The molluscicidal properties of Ambrosia maritima L. (Compositae). 4. Temporal and spatial distribution of Biomphalaria alexandrina in Egyptian village irrigation systems with reference to schistosomiasis transmission control.

Data on the temporal and spatial distribution of Biomphalaria alexandrina were collected throughout one year in two village irrigation systems near Alexandria, Egypt. Snails bearing Schistosoma mansoni cercariae were found from May to December but about 80% of annual transmission potential was confined from June to August. Although snail infection rates were generally 0-2%, higher rates were recorded and tended to be associated with smaller number snail collections. Infected snails were found in most canal and drain types and were judged to be present across most of the irrigated area of each village. In view of the dispersed nature of both infected snails and human-water contact in the area, it is suggested that schistosomiasis transmission is more widespread than focal. Effective transmission control using molluscicides should therefore require application to all village irrigation water-courses. The difficulties these various findings might impose on the use of commercial molluscicides are discussed and some possible advantages of the alternative plant molluscicide, Ambrosia maritima are noted.