Studies on a Microsporidian Hyperparasite of Strigeoid Trematodes. I. Prevalence and Effect on the Parasitized Larval Trematodes

Abstract

A large proportion of the strigeoid infections in the snails in the area surrounding the University of Michigan Biological Station is hyperinfected with a species of Nosema. Only a small proportion of the hyperinfections are spore-producing. Mother and daughter sporocysts and cercarial embryos of all stages contain the stages of this microsporidian. Emerged cercariae are free of infection because large cercarial embryos when infected are so injured and distorted that they cannot escape from the snail. The details of cytological structure of the stages in the life cycle of this microsporidian have not yet been worked out. Early stages of schizogony appear as small vesicles, which can best be seen in cercarial embryos, and the stages of sporogony in protuberances on the surfaces of sporocysts and cercarial embryos appear to be like those of other spp. of Nosema. So far this hyperparasite has been found only in the Douglas Lake area at the northern tip of the southern peninsula of Michigan. Examinations have shown that 12 different spp. of strigeoids in the area harbor this infection. While heavy infections may kill the larval trematodes, in most cases the injury is slight and does not greatly decrease cercarial production.