China has outbreak of snail fever (schistosomiasis), a potentially lethal parasitic worm

Main Category: Public Health
Article Date: 14 Jun 2004 - 16:00 PDT

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It is believed that over 1m people in China are afflicted by snail fever, a potentially lethal parasitic worm which is carried by freshwater snails. The worm attacks the blood and liver of people. As the snail is quite prevalent in China, some experts are saying that over 65 million people are at risk of being infected.

The state media has reported that China has vowed to take stronger measures to stop the spread of the parasitic worm.

Snail fever, or schistosomiasis, is being targeted by research laboratories in China. Scientists plan to identify sources of infection and develop prevention strategies.

Last year 843,000 people were officially infected with schistosomiasis. The areas along the Yangtze River, which often get flooded, seem to be where many of the people get infected. The area is ideal for the snail to live and breed.

Twenty years ago the problem was brought under control by the Chinese authorities. Health officials at the time thought they had permanently stopped the chances of any large-scale outbreaks ever happening again. Unfortunately, as a result of natural disasters and man-made bungles, the fever to make a comeback.

Huge flooding along the Yangtze River six years ago has helped the disease spread into new areas. As China's healthcare system has been scaled down over the last twenty years the disease has been creeping back. Human ignorance has also played a part.

About 200 million people each year are infected with schistosomiasis worldwide.

WHAT IS SCHISTOSOMIASIS?

From: Department of Infectious Disease Epidemiology Imperial College London

Schistosomiasis (also known as Bilharzia) is the disease caused by a blood born fluke (trematode) of the genus Schistosoma. The intermediate hosts of all digenetic trematodes are snails, and schistosomes are no exception. Adult schistosome worms live in a mammalian host, and these adult worms were first discovered in an Egyptian patient in 1851 during an autopsy carried out by Dr. Theodore Maximilian Bilharz. The snail link was not discovered until after the turn into the 20th century.

Schistosomiasis is the second most prevalent tropical disease in Africa after malaria and is of great public health and socio-economic importance in the developing world. There are five major species of schistosomiasis which infect man. One is found in Africa and in South America, two are confined to Africa, and the other two are found only in the Far East in China and the Philippines.

Schistosoma mansoni - causative agent of intestinal bilharzia - originated in Africa but was carried to South America, with the slave trade, where, because a suitable snail host existed, it became established, particularly in Brazil and the Caribbean. It is transmitted by snails of the genus Biomphalaria snails (link), aquatic snails that thrive in irrigation canals, and along lake shores.

Schistosoma haematobium - which causes urinary bilharzia - is transmitted by snails of the species Bulinus, which inhabit less permanent water bodies, because during their life cycle they prefer a period of aestivation (hibernation) in mud, during a dry season.

The third major species of schistosome is S. japonicum, used to be widespread in Japan, China and the Far East and was the cause of widespread and gross morbidity and mortality. It affects not only man but also domestic and wild animals. However due to effective control measures carried out in Japan during the 1940's and 1950's this parasite has been eradicated in Japan.

Snail control and socio-economic development in China, has reduced the prevalence in most areas, and this species is now found only in isolated foci in China and some islands in the Philippines. S .japonicum is transmitted by an amphibious snail (species Oncomelania) which makes snail control relatively easy.

The are two 'minor' species of schistosomiais, First, S. intercalatum which is confined to West Africa, and lives in the mesenteric vessels of man causing abdominal pain and bloody diarrhoea. The second is S.mekongi, which is another form of intestinal schistosoma is found predominantly in Southeast Asia. The main reservoir for this species is dogs.