The major forms of human schistosomiasis are caused by five species of water-borne flatworm, or blood flukes, called schistosomes:
· Intestinal schistosomiasis caused by Schistosoma mansoni occurs in 53 countries in Africa, the Eastern Mediterranean, the Caribbean and South America.
· Oriental or Asiatic intestinal schistosomiasis, caused by the S. japonicum group of parasites (including S. mekongi in the Mekong river basin), is endemic in seven countries in South-East Asia and in the Western Pacific region.
· Another form of intestinal schistosomiasis caused by S. intercalatum has been reported from 10 central African countries.
· Urinary schistosomiasis, caused by S. haematobium, is endemic in 54 countries in Africa and the Eastern Mediterranean.
Schistosomes enter the body through contact with infested surface water, mainly among people engaged in agriculture and fishing. But rural-urban migration is introducing the disease into peri-urban areas in northeast Brazil and Africa, and refugee movements are spreading it in Somalia and Cambodia. More tourists are contracting schistosomiasis with the rise in "off-track" tourism, at times with severe acute infection and unusual sequelae including paralysis of the legs.
The WHO International Agency for Research on Cancer has confirmed that in countries where urinary schistosomiasis is endemic, it causes a specific type of bladder cancer. In some areas of Africa the incidence of bladder cancer linked with schistosomiasis is 32 times higher than that of simple bladder cancer in the USA.
The economic and health effects of schistosomiasis should not be underestimated. School performance and growth patterns of infected children are retarded, although the effects are 90% reversible on average with treatment. In Egypt, Sudan and northeast Brazil, the work capacity of rural inhabitants is severely reduced due to weakness and lethargy caused by the disease.