Schistosomiasis
Schistosomiasis or bilharzia is caused by a helminth called Schistosoma. Five species are known to infect human: Schistosoma haematobium, S. mansoni and S. japonicum. There are a few other species that infect humans, but these are less widespread: S. mekongi and S. intercalatum.
Transmission
Vector
Snails are the intermediate host. Each species has its own snail species as a vector.
Life cycle
Transmission occurs when the larval cercaria infects a human by penetration through the skin. Cercariae live in fresh water. They do not survive in brackish or salty water.
Risk for traveller
Travellers are at risk when they swim, bath, wade or engage in waterbased activities such as rafting or snorkelling in contaminated fresh water. Migrants, visiting friends and relatives (VFR), expatriates and adventurous travellers are more at risk. Most travellers are infected in Africa.
Incubation period
The incubation period for acute schistosomiasis is between 2 and 8 weeks.
Symptoms
Occasionally, a rash known as "swimmer's itch" may develop a few hours to a few days after cercariae have penetrated the skin. This will disappear spontaneously.
Acute schistosomiasis, also known as Katayama fever, is a hypersensitivity reaction to maturing schistosomules and typically appears several weeks (usually 3 to 8 weeks) after the initial infection. It present with one or more of the following symptoms:
- fever
- eosinophilia (> 1000/µL, almost always)
- abdominal pain, diarrhoea, or vomiting
- muscle and joint pain
- urticaria
- severe dry cough or wheezing
- headache
- hepatosplenomegaly
Chronic schistosomiasis occurs as a result of the immunologic reaction of the host to the presence of the eggs. Eggs are secreted by the adult worm and migrate through the blood. S. mansoni and S. japoni eggs often migrate to the liver or intestine, where they can cause a granulomatous reaction which can result in constipation, bloody stools, diarrhoea, ulceration or portal hypertension.
S. haematobium eggs can lodge in the urogenital system and surrounding organs and can cause haematuria, proteinuria, genital lesions, ureteral strictures and has been associated with bladder cancer.
Sporadically ectopic locations of the worms or eggs in the central nervous system are possible, causing neurological symptoms.
Risk areas
- Africa: S. mansoni and S. haematobium occurs
- Middle East: S. haematobium
- Parts of Brazil, Suriname and Venezuela: S. mansoni
- Caribbean: Guadeloupe, Martinique, Saint Lucia: transmission of S. mansoni is probably interrupted, but this is not yet verified
- China, Indonesia, Philippines: S. japonicum
- Cambodia and Laos: S. mekongi
- Parts of Central and West Africa: S. intercalatum
Prevention
There are no vaccines or drugs for prevention of infection.
Travellers can prevent schistosomiasis by avoiding activities such as bathing, swimming, wading, or any direct contact with freshwater in endemic countries. If household water is sourced from freshwater that could contain cercariae, it should be left to stand for minimum 24 hours before use or treated through boiling, filtration, chlorination, or exposure to UV light.
Brief contact with fast-flowing water poses a lower risk than contact with moderately flowing or still water, but the possibility of transmission still exists.
The use of Praziquantel directly after potential exposure does not prevent infection, as it is only active against adult worms.
Any traveller who was potentially exposed to contaminated water, should consult an infectious disease specialist three months after exposure, or sooner in case of symptoms.
Additional information
- CDC yellow book: schistomiasis (yellow book 2024)
- Wikitropica: background information on schisomiasis for medical professionals
References
- PAHO: schistosomiasis, accessed 27/9/2024