole and metrifonate, prevalence rates of S. haematobium were increasing to precontrol levels in many villages in the early 1980s (Ref. 20 and 21). Moreover, Biomphalaria alexandrina started infesting Fayoum canals, and partly because of the lack of experience with focal mollusciciding and rapid case detection, S. mansoni was becoming a community problem by 1985. From 1977 to the present, area-wide mollusciciding has been conducted in all the major canal networks of the Nile Valley from Assiut to Beni Suef (excluding Fayoum) in a well-organized Ministry of Health project which also involved ongoing selective population chemotherapy at rural health centres and units 22. The mollusciciding was applied three times a year from 1977 to 1980, twice a year from 1981 to 1983, and once a year from 1984. Although prevalence and intensity of S. haematobium dropped significantly in all 3 governorates after 6 years of intervention, a 1985 evaluation study found that after 1981, area-wide mollusciciding had done little to reduce transmission in Assiut and E1 Minya 2°.

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Focal Mollusciciding in Irrigation Schemes There are several reasons for the superior effectiveness of focal mollusciciding. First, metrifonate and praziquantel are becoming available on a large scale in the main endemic areas of Egypt and the Sudan, and recent experience in these countries has revealed that on-going, selective population chemotherapy with these drugs alone can control schistosomiasis in all but the highest transmission areas. Therefore, there is no need to apply molluscicides over vast areas in any phase of a control scheme or project. Mollusciciding should be targeted only to those foci or selected areas where adequate delivery of drugs cannot reduce indices of infection to acceptable levels. Second, most developing countries cannot afford to import niclosamide in large amounts. Focal mollusciciding not only economizes on this chemical, it also encourages the use of other molluscicides, which, while not effective for area-wide applications, are adequate for small-scale use. Third, even in Egypt, where the cost of importing niclosamide has until recently not been a problem, this chemical is toxic to fish and so ks use is limited in many areas because of the increasing popularity of fish farming in canals and large drains. Fourth, area-wide mollusciciding cannot be performed in large carrier canals and rivers. Yet it has been demonstrated that numerous important foci exist in these habitats in the Nile River and Ibrahimiya canal in Egypt 20. The same study showed that in these locations focal mollusciciding was possible with copper sulphate (along semi-exposed shorelines of the Nile and large canals), and niclosamide (in sheltered water contact points of slower water circulation). Cost Considerations There has never been a strict parallel evaluation of the cost effectiveness of focal and area-wide mollusciciding in a given area, and there is inadequate information for scientific comparisons between projects. However, the Middle Egypt Project demonstrates how expensive large scale mollusciciding can be. From 1977 to 1984, Hoogstraal Centre for Tick Research

~---

Dr- Harry Hoogstraal was the world's foremost authority on ttcks and l~ck borne diseases.He died in February 1986 leaving his library and vast tick collec~on, along with field notes, manuscripts and original drawings, to the Smithsonian Institution. To continue Dr Hoogstraal's work, the Smithsonlan is establishingthe Hoogstraal Centre for Tick Research, and is seeking to raise a US$1.5 million fund for its support. Cheques for any amount, payable to the Smithsontan Institution, should be sent to The Chairman, Department of Entomology, Smithsonian Institution, Washington D C 20560, USA, from whom further information is available,

about 1350 metric tons of niclosamide were applied in an area of approximately 1.02 million feddan (about 1 million acres) containing 3-4 million rural inhabitants. The total cost of the consumed chemical alone was about 12.7 million Egyptian Pounds 14 times higher than the total cost of the metrifonate and praziquantel used in the project. Despite the huge expense of large-scale mollusciciding, it is still carried out in much of North Africa and the Middle East. The 1985 Report of a WHO Expert Committee on the Control of Schistosomiasis23was a little ambiguous on the future of this kind of mollusciciding. On the one hand it stated, "Population-based chemotherapy combined with health education and focal and seasonal mollusciciding are likely to be the most important features of schistosomiasis control in high priority endemic loci". It also said, "Even in some large irrigation schemes, where human population density is high and where water-management mechanisms are sophisticated, area-wide molhisciciding can be cost-effective". For all the reasons we have given, we hope this last statement will not be rigidly followed, and believe that focal mollusciciding is the better option in all areas. References

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