Biomphalaria pfeifferi of different sizes were subjected to fluctuating temperature regimes incorporating quantities of above-optimal heat ranging from approximately 2 to 75 degree hours greater than 27 degrees C per day. Fecundity over this range was found to be similar from 2 to 45 deg.h. greater than 27 degrees C/day but above this level egg production fell sharply. Histological examination of the gonads of experimental and control snails showed that not only was gametogenesis retarded above 45 deg.h. greater than 27 degrees C/day but also, and perhaps more importantly, development of the gonad was considerably impaired. It is hoped that this kind of information, which is based on field data, will be useful in assessing the likelihood of Schistosoma mansoni transmission becoming established in newly impounded waters in non-endemic areas.

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