[Epidemiologic study of S. haematobium bilharziasis in the rice fields of Yagoua (North Cameroon) II. Distribution and ecology of intermediate hosts].

While inventoring the local malacological fauna and evaluating the density of species acting as intermediate hosts of human schistosomes it has been shown that Bulinus truncatus is predominant in the study area. Bulinus globosus is scarcely encountered. Biomphalaria pfeifferi is only found in the Mayo Guerleo, in irrigation canals and draining ditches. But B. globosus and B. pfeifferi have not yet invaded the recent aquatic network and are not found in the lake nor in the newly created irrigation systems. Snails shedding cercariae of S. haematobium and S. mansoni have been collected in the area. Bulinus forskalii is found in all types of permanent of temporary waterbodies but S. intercalatum has not yet been reported from North-Cameroon. Lymnaea natalensis, the intermediate host of Fasciola gigantica, is found in the lake and in the canals, in the Mayo Guerleo and in the Toupasko pool. The rice field does not form by itself a favorable environment for snails to develop in it. Spots where transmission occurare more often in the area the secondary of tertiary irrigation canals and the brains. The ditches obstructed by plants are the places where the snails are developing best. Bulinus spp. are found in large numbers in the lowest parts of the aquatic network and they accumulate there to survive drought conditions in the mud. The recently built irrigation systems are not massively invaded by the snails yet and the prevalence of bilharziasis in the area correspond to the distribution of the intermediate hosts of schistosomes and to the density of the snails populations.