Distribution of freshwater snails in the river Niger basin in Mali with special reference to the intermediate hosts of schistosomes

Snail surveys were carried out in various parts of Mali. All areas surveyed are part of the Niger basin being either affluents or irrigation schemes fed by this river. The snail species present varied greatly between areas. The following potential hosts of schistosomes were recorded: Biomphalaria pfeifferi, Bulinus truncatus, B. globosus, B. umbilicatus, B. forskalii and B. senegalensis. In the large irrigation schemes, i.e. ‘Office du Niger’ and Baguinéda, only B. pfeifferi and B. truncatus appear to be intermediate hosts. Snail distribution appeared to some extent to be focal and high snail densities appeared to be associated with human water contact activities, which apparently create favourable biotopes for the snails. This is probably due to an alteration of the vegetation and an increase of the trophic status of the site by contamination with food remnants and other debris. The larger irrigation canals or lakes in these schemes play an important role in the transmission of human schistosomes and transmission appears to be very focal in these habitats. Infected snails are almost exclusively found in well-defined human water contact sites (WCS). Local infection rates with schistosomes were often high (i.e. up to 27% in B. pfeifferi). In urban areas (i.e. Bamako), transmission patterns are more variable. In Bamako schistosome-infected B. truncatus were found in the Niger river. A number of smaller semi-permanent or permanent streams are very important transmission sites, and schistosome infections were recorded from B. pfeifferi, B. truncatus and B. globosus. Schistosome infection rates in B. pfeifferi were often high (up to 30%). In a new lake at Sélingué, B. truncatus was found to be widely distributed only about a year and a half after the dam was constructed, and in some sites schistosome infections were recorded.