The survival of the cercariae of Schistosoma mansoni in relation to water temperature and glycogen utilization

Abstract

Survival curves for cercariae of Schistosoma mansoni maintained at different temperatures were determined. Between 15 and 35 °C the curves were of reverse sigmoid form and the probit transformation gave a statistically good fit. The slope of the regression lines fitted to the probit transformations increased exponentially as the temperature rose. Above 35 °C heat intolerance became important and increased exponentially as the temperature rose. Below 15 °C, cold intolerance caused an initial high level of mortality followed by an extended period with insignificant mortality. The glycogen content of whole cercariae and separated cercarial bodies and tails was measured during ageing. The glycogen content of whole cercariae declined exponentially during ageing. This was a product of exponential decreases in the glycogen content of both the body and the tail of the organism. The cercarial tail was shown to contain slightly over half of the total glycogen content. The rate of glycogen use was higher in the tail than in the body. A computer simulation model was used to demonstrate that the observed exponential use of glycogen could generate survival curves similar to those observed experimentally.