Figure 1.

Schistosoma mansoni life cycle. The life cycle involves both an aquatic snail intermediate (Biomphalaria spp.) and a human definitive host. Mice and hamsters can be used to maintain the life cycle in the laboratory. Male (broad pink and red) and female (skinny pink) adult worms are found in the venules draining the intestine. Eggs pass through the intestine and out of the body with the feces. The eggs hatch in fresh water, and motile miracidia actively search for snails. Following penetration into the snail host, miracidia differentiate into sporocysts. Sporocysts proliferate asexually in the snail, eventually releasing motile clonal cercariae into the water. Cercariae penetrate the unbroken skin of a mammalian host, and then migrate through the bloodstream to the hepatic portal system where they develop into adults. In the laboratory, the entire life cycle takes 75 to 90 days to complete. S. mansoni is a conventional dioecious diploid, except for the fact that larval forms replicate asexually within the snail intermediate host. This aids in the staging of genetic crosses because clonally generated male and female larvae from different snails can be used to infect mice.