Schistosomiasis, also known as bilharziasis, results from long-lived infection by multicellular intravascular parasites of one of five trematode species — Schistosoma japonicum, S. mansoni, S. haematobium, S. intercalatum, or S. mekongi.
Parasite transmission and the consequent risk of human infection are strongly linked to specific geographic locations, because the parasite goes through several developmental stages that must occur in fresh water, including a period of growth within particular species of intermediate host snails.
- Infection of humans with schistosoma species causes chronic hepatic and intestinal fibrosis or fibrosis, stricturing, and calcification of the urinary tract.
- Infection follows contact with fresh water harboring larval parasites called cercariae, which penetrate humans' skin, become schistosomula, and enter capillaries and lymphatic vessels.
- The worms migrate to the portal venous system, where they mature and unite, and then to the superior mesenteric veins, the inferior mesenteric and superior hemorrhoidal veins, or the vesical plexus and veins draining the ureters.
- Eggs are produced and pass into adjacent tissues; many are shed in feces or urine.
- The eggs hatch, releasing miracidia that infect freshwater snails, which ultimately release cercariae.
Check this interactive graphics of Schistosomiasis.