Schistosoma Life Cycle
1. In the human host, eggs containing miracidia are eliminated with feces or urine into water.
2. Under appropriate conditions, the eggs hatch and release miracidia.
3. The miracidia swim and penetrate a snail (intermediate host).
4 and 5. Within the snail, the miracidia progress through 2 generations of sporocysts to become cercariae.
6. The free-swimming cercariae are released from the snail and penetrate the skin of the human host.
7. During penetration, the cercariae lose their forked tail, becoming schistosomula.
8 and 9. The schistosomulae migrate via venous circulation to lungs, then to the heart, and then develop in the liver, exiting the liver via the portal vein system when mature.
10. The paired (male and female) adult worms migrate (depending on their species) to the intestinal veins in the bowel or rectum or to the venous plexus of the genitourinary tract, where they reside and begin to lay eggs.
S. japonicum is more frequently found in the superior mesenteric veins draining the small intestine (see A).
S. mansoni occurs more often in the inferior mesenteric veins draining the large intestine (see B).
However, both species can occupy either location and are capable of moving between sites.
S. intercalatum and S. guineensis also inhabit the inferior mesenteric plexus but lower in the bowel than S. mansoni.
S. haematobium most often inhabits in the vesicular and pelvic venous plexus of the bladder (see C), but it can also be found in the rectal venules.
The females (size ranges from 7 to 28 mm, depending on species) deposit eggs in the small venules of the portal and perivesical systems. The eggs are moved progressively toward the lumen of the intestine (S. mansoni, S. japonicum, S. mekongi, S. intercalatum/guineensis) and of the bladder and ureters (S. haematobium), and are eliminated with feces or urine, respectively.
Image from the Centers for Disease Control and Prevention, Global Health, Division of Parasitic Diseases and Malaria.