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Schistosomiasis

Schistosomiasis (also called bilharzia) is endemic in 74 developing countries, infecting more than 200 million people in rural and peri-urban areas. Of these, 120 million have symptoms of the disease of whom 20 million have severe consequences. In many areas, schistosomiasis infects a large proportion of children under 14. An estimated 650 million people worldwide live in endemic areas.

Characteristics

The major forms of human schistosomiasis are caused by five species of water-borne flatworm, or blood flukes, called schistosomes:

  • Intestinal schistosomiasis caused by Schistosoma mansoni occurs in Africa, the Eastern Mediterranean, the Caribbean and South America.
  • Oriental or Asiatic intestinal schistosomiasis, caused by the S. japonicum group of parasites (including S. mekongi in the Mekong river basin), is endemic in South-East Asia and in the Western Pacific region.
  • Another form of intestinal schistosomiasis caused by S. intercalatum has been reported from central African countries.
  • Urinary schistosomiasis, caused by S. haematobium, is endemic in Africa and the Eastern Mediterranean.

Schistosomes enter the body through contact with infested surface water. This particularly affects people engaged in agriculture and fishing. But rural-urban migration is introducing the disease into peri-urban areas in northeast Brazil and Africa, and refugee movements are spreading it to other areas. More tourists are contracting schistosomiasis with the rise in eco-tourism and travel off the beaten track, at times with severe acute infection and unusual problems including paralysis of the legs.

The economic and health effects of schistosomiasis are considerable. School performance and growth patterns of infected children are impeded, although the effects are usually reversible with treatment. In some situations schistosomiasis may affect people's ability to work.

Diagnosis

To diagnose urinary schistosomiasis, a filtration technique using filter paper or polycarbonate or nylon filters is used. Children with S. haematobium nearly always have microscopic or visual blood in their urine (haematuria). Children needing treatment can be also be identified by looking at urine specimens or checking for microscopic blood with chemical reagent strips. In addition, asking children about a history of blood in urine can be used to identify communities at high risk of infection. The eggs of intestinal schistosomiasis can be detected in faecal specimens through a technique using cellophane soaked in glycerine, or between glass slides.

Treatment

Praziquantel is the only available and effective treatment against all forms of schistosomiasis with few, and only transient, side effects. The cost of a single 600-mg tablets is about US$ 0.08 and of an average treatment is about US$ 0.20–0.30. Even though re-infection may occur after treatment, the risk of developing severely diseased organs is diminished and even reversed in young children. In most areas, a reduction in the overall number of cases is maintained for 18-24 months and in other areas for up to five years without further intervention.

Control strategy

The strategy for schistosomiasis control aims to reduce disease through treatment with praziquantel, which is the only available drug. Praziquantel has been used successfully over the past 20 years to control schistosomiasis in Brazil, Cambodia, China, Egypt, Morocco and Saudi Arabia. Treatment at least three times during childhood is likely to prevent disease in adulthood.

The recent focus and consensus on neglected tropical diseases has highlighted that some diseases can be treated on a large scale with safe and effective drugs, and at regular intervals. Such treatment is done without individual diagnosis, once the endemic area is defined and the overlap of diseases determined.

Recommended intervention strategy and aim

Targeted distribution of praziquantel is the norm. Intervention frequency is determined by the prevalence of infection or of visible haematuria (for urinary schistosomiasis only) among school-age children. The aim is morbidity control: periodic treatment of at-risk populations will cure subtle morbidity and prevent infected individuals from developing severe, late-stage morbidity due to schistosomiasis.

Eligible population

  • School-age children.
  • Adults considered to be at risk, from special groups (pregnant and lactating women; groups with occupations involving contact with infested water, such as fishermen, farmers, irrigation workers, or women in their domestic tasks), to entire communities living in endemic areas.

Sources: US Department of Health; The World Health Organization


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