bilharzia

Disease that causes anaemia, inflammation, formation of scar tissue, dysentery, enlargement of the spleen and liver, cancer of the bladder, and cirrhosis of the liver. It is contracted by bathing in water contaminated with human sewage. Some 200 million people are thought to suffer from this disease in the tropics, and 750,000 people a year die.

Freshwater snails act as host to the first larval stage of blood flukes of the genus Schistosoma; when these larvae leave the snail in their second stage of development, they are able to pass through human skin, become sexually mature, and produce quantities of eggs, which pass to the intestine or bladder. Numerous eggs are excreted from the body in urine or faeces to continue the cycle. Treatment is by means of drugs, usually containing antimony, to kill the parasites.

Researchers have attempted to control the snail host by introducing Louisiana crayfish into ponds, canals, and cattle tanks around three Kenyan villages where the disease is endemic. Crayfish accidentally released from a local farm had been observed to feed voraciously on the snails. The results reported in 1999 suggest that only under certain environmental conditions the introduction of the crayfish leads to a significant reduction in the spread of Schistosoma. As of 2006, several vaccine candidates had been tested in clinical trials, but none of them provided complete protection.