Geology and geomorphology in relation to the distribution of snail intermediate hosts of bilharzia in South Africa

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Abstract
Practically the entire endemic area of the bilharzia intermediate host snails Biomphalaria pfeifferi and Bulinus (Physopsis) sp. is over rock formations which are resistant to erosion. Only habitats characterized by being permanently lentic and not drastically disturbed by floods apparently are potentially suitable for the establishment of host snail populations. The only formations with erosional properties which permit the creation of such habitats are those making up most of the endemic area. Anomalous areas where snails occur on non-resistant rocks were noted and, where possible, tentative explanations based on geomorphological characteristics of the areas, were given. If it is accepted that the geology of an area conditions the suitability of watercourses to colonization by bilharzia host snails, this can be of immediate benefit in the selection of village sites in endemic areas. There is nothing to be lost by avoiding areas of hard rock when siting new villages to accommodate expanding human populations.