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Author: Marie Hartley, Staff Writer, 2009.
Schistosomiasis (also known as bilharzia) is a disease caused by infection with parasitic worms called schistosomes. Humans are the primary hosts of 3 major species of schistosomes:
Humans usually become infected by schistosomes via contact with freshwater lakes and streams in endemic areas:
Schistosomiasis is an extremely common illness in endemic areas. It is estimated that more than 200 million people worldwide are infected. Although the majority of infected people are symptom-free, schistosomiasis causes more than 200,000 deaths per year worldwide.
Other schistosomes can also cause disease in humans (swimmer's itch – see below).
Schistosomiasis is passed on by infected people urinating or defaecating into freshwater. Schistosoma eggs hatch in the water and develop inside particular snail species. These snails release thousands of larvae back into the water. Larvae can penetrate unbroken human skin when humans enter the water to bathe or swim. Within a few weeks, worms grow inside blood vessels and produce eggs. Some of these eggs migrate to the bladder or bowel and are released in the urine or faeces. Other eggs become trapped in body tissues, producing an immune reaction.
Schistosomiasis can cause acute and chronic complications.
Worldwide chronic schistosomiasis is far more common than the acute illness. Chronic symptoms arise months to years after exposure. Chronic features are due to an intense immune response against the schistosomal eggs, with granuloma (ball-like collection of immune cells) formation and scarring.
There is no vaccine currently available for schistosomiasis.
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