Field studies on the transmission and survival of Cyclocoelum mutabile (Digenea) infections in natural snail populations in southern Manitoba, Canada.

The transmission of Cyclocoelum mutabile to snails was examined under natural conditions by sampling the snail communities of 4 natural ponds that had been exposed experimentally to infection by laboratory-infected coots (Fulica americana). Five of 6 snail species in the ponds, Physa jennessi, Promenetus exacuous, Armiger crista, Gyraulus parvus, and Stagnicola elodes, became infected. No natural infections were found in the few Helisoma trivolvis examined. The second most abundant species Promenetus exacuous was infected most often, whereas Physa jennessi, the most abundant species present, was rarely infected. The temporal pattern of infections in the snail community suggests the transmission window of this parasite in southern Manitoba is limited by both the 14 C hatching threshold of the fluke eggs and the seasonality of ovigerous infections in the coot host. No naturally overwintering infections were found in snails from these ponds, which were examined the following spring. None of the 1,120 laboratory-infected snails placed in cages and held overwinter in the ponds survived, whereas 14 of the 1,120 uninfected control snails kept in the same cages survived. These results are consistent with the hypothesis that C. mutabile must be reestablished in northern waterfowl breeding areas each spring.