schistosoomid

Schistosoomid, commonly called schistosomes, are parasitic flatworms (trematodes) in the family Schistosomatidae. They are dioecious, with distinct male and female worms that pair in the host’s blood vessels. The most important human species belong to the genus Schistosoma (notably S. mansoni, S. haematobium, and S. japonicum), and several other schistosome species infect animals or cause zoonoses. Transmission is via freshwater snails that serve as intermediate hosts.

Lifecycle: Eggs released in urine or feces hatch into miracidia in water and infect suitable snail species. Inside the snail, development yields cercariae, which are released and can penetrate human skin during contact with contaminated water. Cercariae migrate to the mesenteric or vesical venous plexuses, where they mature into adults. The female lays eggs, some of which are excreted in stool or urine, while others become lodged in tissues, provoking granulomatous inflammation and fibrosis.

Disease manifestations: Acute schistosomiasis (Katayama fever) can occur after initial infection. Chronic disease is determined by the site of egg deposition: intestinal schistosomiasis (S. mansoni, S. japonicum) causes abdominal pain, diarrhea, and hepatosplenic fibrosis with portal hypertension; urogenital schistosomiasis (S. haematobium) causes hematuria, dysuria, and may lead to hydronephrosis and increased risk of bladder cancer. Eggs’ spine morphology helps species identification in stool samples.

Diagnosis and treatment: Diagnosis relies on detection of eggs in stool or urine, with serology or antigen tests as adjuncts. Praziquantel is the drug of choice for most schistosome infections; alternative agents include oxamniquine in certain settings. Prevention focuses on reducing water contact, improving sanitation, and controlling snail populations, alongside mass drug administration in endemic areas.

Epidemiology: Schistosomiasis remains a major tropical disease, endemic in parts of Africa, the Middle East, South America, and Asia, affecting millions and causing substantial morbidity.