Schistosomiasis

Schistosomiasis is a disease that is caused by parasites (genus Schistosoma) that enter humans by attaching to the skin, penetrating it, and then migrating through the venous system to the portal veins where the parasites produce eggs and eventually, the symptoms of acute or chronic disease (for example, fever, abdominal discomfort, blood in stools). This disease is also known as bilharziasis, bilharzia, bilharziosis, and snail fever or, in the acute form, Katayama fever. Theodore Bilharz identified the parasite Schistosoma hematobium in Egypt in 1851. Schistosomiasis is the second most prevalent tropical disease in the world; malaria is the first. The disease is found mainly in developing countries in Africa, Asia, South America, the Middle East, and the Caribbean. About 207 million people in at least 74 countries are estimated to have the disease. In the U.S., it is diagnosed in tourists who have visited these developing countries and in visitors from these countries, or from lab accidents. The type of snail that is part of the parasite's life cycle (see below) is not endemic to U.S. freshwater sources, so the disease is not endemic in the U.S. Acute schistosomiasis may reach a death (mortality) rate of 25%, although most areas report lower rates.

Parasites of the genus Schistosoma (S. mansoni, S. mekongi, S. intercalatum, S. hematobium, and S. japonicum) cause the disease. The disease in humans is part of the complicated life cycle of the parasites that is illustrated in the figure below. Humans enter freshwater areas that contain snails that grow Schistosoma sporocysts that develop into free-swimming cercariae. The cercariae can attach to and penetrate the human skin, migrate to blood vessels, and through lung blood capillaries reach the portal blood or vesicular (bladder) blood systems. During this migration, the cercariae change and develop from schistosomula into male and female adult parasitic worms. The worms incorporate human proteins into their surface structures, so most humans produce little or no immune response to the parasites. After parasite mating occurs in the portal or vesicular blood system, egg production occurs. In contrast to the adult parasites, the parasite's eggs stimulate a strong immune response by most humans. Some eggs migrate through the bowel or bladder tissue and are shed in feces or urine, while other eggs are swept into the portal blood and lodge in other tissue sites. Eggs shed into urine or feces may reach maturity in freshwater and complete their life cycle by infecting susceptible snails. In addition, some adult worms may migrate to other organs (for example, eyes or liver). This life cycle is further complicated by S. japonicum species that may also infect domesticated and wild animals, which can then serve as another host system. S. hematobium is the species that usually infects the human bladder tissue, while the other species usually infect the bowel tissue.

The acute and chronic symptoms of schistosomiasis are thought to be mainly due to the egg migration through tissue and the human immune response to the eggs. Chronic symptoms are mainly due to eggs that are not shed from the body. Complications (for example, hepatomegaly or enlarged liver and bladder cancer) related to the disease are thought to occur due to long-term exposure to the highly antigenic eggs.

Source: MedicineNet.com
http://www.medicinenet.com/schistosomiasis/article.htm