Several species of parasitic blood flukes (trematodes), of which the most important are Schistosoma mansoni, S. japonicum, S. mekongi and S. haematobium.
Infection occurs in fresh water containing larval forms (cercariae) of schistosomes, which develop in snails. The free-swimming larvae penetrate the skin of individuals swimming or wading in water. Snails become infected as a result of excretion of eggs in human urine or faeces.
Nature of the disease
Chronic conditions can develop when adult flukes live for many years in the veins (mesenteric or vesical) of the host where they produce eggs, which cause damage to the organs in which they are deposited. The symptoms depend on the main target organs affected by the different species, with S. mansoni, S. mekongi and S. japonicum causing hepatic and intestinal signs and S. haematobium causing urinary dysfunction. Advanced intestinal schistosomiasis may result in hepatosplenomegaly, liver fibrosis and portal hypertension. Severe disease from uro-genital schistosomiasis may include hydronephrosis and calcification of the bladder. The larvae of some schistosomes of birds and other animals may penetrate human skin and cause a self-limiting dermatitis, “swimmer’s itch”. These larvae are unable to develop in humans.
S. mansoni occurs in many countries of sub-Saharan Africa, in the Arabian peninsula, and in the Bolivarian Republic of Venezuela, Brazil and Suriname; transmission has also been reported from several Caribbean islands. S. japonicum is found in China, in parts of Indonesia and in the Philippines. S. haematobium is present in sub-Saharan Africa and in eastern Mediterranean areas. S. mekongi is found along the Mekong River in northern Cambodia and in the south of the Lao People’s Democratic Republic. (Map)
Risk for travellers
In countries or areas at risk while swimming or wading in fresh water.
Avoid direct contact (swimming or wading) with potentially contaminated fresh water in countries or areas at risk. In case of accidental exposure, dry the skin vigorously to reduce penetration by cercariae. Avoid drinking, washing or washing clothing in water that may contain cercariae. Water can be treated to remove or inactivate cercariae by paper filtering or use of iodine or chlorine.