Life Cycle of Schistosoma mansoni
The dioecious adults of Schistosoma mansoni live in the portal veins, such as the posterior mesenteric which drains the large intestine. The females spend much of their time, housed in the gynecophoric groove of the male where copulation takes place. Egg capsules are deposited by the female which work their way through the wall of the vein into the adjacent large bowel. The mechanism of this migration through the vein and colon wall is still in question. Once in the large intestine, the egg capsules with the contained miracidium, exit along with the feces. Many ova (egg capsules) do not enter the lumen but remain surrounded by eosinophils and macrophages as a granuloma. If deposited in freshwater, the ciliated, miracidium will hatch and swim from 2-6 hours in search of a specific snail host. The principle snail hosts are of the genera, Biomphalaria spp., mostly in the old world, and Tropicorbis centrimetralis in Brazil. Once in the proper snail host, the miracidium sheds its epidermal plates and transforms into a mother sporocyst. The mother in turn produces daughter sporocysts in 6-7 weeks. The daughter sporocysts produce ciliated, furcocercous cercariae with typical forked tails. The furcocercous cercariae of the various schistosomes are indistinguishable. Schistosomes have no second intermediate host, but instead, the cercariae swim to the surface and sink to the bottom over the course of about 3 days in search of a human host. If the cercariae come in contact with human skin, they penetrate the skin, aided by digestive secretions of the penetration glands. The cercariae drop their tails in the penetration process and are then referred to as schistosomules. These schistosomules enter the circulation within 24 hours of penetration, following the venous return to the right heart though some follow the lymphatic pathway to the subclavian before entering the right heart. In passing through the capillaries of the lungs, many are lost. Those which successfully navigated through the lung capillaries are carried by the left heart to the mesenteric arteries. From there, the schistosomules must be able to pass through the capillary bed to the hepatic portal system, reaching the liver. There, males and females pair up and migrate to their destination in the posterior mesenteric veins, bordering the large intestine.