Penetration and migration routes of Schistosoma japonicum miracidia in the snail Oncomelania hupensis

The routes of penetration and the strategies of invasion of Schistosoma japonicum miracidia in the snail vector Oncomelania hupensis were observed in a histological study. In all species of the genus Schistosoma, it is usually assumed that the miracidia achieve penetration through the tegument. Our results showed that at least 57% of S. japonicum miracidia penetrated the snail by natural openings (branchial cavity, mouth and rectum). Throughout the invasion phase, the larvae were observed in all the tissues and organs with the exception of the genital gland. The spatial distribution of parasites in the snail revealed that the migration towards the visceral organs such as the kidney, heart and sinuses (which are the most usual microhabitats of the mother sporocysts of S. japonicum) appeared to take place via the circulatory system. Using natural openings as routes for penetration probably provides a selective advantage in a host-parasite system in which the target mollusc is amphibious: we presume that the miracidia inside these natural openings are protected against desiccation when the snail leaves the water, and that they can subsequently invade the tissues.