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Definition: bilharzia (schistosomiasis) from Environmental History and Global Change: A Dictionary of Environmental History

A disease caused by parasitic flatworms or flukes which use humans as a primary host and snails as a secondary one. Irrigation ditches are especially good habitats for the snails. Around 1 billion people are at risk and c.300 million infected, especially in Africa, parts of S America, the Caribbean, Middle East and Asia. The mortality rate is low but it is a source of chronic illness. There is currently no vaccine (Farley 1991).


Summary Article: schistosomiasis
from The Columbia Encyclopedia

(shĭs'tӘsōmī'Әsĭs), bilharziasis (bĭl´´härzī'Әsĭs), or snail fever, parasitic disease caused by blood flukes, trematode worms of the genus Schistosoma. Three species are human parasites: S. mansoni, S. japonicum, and S. haematobium. The disease is prevalent in Asia, some Pacific islands, Africa, the West Indies, South America, Spain, Puerto Rico, and Cyprus. The larvae of the parasite are harbored by snails, which serve as intermediate hosts, and infect humans who bathe in or otherwise come in contact with infested waters. The larvae enter through the skin, migrate via the blood vessels, and mature in the lungs. From there they travel to the veins of the upper or lower intestine or bladder and reproduce. Some eggs pass out in the feces. Others are carried into the liver, where the body surrounds them with white blood cells, forming hundreds of tiny ball-like granulomas that eventually impair the liver's ability to function. It is believed that the flukes settle in blood vessels that have a particular human immune substance, tumor necrosis factor, that they require in order to reproduce.

The disease is characterized by a skin eruption at the site of entry, fever, diarrhea, and other symptoms, depending on the tissues affected; cirrhosis of the liver is common. The disease can be cured with the drug praziquantel, but reinfection can occur. Although symptoms vary according to the species of infecting fluke, all forms can result in general weakening and eventual death. Control of the disease is difficult, but control of the snail populations that serve as intermediate hosts is effective in reducing the incidence of the disease. Proper sanitation and disposal of human wastes are also important.

The Columbia Encyclopedia, © Columbia University Press 2018

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