ECOLOGY OF BULINUS TRUNCATUS AND BIOMPHALARIA ALEXANDRINA AND ITS IMPLICATIONS FOR CONTROL OF BILHARZIASIS IN EGYPT-49 PROJECT AREA

Abstract

The respective vectors of the 2 forms of bilharziasis hi Egypt do not have the same ecological distribution. B. truncatus is most abundant in large canals, and decreases in density as the water approaches and flows into drains. B. alexandrina is more abundant in the presence of aquatic vegetation, but they differ in their respective associations with the water hyacinth, Eichhornia crassipes. B. alexandrina reaches maximum abundance hi the presence of this plant, but B. truncatus is as uncommon in the absence of plants as hi the presence of plants as in the presence of E. crassipes. Calculation of life-table parameters from field data shows that, under optimum field conditions, both species can double their populations hi 14-16 days. The reproductive rates of both species are greatest hi March and the death rates in midsummer. The observed peak densities in May and June give a false impression of optima because of undercollection of young snails, which are most abundant hi March and April. Control operations should take advantage of the findings on population parameters. A single area-wide treatment with molluscicide hi April is recommended. During the remainder of the year, search for isolated foci of snail breeding and individual treatment of these will effect large savings of chemical and will be effective in controling the transmission of the parasites.