The prepatent period and cercarial production ofSchistosoma haematobium inBulinus truncatus (Egyptian field strains) at different constant temperatures

Abstract

The developmental time ofSchistosoma haematobium inBulinus truncatus snails (field strains) was determined in the laboratory at different constant temperatures between 18 and 32°C. The basic relationship between the length of the minimum prepatent period (y, in days) and the temperature (x, in °C) is given by the hyperbolic formulay=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° C; at 18° 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° or 33° C. The cercarial release per snail at weekly exposures showed a maximum at 25°C with a geometric mean of 109 cercariae (95% confidence limits 79–149), decreasing to 8 (2–30) at 18° C and 62 (38–100) at 32° C. The absolute maximum of cercariae shed by one snail during 5 h “stimulation” was 2,150 in a 25° C batch, 48 at 18° C and 529 at 32° C. The epidemiological application, the prognosis of the transmission period and the estimation of the transmission potential in relation to climatic conditions are discussed.