Schistosomiasis
Schistosomiasis, bilharzia or snail fever is a disease affecting many people in developing countries. Although it has a low mortality rate, schistosomiasis is very debilitating. An estimated 200 million people have the disease, 120 million symptomatic. A few countries have eradicated the disease, and many more are working towards it. The World Health Organization is working towards this goal.
There are 5 species of flatworms that cause schistosomiasis. Each causes a different clinical presentation of the disease
- Schistosoma mansoni and S. intercalatum cause intestinal schistosomiasis
- Schistosoma haematobium causes urinary schistosomiasis
- Schistosoma japonicum and S. mekongi cause Asian intestinal schistosomiasis
The eggs of the above schistosomes open upon contact with water, releasing a miracidium parasite, which then infects a snail. After infection, the miracidium divides into thousands of new parasites, known as cercariae, which are the infective larvae. The cercariae, capable of penetrating a human being's skin, are excreted by the snail into the surrounding water. Once the infective larvae penetrate the host organism's skin and enter its blood vessels, it is 30 to 45 days before a long worm, which is capable of laying between 200 and 3000 eggs per day over the course of its 5 year lifetime, appears. S. japonicum lays about 3000 eggs daily and S. mansoni lays 350 eggs.
The disease is found in tropical countries in Africa, Caribbean, eastern South America, east Asia and in Arabia.
Controlled urbanization has reduced exposure sites, with a subsequent decrease in new infections.
External links
- World Health Organization Partners for Parasite Control website (http://www.who.int/wormcontrol/en/)
- World Health Organization fact sheet on the disease (http://www.who.int/wormcontrol/documents/fact_sheets/schistosomiasis/en/)