The influence of stream geology on the distribution of the bilharzia host snails,Biomphalaria pfeifferiandBulinus (Physopsis)sp

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Abstract
The Gladdespruit and Komati River surveys have shown that similar environmental conditions have developed where the watercourses flowed over particular rock types. In these watercourses a potentially useful association was found between the occurrence of permanent, lentic habitats produced by the weathering of bedrock with a hardness above 5 in Mohs' Scale of Hardness and the longitudinal distribution of persistent populations of the bilharzia intermediate host snails Biomphalaria pfeifferi and Bulinus (Physopsis) sp. Bilharzia transmission follows this pattern in these waterways. Weathering of bedrock with hardness below 5 in Mohs' scale, and/or flow over beds of alluvial sand, produced perpetually lotic (flowing) environments without persistent host snail populations because of the snail's inability to tolerate high current speeds. However a few temporary populations survived in temporarily lentic habitats during the dry winter season.