Parasitology Today, vol. 4, no. 7, 1988 LETTER5 Schistosomiasis in Cameroon Sir- We agree fully with the call for adequate ecological and medical impact studies for water development projects it, the tropics, as outlined in your September 1987 Alternative Answers column ~.But the autl~,ors ' choice of northern Cameroon as an example of why such studies are important, also illustrates a common problem in impact evaluation-the lack of adequate baseline data. Resultsfrom our recently completed nationwide schistosomiasissurveys in Cameroon (soon to be submitted for publication) illustrate why evaluations based on inadequate baseline data are usually unsatisfactory. Much attention has been focused recently on problems of transmission of schistosomiasisattributed to water development projects in the Extreme North Province of Cameroon, in particular the SEMRY* rice schemes developed alongthe Logone River near Chad2'3' and small reservoirs built in the Mandara Mountains near the border with Nigeria 4-a. These studies have concentrated on villageswithin or adjacent to these'schemes and have commonly found elevated prevalences of urinary schistosomiasis.With no broad- based studies to rely on, the irr~pressionleft from these studies and the rewew presented in Parasitology Today i isthat perrnanent waters found in the development schemes constitute the major source of transmission in the Sahelianregion of Cameroon. Resultsfrom our national survey of schoolchildren from schools randomly selected from each subdivisior, of Cameroon, show relatively high prevalences of S. haematobium and S. mansoni throughout much of the North and Extreme North Provinces. For more than two-thirds of subdivisions where prevalences were greater than 30% there are no water development schemes and only rarely is year-round surface water present. Our malacological surveys show that Bulinus globosus and B. senegalensis- hosts for S. haematobium - are the most common snailsin the Sahelian region, where they occur almost exclusively in temporary ponds and streams. Biomphalaria pfeifferi- a S. rnansoni host - is also common in temporary water bodies in this region. Our results clearly show that the major problems with schistosomiasis in the Sahelian region exist outside the water development schemes. Certainly efforts to monitor infection levels in the schemes should continue, but results from such studies must be evaluated in view of the broader regional problem of schistosomiasis. Furthermore, there have been no studies within the *SEMRY- Soci6t6d'Expansion et de Modernisationde la Riziculturede Yagoua. ~) 1988, ElsevierPublications, Cambridge 0169-4758188/$02.00 schemes to determine the contribution to transmission of man-made water bodies as compared with temporary bodies of water found in the same villages. We hope that results from our current investigations in the region will help answer this latter question and further clarify the importance of these water development schemes to transmission of schistosomiasisin that region. Reply Sir-The model of North Cameroon was chosen to evaluate the impact of dams 4 and irrigation schemes on human schistosomiasis. Man-made water bodies- deep artificial lakes in the granite Mandara Mountains7, shallow lakes in the plains resulting from the construction of earth dykes or rice field irrigation schemes in the Logone Valley 3- demonstrate that the effect of water resource projects on schistosomiasiscaused by Schistosoma mansoni or S. haematobium varies with the nature of the artificial body of water studied. It is not the measurement of schistosomiasisbaseline prevalence data (or other indices) in the area, before the construction of dams or canals,that can predict the future intensity of the disease. This is directly linked to the newly created hydrobiological conditions, producingthe rapid multiplication of freshwater snails or, in some casesthe converse, the stagnation of snail populations. Prevalences for both intestinal and urinary schistosomiasisin the Extreme North of Cameroon are high 3'6'8'9, and the sites where transmission occurs are numerous. There are small water bodies of all kinds, isolated or related to the hydrographical network. The snails which are the local intermediate hosts for schistosomes are Biomphalaria pfeifferi, Bulinus truncatus and B. globosus I°. B. senegalensis is transmitting schistosomiasiscaused by S. haematobium in the area too, but this snail, abundant in seasonally filled rain pools, has not been collected in the artificial lakes or irrigation canals.What is important to consider is the impact of the newly created water reservoirs on snail population dynamics. Irrigation schemes in Yagoua (SEMRY* I)and Maga(SEMRY* II) have produced an intensive and regular increase of snails,chiefly B. truncatus, in the drainage canalsblocked with aquatic plants2, and similar swarming of B. pfeifferi and B. globosus has been observed in the shallow lake of Douvar (water supply reservoir for the town of Mokolo). On the other hand, it is obvious that snails do not develop in just any kind of water. No snailsare found in the oligotrophic waters of the deep lakes of the Mandara Mountains- characterized by steep rocky banks, absence of aquatic plants, and great variations in water levels~ ~. An artificially created water body is 199 R. Ratard, M.M. Ekani, G.J. Greet, L. Kouemeni, R. Mimpfoundi* and E. Ngonseu TulaneUniversity-IMPM Schisto Project USAID BP817 and*Faculty of Science Yaounde, University of Yaounde Cameroon Cameroon thus not always propitious for snailsto develop in; this depends on the geological formation in the area and on the pH and ionic composition of the water 12. Similar observations have been made for malaria in North Cameroon ~ 3. Large water surfaces with no vertical aquatic plants are not suitable for the development of Anopheles gambiae or An. funestus larvae, but in the rice fields of the Logone Valley where a rapid multiplication of anopheline mosquitoes has been noted ~4, this swarming has not produced an increase in malaria prevalence. Isthis due to female mosquitoes switching to feed on livestock? Do the Anopheles cytotypes in rice fields differ from those hatching from natural waterbodies? Or are the malaria indices decreasing in the rice fields area of Yagoua because of increasing usage of chloroquine? Becauseof this latter question, measuring prevalence alone-even in a longitudinal survey- will not clarify trends related to the biology and ecology of vectors or aquatic intermediate hosts. C.L. Ripert C.P. Fl=_.ccurt Laboratoirede Parasitologie M~dicale Universit~ de Bordeaux II 146rue Leo Saignat F-33076Bordeaux Cedex France References I Ripert,C.L.and Raccurt, C.P.(1987) ParasitologyToday 3,287-289 2 Wibaux-Charlois, M. et al. (1982) Bull. Soc. Pathol. Exot. 75, 72-93 3 Yelnik, P.A. etal. (I 982)Bull. Soc.Pathol.Exot. 75, 62-7 I 4 P~pert,C. et al. (1979) Bull. Soc.Pathol. Exot. 72, 324-329 5 PJpert, C.etal.(t983)Geogr. Trop.48, 177- 186 6 Granier,H. et al. ( 1985)Med. 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