Ecological Observations on the Molluscs of Significance in the Transmission of Bilharziasis in Kenya

Further knowledge of the bionomics of bilharziasis vectors is necessary before more effective measures can be taken to control them. Such knowledge, acquired from long-term observations in a variety of habitats, should help to fill some of the gaps in knowledge of molluscan ecology. The present studies were conducted over a period of three to five years and included observations on the proved and suspected vectors of bilharziasis in Kenya. Records were kept of seasonal and regional distribution of the snails, population dynamics and infection rates. Seasonal distribution normally conformed to a definite pattern but was often so influenced by unseasonable weather and interference by man as to render it unrecognizable. Temperature, water fluctuations, types and amount of vegetation, dissolved oxygen and other physical and chemical characteristics of the water were recorded and an attempt was made to assess the importance of the changes in such environmental factors on the prevalence of the snails studied. No correlation could be found between vegetational changes in the habitat and the number of snails present, but complete coverage of the water surface with horizontal vegetation tended to an absence of snails. Except for torrential floods, water fluctuations seemed to have no effect on snail numbers and the chemical and physical properties of the water of negative habitats appeared to be within the range of tolerance of the species observed.It was not found possible to assess the effects of small temperature changes on reproduction in snails; it is suggested that further trials are justified.There was no obvious effect on numbers when snails were found with predators in the same habitat. Many otherwise adequate habitats were unintentionally rendered unsuitable for vector snails by direct interference by man.