Schistosomiasis

Schistosomiasis is a parasitic infection caused by trematode flukes of the genus Schistosoma. It is acquired by direct skin penetration by free swimming cercariae released from infected freshwater snails.

The infecting species determines the predominant pattern of chronic disease. Schistosoma haematobium causes urogenital disease. Schistosoma mansoni and Schistosoma japonicum, together with several other regionally important species, cause intestinal and hepatosplenic disease.

It is one of the most prevalent parasitic infections worldwide and a major cause of chronic morbidity in endemic tropical and subtropical regions.

Disease pathology reflects the host immune response to deposited eggs rather than direct damage from the adult worms.

Pathophysiology and life cycle

• Cercariae released from infected freshwater snails penetrate intact human skin during freshwater contact. They migrate through the tissues and bloodstream to the venous plexus specific to the infecting species.
• The vesical and pelvic venous plexus is the site for Schistosoma haematobium. The mesenteric venous plexus is the site for Schistosoma mansoni and Schistosoma japonicum. Adult worm pairs reside there and produce eggs.
• Chronic disease results predominantly from the host granulomatous immune response to eggs trapped within tissue. In urogenital disease these lie in the bladder and ureteric wall. In intestinal and hepatosplenic disease they lie in the intestinal wall and, after venous embolisation, the liver.
• Progressive fibrosis around trapped eggs over years of ongoing or repeated infection underlies the chronic organ damage. This includes bladder fibrosis and calcification, and portal hypertension from periportal (Symmers) fibrosis.
• Schistosoma haematobium has an established causal association with squamous cell carcinoma of the bladder.

• Cercarial dermatitis ("swimmer's itch") is a pruritic papular rash at the site of skin penetration. It can occur within hours of exposure, particularly with repeated prior sensitisation. Many infections produce no symptomatic acute skin reaction.
• Acute schistosomiasis (Katayama syndrome) occurs several weeks after heavy exposure, particularly in previously unexposed travellers. It presents with fever, urticarial rash, myalgia, cough and marked eosinophilia.
• It reflects an acute hypersensitivity response to migrating schistosomula and early egg deposition. It can occasionally be severe.
• Chronic urogenital schistosomiasis (Schistosoma haematobium) presents with terminal haematuria (blood appearing at the end of micturition, a relatively characteristic feature) and dysuria.
• Chronic disease leads to bladder wall fibrosis, hydronephrosis from ureteric involvement, and an elevated risk of squamous cell bladder carcinoma over years of untreated or repeated infection.
• In women, genital involvement can affect fertility and is associated with increased susceptibility to HIV acquisition.
• Chronic intestinal and hepatosplenic schistosomiasis (Schistosoma mansoni, Schistosoma japonicum) presents with abdominal pain and diarrhoea (sometimes bloody).
• With more significant hepatic egg deposition and periportal fibrosis over years, progressive portal hypertension develops, with splenomegaly, oesophageal varices and variceal bleeding.
• Hepatic synthetic function is generally preserved, because the fibrosis is periportal rather than diffuse parenchymal.
• Specifically seek a history of freshwater exposure (swimming, wading, agricultural work involving irrigation contact) in an endemic region in any patient with compatible symptoms or unexplained eosinophilia.

• Microscopy demonstrating characteristic eggs confirms the diagnosis. Use urine microscopy for Schistosoma haematobium and stool microscopy for intestinal species.
• For Schistosoma haematobium, terminal urine collection optimises egg detection because of the characteristic pattern of egg excretion.
• Sensitivity is imperfect with light infection. Testing at different times of day can improve yield. Urine egg excretion varies diurnally, typically peaking around midday.
• Serological testing has good sensitivity. It is particularly valuable for detecting infection in travellers with light egg burden, or in the early phase before egg production is established.
• It cannot distinguish current from past, treated infection, because the antibody response persists.
• Eosinophilia on full blood count is a useful supportive finding, particularly in returning travellers.
• Cystoscopy (for suspected urogenital disease) supports staging and assessment of chronic organ involvement.
• For hepatosplenic disease, ultrasound of the liver (showing the characteristic periportal fibrosis pattern) and spleen supports assessment. Upper gastrointestinal endoscopy assesses varices where relevant.
• Consider screening for schistosomiasis in any traveller or migrant with relevant freshwater exposure in an endemic region, even without specific symptoms.
• Early or mild infection is frequently asymptomatic or non specific, yet progressive chronic organ damage can continue.

• Praziquantel 40 mg per kg as a single dose is the treatment of choice across all Schistosoma species.
• Some regimens divide the dose into two on the same day. For Schistosoma japonicum, a slightly higher total dose of 60 mg per kg divided over the day is sometimes used.
• Praziquantel is most effective against adult worms and has reduced efficacy against the earlier schistosomula stage.
• Treatment given very soon after exposure (within six to eight weeks) may therefore need to be repeated to ensure adequate efficacy against organisms that have since matured.
• Manage acute schistosomiasis (Katayama syndrome) with supportive care. Corticosteroid therapy may be used to control significant acute hypersensitivity symptoms. Praziquantel can exacerbate the acute inflammatory response if given before adequate antigen exposure has occurred.
• Give praziquantel either at presentation or, in some protocols, deferred by several weeks to optimise efficacy against the now more mature worm burden. Disease severity and specialist tropical medicine input guide the approach.
• Chronic disease with established organ complications needs management of the complication alongside antiparasitic treatment.
• Manage portal hypertension and variceal bleeding following standard principles. Arrange urological assessment and management for significant bladder or ureteric fibrosis and hydronephrosis.
• Keep specific surveillance for bladder malignancy in patients with longstanding Schistosoma haematobium infection, given the well established carcinogenic association.
• For travellers, prevention is primarily avoidance of freshwater contact in known or potentially endemic areas. Where contact is unavoidable, dry the skin rapidly with a towel after any water contact, which reduces cercarial penetration risk relatively rapidly though incompletely.
• At population level in highly endemic regions, prevention uses mass drug administration programmes, improved sanitation to reduce environmental egg contamination of water sources, and snail intermediate host control.