In Asia, cattle and water buffalo can be important reservoir hosts. Infection occurs when free-swimming larvae penetrate human skin. The larvae develop in fresh-water snails. Humans are infected when they enter larvae-infested water for domestic, occupational and recreational purposes. After skin penetration, the larvae transform and are carried by the blood to the veins draining the intestines or the bladder where they mature, mate and produce eggs.
Eggs cause damage to various tissues, particularly the bladder and liver.
The reaction to the eggs in tissues causes inflammation and disease. When infected humans excrete parasite eggs with feces or urine into water, the eggs hatch releasing larvae that in turn infect aquatic snails. In the snail the parasite transforms and divides into second-generation larvae which are released into fresh water ready to infect humans. Those who work in irrigation or fishing are at increased risk for schistosomiasis. Distribution of the disease Schistosomiasis is endemic in 76 countries, most of which are in Africa.