What is Schistosomiasis?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 1940s and 1950s 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. High Risk GroupsSchistosomiasis is widespread among the poor populations in less developed countries, who live in conditions that favour transmission and who have no access to proper health care or effective prevention measures. The occurrence is particularly linked to agricultural and water development schemes, plus the African Lakes and rivers. Infection is predominant in school age children, in special occupation groups (fishermen, irrigation workers, farmers) and in females and other groups using infested water for their domestic purposes. Children swimming are at particularly high risk, because of their prolonged and complete body exposure.
Health Consequences of SchistosomiasisLight infections with schistosomiasis can be asymptomatic, and many people may live their lives without knowing they have ever been infected. However, globally, up to 120 million of the estimated 200 million infected people are believed to be symptomatic, and as many as 20 million may well be suffering severe consequences of their infection. The annual deaths associated with schistosomiasis are estimated at 20,000. The first obvious symptom of Schistosoma haematobium infection is blood in the urine (haematuria). Early signs of morbidity common to S.haematobium and S.mansoni, and which manifest in school age children are anaemia, impaired growth, impaired development, poor cognition, and substandard school performance. Since S.mansoni eggs are voided through the faeces, intestinal symptoms such as diarrhoea with or without blood are found in heavy infections. However none of these signs and symptoms are due solely to schistosomiasis, and so a diagnosis on clinical presentation is difficult. Due to the non-specific signs and symptoms, many S.mansoni affected persons may never realise they have the infection, and thus do not seek treatment. Equally, many health decision makers do not perceive schistosomiasis as a serious health problem, which means the disease can progress untreated to the development of late, irreversible sequelae. The late and life threatening consequences of schistosomiasis
include bladder cancer or serious kidney malfunction, caused by S.haematobium,
and severe complications of the liver and spleen in the case of intestinal
schistosomiasis. Mortality is rarely acknowledged to be caused by
schistosomiasis, because there is no recognition of the link between
schistosomiasis infection in early life and later development of bladder cancer
or renal failure associated with urinary schistosomiasis and liver fibrosis and
portal hypertension associated with intestinal schistosomiasis. |
