Hourly change in cercarial densities of Schistosoma haematobium and S. bovis at different depths in the water and distances from the shore of a dam in Kwale District, Kenya

Hourly change in cercarial densities was studied at different depths in the water and distances from the shore at a dam in Kwale District (Kenya), where Schistosoma haematobium is highly endemic, by using a filtration apparatus for detecting cercariae. The peak of cercarial density at the surface of water (2-3 cm deep) was at 11: 00 hours. Those at the middle point (25 cm deep) and the bottom (50 cm deep) were at 12: 00 and 13: 00 hours respectively. In the morning, the majority of cercariae (79 % of the total detected) was obtained at the surface of water, but none at the bottom. After midday, 40 % of the cercariae were obtained at the bottom. Cercariae seemed to sink with time resulting in a wider distribution in the water