Schistosomiasis is one of the most prevalent parasitic diseases globally, second only to malaria in terms of human morbidity in tropical regions. It affects an estimated 240 million people worldwide and causes significant chronic morbidity in endemic populations. For UK travellers, it is a clinically important risk that is often overlooked in travel health consultations, particularly for those planning activities involving freshwater exposure in sub-Saharan Africa, the Middle East, Southeast Asia or parts of South America.
Unlike many travel-related infections, schistosomiasis is not prevented by vaccination (none exists) and not treated by mosquito bite avoidance. Its prevention relies entirely on avoiding freshwater exposure in endemic areas, a message that is clinically important to deliver clearly and that can be challenging to communicate effectively when the relevant water source is a visually pristine lake in a stunning landscape.
What Is Schistosomiasis and How Is It Acquired?
Schistosomiasis is caused by parasitic flatworms of the genus Schistosoma. The species most clinically relevant to travellers are Schistosoma haematobium (urogenital schistosomiasis, endemic in sub-Saharan Africa and parts of the Middle East), Schistosoma mansoni (intestinal schistosomiasis, endemic in sub-Saharan Africa and South America) and Schistosoma japonicum (endemic in Southeast Asia, predominantly China, Philippines and Indonesia).
The life cycle requires two hosts: humans (or other mammals) and specific freshwater snails. Infected humans excrete schistosome eggs in urine or faeces into water. Eggs hatch into free-swimming cercariae, which infect freshwater snails, multiply, and are released as free-swimming cercariae that can penetrate intact human skin on contact with contaminated water. There is no insect vector: the infective stage penetrates the skin directly during wading, swimming or contact with contaminated freshwater.
The clinical significance of this transmission route for travellers is profound: any freshwater contact, including wading, swimming, showering with untreated river or lake water, or even washing in water drawn from endemic sources, constitutes a potential exposure. The cercariae are microscopic and invisible to the naked eye. A beautiful freshwater lake in Uganda or a slow-moving river in Mali may be highly contaminated despite appearing pristine.
High-Risk Destinations and Activities
Lake Malawi is among the best-known examples for UK travellers: a popular and beautiful destination where schistosomiasis transmission is well documented in the tourist literature. Studies of travellers to Lake Malawi have found schistosomiasis infection rates of 40 percent or more in those who swam in the lake without prophylaxis. Other well-documented high-risk freshwater bodies for UK travellers include Lake Tanganyika, the River Nile and associated canals, the Zambezi River and its tributaries, Lake Victoria, and numerous other bodies of water across sub-Saharan Africa.
Activities associated with the highest risk include swimming and wading in endemic lakes and rivers, water sports including kayaking, white-water rafting and sailing on endemic freshwater bodies, and washing with unfiltered freshwater from endemic sources.
Saltwater (the sea) is not a transmission route for schistosomiasis: the freshwater snail intermediate host does not survive in salt water. Adequately chlorinated swimming pools are safe. The risk is specific to freshwater bodies in endemic regions.
Clinical Presentation: Acute and Chronic Disease
Swimmer’s itch (cercarial dermatitis): Within hours of cercarial penetration, many travellers develop a pruritic (itchy) maculopapular rash at the site of skin penetration. This is usually self-limiting within days and does not confirm schistosomiasis infection (similar reactions can occur with other cercariae that do not cause human disease).
Katayama syndrome (acute schistosomiasis): Four to eight weeks after primary infection, some travellers develop a serum sickness-like illness with fever, headache, myalgia, fatigue, cough, urticaria and eosinophilia. This represents an immune response to migrating schistosomula (larval worms). Katayama syndrome is particularly common in travellers and those not previously exposed, as it represents an inflammatory response to first-ever infection that does not occur in populations with lifelong exposure and partial immunity.
Chronic schistosomiasis: Untreated infection can persist for years (schistosomes are long-lived) and cause progressive organ damage: haematobium infection causes haematuria (blood in urine), bladder fibrosis, and an increased risk of bladder cancer; mansoni infection causes hepatic fibrosis, portal hypertension and splenomegaly over years.
| Testing and Treatment After Freshwater Exposure in Endemic AreasTravellers who have had any freshwater exposure in endemic areas should inform their travel health clinician or GP on return. Testing for schistosomiasis involves serology (antibody testing) which becomes reliably detectable approximately 6 to 8 weeks after exposure, and urine or stool microscopy for eggs. Testing before this window may produce a false negative result. Treatment with praziquantel (a single oral dose, usually given as 40 mg per kg) is highly effective and well tolerated. Early treatment before heavy egg-laying begins reduces the risk of organ damage. Travellers should not wait for symptoms to develop before requesting testing after a known exposure. |
Prevention: The Message That Must Be Clear
There is no vaccine against schistosomiasis and no reliable prophylactic medication. Prevention is based entirely on avoiding contact with freshwater in endemic regions. Specifically:
- Do not swim, wade or allow skin contact with freshwater lakes, rivers, canals or streams in endemic regions.
- Shower and dry the skin briskly after any accidental exposure (drying the skin before cercariae fully penetrate has limited but some protective effect).
- Boiled or bottled water for drinking and cooking is safe.
- Seawater and adequately chlorinated pool water are safe.
This message should be delivered clearly and specifically at the travel health consultation. Travellers who have not been told about schistosomiasis risk in sub-Saharan Africa, or who have been told only about malaria and food hygiene, are at risk of freshwater exposure they would have avoided with accurate information.
This article is for general information and does not constitute individual clinical advice. For travel health advice including schistosomiasis risk assessment for your specific destination, please book a consultation at Priory Pharmacy in York.
Frequently Asked Questions
Schistosomiasis is a parasitic infection acquired through contact with contaminated freshwater. The parasites can enter the body through the skin while swimming, paddling or bathing in affected lakes and rivers.
Yes. Lake Malawi is a well-known risk area for schistosomiasis. Travellers are generally advised to avoid freshwater activities in areas where the parasite may be present.
If you have had freshwater exposure in a risk area, testing is often recommended even if you have no symptoms. Blood tests are typically most reliable several weeks after exposure.