Distribution of Helisoma duryi, an introduced competitor of intermediate hosts of schistosomiasis, in an irrigation scheme in northern Tanzania.

Helisoma duryi is a planorbid snail, which has been suggested as biological control agent against the intermediate hosts of schistosomiasis. This snail species has been present in a sugar estate in northern Tanzania since 1972. In January 1981 a snail survey was done in this area in order to determine the distribution and abundance of H. duryi relative to other freshwater snail species. The distribution of H. duryi was found to be restricted to a few drains, while Biomphalaria pfeifferi, Bulinus natalensis, and Lymnaea natalensis were widespread. However, where H. duryi was found, no or very few B. pfeifferi were recorded indicating either competitive interactions between the two species or perhaps differences in the ecological preferences of the two species. No competitive interactions were indicated between H. duryi and B. natalensis. The apparent failure of H. duryi to spread in the area might be due to regular molluscicide application to the canal system and drainage canals, where H. duryi did not occur.