The prepatent period and cercarial production of Schistosoma haematobium in Bulinus truncatus (Egyptian field strains) at different constant temperatures.

The developmental time of Schistosoma haematobium in Bulinus truncatus snails (field strains) was determined in the laboratory at different constant temperatures between 18 and 32 degrees C. The basic relationship between the length of the minimum prepatent period (y, in days) and the temperature (x, in degree C) is given by the hyperbolic formula y = 295/(x-15.3), 15.3 being the theoretical "developmental null point" and 295 the constant time-temperature product. The shortest prepatency was 17-19 days at 30, 31 and 32 degrees C; at 18 degrees C, cercarial development required at least 106-113 days. The maturation time frequently exceeded the possible minimum by several weeks. No schistosome matured in our experiments at 17 degrees or 33 degrees C. The cercarial release per snail at weekly exposures showed a maximum at 25 degrees C with a geometric mean of 109 cercariae (95% confidence limits 79-149), decreasing to 8 (2-30) at 18 degrees C and 62 (38-100) at 32 degrees C. The absolute maximum of cercariae shed by one snail during 5 h "stimulation" was 2,150 in a 25 degrees C batch, 48 at 18 degrees C and 529 at 32 degrees C. The epidemiological application, the prognosis of the transmission period and the estimation of the transmission potential in relation to climatic conditions are discussed.