Schistosomes have a complex lifecycle, with sexual reproduction occurring within the definitive vertebrate host and asexual reproduction occurring in the intermediate snail host. Reproduction occurs post-pairing of adult worms in the venous/arterial system of the vertebrate host. Females may produce over 1000 eggs a day, which are passed in urine and/or stool depending on species. On contact with water they hatch into miracidia, the first (free living, non-feeding) larval stage. To survive, the miracidium must locate a susceptible snail and penetrate it within 24 hours, where it transforms into the first stage sporocyst. Second stage sporocysts, produced from the primary, develop in the hepatopancreas of the snail and divide into cercariae; the second larval stage (also free living).
Cercariae pass from the snail into the water column and use light and chemical cues to find a suitable definitive host within 24 hours. The cercariae penetrate the definitive host through the skin, and developing into schistosomula within a capillary, they migrate to the liver via the lungs where they begin to feed on red blood cells and to pair together. This process takes around 10 days. Next they migrate to the capillaries around the intestines in the case of S. mansoni and S. japonicum, or to the capillaries around the bladder wall or kidney for S. haematobium. The worms reach maturity at 6- 8 weeks, and adult worms generally live for 4 years, but can persist for much longer.
Transmission requires contact with freshwater together with the presence of a suitable intermediate snail host in the water body. Children generally have higher rates of infection, as they generally have more frequent exposure by swimming and playing in infected water, also the immune response to schistosomiasis tends to increase with age.
Potential transmission site for S. haematobium shown, Angola 2013
The life cycle of schistosomes