Schistosomiasis affects 230 million each year
Schistosomiasis is a chronic, parasitic disease caused by blood flukes (trematode worms) of the genus Schistosoma. At least 230 million people require treatment every year. Schistosomiasis transmission has been documented in 77 countries. However those at most risk of infection are in 52 countries.
People become infected when larval forms of the parasite – released by freshwater snails – penetrate their skin during contact with infested water.
In the body, the larvae develop into adult schistosomes. Adult worms live in the blood vessels where the females release eggs. Some of the eggs are passed out of the body in the faeces or urine to continue the parasite life-cycle. Others become trapped in body tissues, causing an immune reaction and progressive damage to organs.
Schistosomiais is prevalent in tropical and sub-tropical areas, especially in poor communities without access to safe drinking water and adequate sanitation. It is estimated that at least 90% of those requiring treatment for schistosomiasis live in Africa.
There are two major forms of schistosomiasis – intestinal and urogenital – caused by five main species of blood fluke,
Schistosomiasis particularly affects agricultural and fishing populations. Women doing domestic chores in infested water, such as washing clothes, are also at risk. Hygiene and play habits make children especially vulnerable to infection.
In north-east Brazil and Africa, refugee movements and migration to urban areas are introducing the disease to new areas. Increasing population size and the corresponding needs for power and water, often result in development schemes and environmental modifications that also lead to increased transmission.
With the rise in eco-tourism and travel “off the beaten track”, increasing numbers of tourists are contracting schistosomiasis. At times, tourists present with severe acute infection and unusual problems including paralysis.
Urogenital schistosomiasis is also considered to be a risk factor for HIV infection, especially in women.
Symptoms of schistosomiasis are caused by the body's reaction to the worms’ eggs, not by the worms themselves.
Intestinal schistosomiasis can result in abdominal pain, diarrhoea, and blood in the stool. Liver enlargement is common in advanced cases, and is frequently associated with an accumulation of fluid in the peritoneal cavity and hypertension of the abdominal blood vessels. In such cases there may also be enlargement of the spleen.
The classic sign of urogenital schistosomiasis is haematuria (blood in urine). Fibrosis of the bladder and ureter, and kidney damage are common findings in advanced cases. Bladder cancer is also a possible late-stage complication. In women, urogenital schistosomiasis may present with genital lesions, vaginal bleeding, pain during sexual intercourse and nodules in the vulva. In men, urogenital schistosomiasis can induce pathology of the seminal vesicles, prostate and other organs. This disease may also have other long-term irreversible consequences, including infertility.
The economic and health effects of schistosomiasis are considerable. In children, schistosomiasis can cause anaemia, stunting and a reduced ability to learn, although the effects are usually reversible with treatment. Chronic schistosomiasis may affect people’s ability to work and in some cases can result in death. In sub-Saharan Africa, more than 200 000 deaths per year are due to schistosomiasis.
Schistosomiasis is diagnosed through the detection of parasite eggs in stool or urine specimens.
For urogenital schistosomiasis, a filtration technique using nylon, paper or polycarbonate filters is the standard. Children with S. haematobium almost always have microscopic blood in their urine and this can be detected by chemical reagent strips. Asking children about a history of blood in their urine can also be used to identify communities at high risk of infection, therefore assisting in mapping priority areas for intervention.
The eggs of intestinal schistosomiasis can be detected in faecal specimens through a technique using methylene blue-stained cellophane soaked in glycerine or glass slides.
For people from non-endemic or low transmission areas, serological and immunological techniques may be useful in the detection of infection.
Prevention and control
Prevention and control of schistosomiasis is based on preventive treatment, snail control, improved sanitation and health education.
The WHO strategy for schistosomiasis control focuses on reducing disease through periodic, targeted treatment with praziquantel. This involves regular treatment of all people in at-risk groups. Treatment should be complemented with health education, as well as access to safe water and good sanitation.
Groups targeted for treatment are:
school-aged children in endemic areas;
adults considered to be at risk in endemic areas, people with occupations involving contact with infested water – such as fishermen, farmers, irrigation workers – and women whose domestic tasks bring them into contact with infested water;
entire communities living in highly endemic areas.
The frequency of treatment is determined by the prevalence of infection or visible haematuria (in the case of urogenital schistosomiasis) in school-age children. In high transmission areas, treatment may have to be repeated every year for several years.
The aim is to reduce disease: periodic treatment of at-risk populations will cure mild symptoms and prevent infected people from developing severe, late-stage chronic disease. However, a major limitation to schistosomiasis control has been access to praziquantel. Available data show that less than 14% of people requiring treatment are reached.
Praziquantel is the only available treatment against all forms of schistosomiasis. It is effective, safe and low-cost. Even though re-infection may occur after treatment, the risk of developing severe disease is diminished and even reversed when treatment is initiated in childhood.
Praziquantel has been used successfully over the past 20 years to control schistosomiasis in Brazil, Cambodia, China, Egypt, Morocco and Saudi Arabia. Subsequently, some countries have succeeded in interrupting schistosomiasis transmission.
WHO’s work on schistosomiasis is part of an integrated approach to the control of neglected tropical diseases. Although medically diverse, neglected tropical diseases share features that allow them to persist in conditions of poverty, where they cluster and frequently overlap.
WHO coordinates the strategy of preventive chemotherapy in consultation with collaborating centres and partners from academic and research institutions, the private sector, nongovernmental organizations, international agencies and other United Nations organizations. WHO also develops technical guidelines and tools for use by national control programmes.
Working with partners and the private sector, WHO has advocated for increased access to praziquantel and resources for implementation. These efforts enabled at least 28 countries to implement preventive chemotherapy for schistosomiasis in 2010. The number of people treated for schistosomiasis increased from 12.4 million in 2006 to 33.5 million in 2010.
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