Experimental epidemiology of schistosomiasis. I. The prepatent period and cercarial production of Schistosoma mansoni in Biomphalaria snails at various constant temperatures.

Laboratory experiments have permitted the quantification of the developmental times (prepatent periods) of Schistosoma mansoni in the snail over the whole possible range of constant temperatures. The basis relationship is satisfactorily described by a hyperbola of the formula y = 268/(x-14.2), y being the minimum time from miracidial infection to cercariae shedding (in days) x the mean temperature, and 14.2 the theoretical temperature threshold (in degrees C). Cercariae production takes place within the limits of +16 degrees C and 35 degrees C, the number of cercariae being low and the mortality of snails high at the extreme values. Long-term alternations between two temperature levels resulted in prepatent periods corresponding exactly to the proportional time-temperature products. However, slight accelerations of up to 7% and more could be observed when the prepatency began in a period of high temperature. The number of cercariae each snail shed during a period of one hour exposure per week decreased from about 1,500 at 18 degrees C to about 250 at 16 degrees C. Shedding ceased completely after 1-2 weeks at 15 degrees C.