ABSTRACT
The S. japonicum life cycle (Fig. 17.1) includes both parasitic and free-living stages. The infective stage for humans is the cercaria (pI. cercariae), which is free-living and free-swimming, but short-lived (24-72 h). The S.japonicum cercariae gain entry to the host through penetration of skin immersed in water. The cercariae then transform into larval schistosomula, which penetrate the circulation through subcutaneous vessels and reach the pulmonary circulation. In the lungs, the schistosomules elongate, break into the pulmonary veins and then travel through the heart to the systemic capillary bed. If the schistosomulum reaches the splanchnic vessels,
it moves across the capillary bed to the portal circulation (otherwise, it returns to the heart to circulate again). From the mesenteric capillaries, the schistosomulum travels to the liver, where it goes into the intrahepatic branches of the portal vein and matures into an adult schistosome. Adult S. japonicum are dioecious, i.e., either male or female and these migrate back to the mesenteric vessels to pair, mate and begin oviposition in the wall of the bowel. Eggs leave the human body in feces and when exposed to fresh water, these hatch to release miracidia, which in turn infect intermediate host snails, which complete the parasite life cycle by releasing cercariae 4-12 weeks after infection.
