Schistosomiasis host snail control in irrigation night storage reservoirs

The control of the host snails of schistosomiasis in irrigation night storage reservoirs is important if the users of irrigation water and the occupants of the villages served by the irrigation scheme are to be protected from excessive transmission of the diseases. This technical note describes two biological control methods to control the host snails of schistosomiasis. Night storage reservoirs were of particular attention as previous studies by the joint HR Wallingford, AGRITEX and Blair Research Institute team in Zimbabwe, had shown that other snail control methods in night storage reservoirs were ineffective. The two biological control methods studied were a) the introduction of competitor snails, and b) the introduction of two species of fish, one to reduce aquatic vegetation and the other to predate snails. The snail competition study examined in the laboratory and semi-natural ponds the interspecific competition between Bulinus globosus (a schistome host species) and Bu/inus tropicus (a non¬susceptable species). B. Tropicus has a faster reproductive rate, predates the egg masses of B. globosus and in the short time period of the study appears to out compete B. globosus. The fish study examined two species of fish, both indigenous to Zimbabwe, Sargochromis codringtoni, a malacophagous species (snail eating) and Tilapia rendalli a mainly herbivorous species (vegetation eating). The study on fish is continuing but early results indicate that the fish if used in combination can eat most species of aquatic snails including the target host species and remove the vegetation which offers some protection. These biological methods have not been put into practice yet and larger scale studies on the practical use are needed. The methods look promising especially if combined with other operational methods previously recommended in the collaborative study, such as fluctuating water levels, desiccating reservoir beds and removing aquatic weeds. The Blair Research Institute is investigating a natural molluscicide, endod. The reported low toxicity of endod to fish may allow it to be used in combination with, for example, the fish predation method. The methods, if properly applied and managed, offer the opportunity of low cost acceptable snail control