Life cycle

Infection follows after activities such as swimming, bathing, fishing, farming, and washing clothes in contaminated water (1).

• free swimming adult schistosome or cercariae (the form infectious for humans) enters through the skin
  • intense itching follows the initial exposure (swimmer's itch) when the parasites initially enter the skin.
• once entered these cercariae shed their forked tail, becoming schistosomulae.
• the schistosomulae migrate through several tissues and stages to their residence in the veins. Maturing larvae (schistosomula) need about 5–7 weeks before becoming adults and producing eggs
• male and female schistosomes mature within the hepatic portal veins and then leave in pairs to lodge in the mesenteric (S. mansoni, S. japonicum) or pelvic (S. haematobium) venules where they mate
  • S. japonicum is more frequently found in the superior mesenteric veins draining the small intestine and S. mansoni occurs more often in the superior mesenteric veins draining the large intestine
    • however, both species can occupy either location, and they are capable of moving between sites, so it is not possible to state unequivocally that one species only occurs in one location
  • S. haematobium most often occurs in the venous plexus of bladder, but it can also be found in the rectal venules
• adult worms may live for several years in the human host. During this time, the female lays eggs continuously. The eggs are excreted in either the faeces or urine.
  • eggs, whether excreted or retained in the body, die within 1–2 weeks after being released by the female worm
• freshwater becomes contaminated by Schistosoma eggs when infected people urinate or defecate in the water.
• once the ova reach water, they release the immature cilia form (miracidia) which then enters its intermediate host, a form of freshwater snail. The miracidia mature in the liver of their snail host. When mature they are released again into the water as a new generation of adult cercarial worms. The mature worms are then ready to infect humans and restart the cycle.
  • cercariae can remain infective in freshwater for 1–3 days (1,2)

Reference:

• (1) Centers for Disease Control and Prevention (CDC). Parasites - Schistosomiasis. Accessed 12/08/2017
• (2) Colley DG et al. Human schistosomiasis. Lancet. 2014;383(9936):2253-64.