Fact sheet: Schistosomiasis
Schistosomiasis, also known as bilharziasis, is a parasitic disease that leads to chronic ill health.
The main forms of human schistosomiasis are caused by five species of flatworms, or blood flukes, known as schistosomes. Intestinal schistosomiasis, caused by the species Schistosoma mansoni, is prevalent in 54 countries and territories of Africa, the Caribbean, the Eastern Mediterranean and South America. Another form, caused by Schistosoma intercalatum is found in 10 African countries. The Asian forms of intestinal schistosomiasis are caused by Schistosoma japonicum and Schistosoma mekongi, and are found in a number of Asian countries. Urinary schistosomiasis, caused by Schistosoma haematobium, affects 53 countries in Africa and the Eastern Mediterranean.
Disease due to schistosomiasis is indicated either by the presence of blood in the urine (urinary schistosomiasis) leading eventually to bladder cancer or kidney problems or, in the case of intestinal schistosomiasis, by initial atypical symptoms such as intermittent (bloody) diarrhoea, which can lead to serious complications of the liver and spleen. Sufferers are seriously weakened by the disease. It therefore has serious consequences on the socio-economic development of tropical and subtropical regions.
People are infected by contact with infested water during their normal daily activities for personal or domestic purposes, such as hygiene and recreation (swimming), or in professional activities such as fishing, rice cultivation, irrigation, etc.
Due to lack of information or insufficient attention to hygiene, infected individuals contaminate their environment. The eggs of the schistosomes in the excreta of an infected person hatch on contact with water and release larvae, the miracidia. In order to survive, the tiny larvae must find a fresh water snail. Once it has found its snail host, the miracidium divides itself in this host in several cycles, to eventually produce thousands of new parasites (cercariae). The cercariae are then excreted by the snail into the surrounding water. They can penetrate an individual's skin within a few seconds, continuing their biological cycle once they have made their way to the victim's blood stream. Within 30 to 45 days, the parasite is transformed into a long worm - either male or female. The female lays from 200 to 2000 eggs per day over an average of 5 years, according to the species.
In the case of intestinal schistosomiasis, the worms reside in the blood vessels lining the intestine. In urinary schistosomiasis, they live in the blood vessels of the bladder. Only about half of the eggs are excreted in the faeces (intestinal schistosomiasis), or in the urine (urinary schistosomiasis). The rest are trapped in the body tissues, damaging other vital organs. It is the eggs - and not the worm - which cause damage to the intestines, the bladder and other organs.