ABSTRACT
Schistosomiasis is the second leading parasitic disease after Malaria. It comprises a group of chronic diseases caused by helminth digenetic trematodes of the Schistosoma genus. Five species of Schistosoma can affect humans. Schistosoma mansoni, S. japonicum, S. mekongi and S. intercalatum inhabit the mesenteric vessels and S. hematobium inhabits the vesical plexus. Two hundred and fifty millions of subjects are at risk of infection and in endemic regions, 5 to 20% of the population develops severe diseases, while 250 000 to 300 000 individuals die annually of the consequences of this infection.1,2
Infective larvae (cercariae) are released by fresh water snails. Human infection results from contact with water. The larva secretes enzymes that break down the skin’s protein to enable its penetration in the skin. During this migration, cercariae transform into schistosomulae. Twelve to 48 hours after infection, this new form circulates in the body for over 15 days, through the pulmonary vasculature where it undergoes further developmental changes. Then, young worms reach their final habitat in the venous mesenteric plexus (S. mansoni, S. japonicum) or in the urinary tract (S. haematobium). Schistosoma lives in his host for 3 to 5 years. Within the portal vasculature, male and female adults pair off and migrate along the endothelium, against portal blood flow, to the mesenteric (S. mansoni, S. japonicum) or vesicular (S. haematobium) veins where the female lay on average 300 to 3000 eggs a day. These eggs migrate through the bowel or bladder wall to be shed in feces or urine. They hatch in fresh water and release ciliated motile miracidia (approximately 10 days). Free-swimming miracidia infect susceptible snail and
multiply asexually. Within 4 to 6 weeks mature cercariae emerge and exit the snail to seek a human host in a circadian rhythm, dependent on ambient temperature and light.
