HOURLY CHANGE IN CERCARIAL DENSITIES OF SCHISTOSOMA-HAEMATOBIUM AND SCHISTOSOMA-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 i n the water and distances from the shore at a dam in Kwale District (K enya), where Schistosoma haematobium is highly endemic, by using a fil tration apparatus for detecting cercariae. The peak of cercarial densi ty 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 12:0 0 and 13:00 hours respectively. In the morning, the majority of cercar iae (79 % of the total detected) was obtained at the surface of water, but none at the bottom. After midday, 40 % of the cercariae were obta ined at the bottom. Cercariae seemed to sink with time resulting in a wider distribution in the water. The numbers of cercariae obtained at a sampling paint 20 cm from the shore, which was inside the wire-scree ned snail-free area, were 3.4-23 times more than those obtained at a s ampling point 340 cm from the shore, indicating that cercariae were ac cumulating immediately near the shore. Winds might cause the accumulat ion.