Abstract

The complex life cycle of trematodes involves a remarkable and highly character­ istic alternation of asexual and sexual reproductive phases in molluscan and vertebrate hosts, respectively. The parasitic worms that cause schistosomiasis ("bilharzia") in humans belong to the trematode genus Schistosoma. The avian schistosome Trichobilharzia ocellata can be studied as a model for the interactions between schistosomes and their host. This avian schistosome is also an important parasite as it causes cercarial dermatitis, and is becoming a serious problem in some freshwater areas. The life cycle of T. ocellata involves two hosts: a duck as the definitive host (Anas platyrhynchos) and a freshwater snail as the intermedi­ ate host (Lymnaea stagnalis). Eggs produced by the adult female parasite are shed with the feces of the duck, hatch in water, and produce a ciliated larva ("miracidium"). The miracidia can penetrate the skin of the mantle or the head­ foot of a snail host to develop into a primary or mother sporocyst near the site of penetration only during a limited period of time. Daughter sporocysts develop in the mother sporocyst and emerge 3-4 weeks postinfection (Amen and Meuleman, 1992), after which they migrate to the hind part of the snail. In the digestive gland/ovotestis area they grow and give rise to the final larval stage, the cercariae. The production of cercariae may continue for the rest of the life of the snail. Upon receipt of an as yet unidentified stimulus, the cercariae leave the snail and must find a suitable definitive host within a few hours. We are interested in the strategies this schistosome parasite uses to manipulate its snail host so that the parasite is able to survive, multiply, and complete its life cycle. Our studies indicate that T. ocellata interferes with two regulatory systems in the host, the internal defense system (IDS) and the neuroendocrine

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