Studies on the ecology of Bulinus globosus, the intermediate host of Schistosoma haematobium in the Ifakara area, Tanzania.

During a period of 2 years, the ecology of Bulinus globosus was studied in 8 habitats in two streams near Ifakara, SE-Tanzania. The relative Bulinus densities were followed monthly. Two different methods for estimating snail densities (man/time vs. palmleaf traps) gave comparable results. Bulinus densities were constantly low throughout the year in the stream, but they showed distinct seasonal fluctuations in adjacent pools, with a density-peak at the end of the small rainy season. B. globosus, identified by starch gel electrophoresis, was found to be the only intermediate host for urinary schistosomiasis in the investigated streams. A correlation of the Bulinus densities with several abiotic and biotic factors revealed that pH, temperature and conductivity had little effect on the Bulinus population, as they oscillated within the tolerated limits. No correlation of the distribution of B. globosus and other snail species was found. Rainfall patterns have a distinct influence on snail densities. They determine the duration of desiccation and affect the snails by fluctuations of the water level and by the fast increase of water velocity after heavy rains. B. globosus shows a clear predilection for the sedge Cyperus exaltatus as support for oviposition. It is also preferred as food and/or food-support. During the dry season, oviposition of B. globosus is concentrated in clearly defined sites ("breeding pockets"), which, due to the lowering of the water level, become isolated from the stream or retain only a small connection to it. These sites form important reservoirs of B. globosus, from where the snails are spread when the sites are flooded during the subsequent rainy season. The significance of these observations for control measures is discussed.