Schistosomiasis

SCHISTOSOMIASIS Definition Schistosomiasis, also known as bilharziasis or snail fever, is a primarily tropical parasitic disease caused by the larvae of one or more of five types of flatworms or blood flukes known as schistosomes. The name bilharziasis comes from Theodor Bilharz, a German pathologist, who identified the worms in 1851. Description Infections associated with worms present some of the most universal health problems in the world. In fact, only malaria accounts for more diseases than schistosomiasis. The World Health Organization (WHO) estimates that 200 million people are infected and 120 million display symptoms. Another 600 million people are at risk of infection. Schistosomes are prevalent in rural and outlying city areas of 74 countries in Africa, Asia, and Latin America. In Central China and Egypt, the disease poses a major health risk. There are five species of schistosomes that are prevalent in different areas of the world and produce somewhat different symptoms: •

Schistosoma mansoni is widespread in Africa, the Eastern-Mediterranean, the Caribbean, and South America and can only infect humans and rodents.

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S. mekongi is prevalent only in the Mekong river basin in Asia.

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S. japonicum is limited to China and the Philippines and can infect other mammals, in addition to humans, such as pigs, dogs, and water buffalos. As a result, it can be harder to control disease caused by this species.

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S. intercalatum is found in central Africa.

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S. haematobium occurs predominantly in Africa and the Eastern Mediterranean.

Intestinal schistosomiasis, caused by Schistosoma japonicum, S. mekongi, mansoni, and S. intercalatum, can lead to serious complications of the liver and spleen. Urinary schistosomiasis is caused by S. haematobium. It is difficult to know how many individuals die of schistomiasis each year because death certificates and patient records seldom identify schistosomiasis as the primary cause of death. Mortality estimates vary related to the type of schistosome infection but is generally low, for example, 2.4 of 100,000 die each year from infection with S. mansoni. Infection occurs when your skin comes in contact with contaminated freshwater in which certain types of snails that carry schistosomes are living. Freshwater becomes contaminated by Schistosoma eggs when infected people urinate or defecate in the water. The eggs hatch, and if certain types of snails are present in the water, the parasites grow and develop inside the snails. The parasite leaves the snail and enters the water where it can survive for about 48 hours. Schistosoma parasites can penetrate the skin of persons who are wading, swimming, bathing, or washing in contaminated water. Within several weeks, worms grow inside the blood vessels of the body and produce eggs. Some of these eggs travel to the bladder or intestines and are passed into the urine or stool. Within days after becoming infected, you may develop a rash or itchy skin. Fever, chills, cough, and muscle aches can begin within 1-2 months of infection. Most people have no symptoms at this early phase of infection.

Eggs travel to the liver or pass into the intestine or bladder, causing inflammation or scarring. Children who are repeatedly infected can develop anemia, malnutrition, and learning difficulties. After years of infection, the parasite can also damage the liver, intestines, lungs, and bladder. Rarely, eggs are found in the brain or spinal cord and can cause seizures, paralysis, or spinal cord inflammation. Symptoms of schistosomiasis are caused by the body's reaction to the eggs produced by worms, not by the worms themselves.

http://www.cdc.gov/ncidod/dpd/parasites/schistosomiasis/factsht_schistosomiasis.htm