[Distribution of intermediate mollusc hosts in human schistosomiasis in the Office of Niger (Mali).II. Role of different habitats in the transmission].

To determine the role of habitats in the schistosomiasis transmission all the year, a study on snail hosts distribution was carried out within various parts in the irrigated system of the "Office du Niger". Bulinus truncatus and Biomphalaria pfeifferi were the only snail hosts recorded. Densities were low and 85% (46/54) of breeding sites contained less than 50 snails collected per man/hour. Primary and secondary canals play the most important role in disease transmission because of the stability of these habitats, the pollution of human water contact by remnants of food and wastes which favourised the snail growth and infestation. Prevalences were 23.3% (46/197) in Bulinus truncatus and 2.5% (6/243) in Biomphalaria pfeifferi. The cercariae shedding was permanent, but the infection varied significantly according to the snail species, the period and to the area.