schistosomiasis

Schistosomiasis is a parasitic disease caused by blood flukes of the genus Schistosoma. Humans become infected when free-swimming cercariae in freshwater penetrate the skin during activities such as bathing, washing, or fishing. The principal species affecting people are S. haematobium (urinary schistosomiasis) and S. mansoni and S. japonicum (intestinal schistosomiasis).

The parasites have a complex life cycle that requires freshwater snails as intermediate hosts. Eggs excreted in urine or feces hatch in water, releasing miracidia that infect suitable snails. Inside snails, they develop into cercariae, which are released back into water and can infect humans by skin penetration. After migration, adults reside in venous plexuses; eggs are released, some pass in urine or stool to continue the cycle, while others become trapped in tissues and drive inflammation and fibrosis.

Clinical presentation ranges from acute to chronic. Acute schistosomiasis (Katayama fever) may present with fever, cough, and urticarial rash. Chronic disease causes organ-specific pathology: S. haematobium primarily affects the urinary tract, causing hematuria, dysuria, and bladder abnormalities; S. mansoni and S. japonicum affect the intestines and liver, leading to abdominal pain, diarrhea, hepatosplenomegaly, fibrosis, and portal hypertension. Long-term infection increases risk of urinary tract cancer with S. haematobium.

Geographic distribution is tropical and subtropical. Endemic areas include parts of sub-Saharan Africa, the Middle East, the Caribbean, South America, and East Asia, with transmission linked to unsafe water and poor sanitation. Prevention emphasizes improved water and sanitation, snail control, safe water, and mass drug administration in endemic communities.

Diagnosis relies on detecting parasite eggs in stool or urine, supported by serology, antigen tests, or PCR in some settings. Treatment is with praziquantel, the first-line drug for all Schistosoma species; oxamniquine or other drugs may be used in specific contexts. Prevention and control also involve health education and environmental measures.