|MicrobiologyBytes: Microbiology Notes: Schistosomiasis||Updated: January 28, 2007||Search|
World Health Organization
Division of Control of Tropical Diseases
The main forms of human schistosomiasis are caused by five species of the flatworm, or blood flukes, known as schistosomes:
People are infected by contact with water used in normal daily activities such as personal or domestic hygiene and swimming, or by professional activities such as fishing, rice cultivation, irrigation.
Due to lack of information or insufficient attention to hygiene, infected individuals may contaminate their water supply with faeces or urine. The eggs of the schistosomes (above) in the excreta of an infected person open on contact with water and release a parasite, the miracidium. To survive, this motile form must find a fresh water snail:
Once it has found its snail host, the miracidium divides, producing thousands of new parasites (cercariae). The cercariae are then excreted by the snail into the surrounding water. They can penetrate an individual's skin within a few seconds, continuing their biological cycle once they have made their way to the victim's blood vessels. Within 30 to 45 days, the parasite is transformed into a long worm which is either male or female. The female lays from 200 to 2000 eggs per day over an average of 5 years, according to the species.
In the case of intestinal schistosomiasis, the worms reside in the blood vessels lining the intestine. In urinary schistosomiasis, they live in the blood vessels of the bladder. Only about a half of the eggs are excreted in the faeces (intestinal schistosomiasis), or in the urine (urinary schistosomiasis). The rest stay in the body, damaging other vital organs. It is the eggs and not the worm itself which cause damage to the intestines, the bladder and other organs.
Schistosomiasis is endemic in 74 tropical developing countries. Some 600 million people are at risk of becoming infected. It is estimated that 200 million people are already infected. Extreme poverty, the unawareness of the risks, the inadequacy or total lack of public health facilities plus the unsanitary conditions in which millions of people lead their daily lives are all factors contributing to the risk of infection.
Schistosomiasis mainly affects adults workers in rural areas, employed either in agriculture or the freshwater fishing sector. In many areas, a high proportion of children between the ages of 10 and 14 are infected. Urinary schistosomiasis affects 66 million children throughout 54 countries. In some villages around Lake Volta in Ghana, over 90% of the children are infected by the disease.
As with other tropical diseases, population movements and refugees in unstable regions contribute to the transmission of schistosomiasis. Rapid urbanization has been accompanied by new foci of transmission. The increase in "off-track" tourism has led to increasingly serious infections with previously unencountered sequelae, including paralysis of the legs.
The large fresh water reservoirs associated with dams such as Akosombo Dam in Ghana, the Kainji Dam in Nigeria and the Kariba Dam in Zimbabwe as well as smaller reservoirs in the Sahel and irrigation systems throughout Africa are major transmission foci and thus endemic areas for schistosomiasis.
Although the majority of people in endemic areas have only light infections or no symptoms, the impact of schistosomiasis on economic conditions and the general health situation should not be underestimated. In the north-east of Brazil, in Egypt and the Sudan, the work capacity of rural workers has been estimated to be seriously undermined. The disease also substantially affects children's growth and school performance. However, medical treatment is rapidly followed by improvement.
There is an association between urinary schistosomiasis and a form of cancer of the bladder in some regions. This link is mainly recorded among the active section of the population, most of whom are farmers. In Egypt, schistosomiasis linked with cancer is the primary cause of death among men aged between 20 and 44 years. In the industrialized countries, cancer of the bladder without schistosomiasis is usually prevalent among workers aged around 65. In some regions of Africa where Schistosoma haematobium is prevalent, the incidence of cancer of the bladder linked to schistosomiasis is 32 times higher than the incidence of cancer of the bladder in the USA.
Schistosomiasis control is far more effective when placed in the context of a general health system. The integration process is slow, but this "horizontal" approach is now becoming an integral part of health care at village level. Schistosomiasis prevention and control measures should be implemented before dam construction work begins. Control approaches for each form of schistosomiasis varies and must be adapted to the epidemiological situation, available financial resources, and the particular local culture. This strategy has produced excellent results; in some regions it has met the planned objectives within 2 years. It is nevertheless essential to plan surveillance and maintenance over periods of 10 to 20 years.
Health education on schistosomiasis has greater importance than ever before. The introduction into schools of diagnosis and treatment has made children and parents much more aware of the problem connected with disease. Schoolteachers and local health workers are effective in explaining the role played by people in the transmission of schistosomiasis. Campaigns in the Egyptian mass media have proved particularly successful in increasing awareness of the need for diagnosis and treatment.
The supply of safe drinking water is fundamental to schistosomiasis control. The beneficial results of chemotherapy - normally quite spectacular - are even more marked in communities with adequate water supplies. The high prevalence of schistosomiasis is clearly a reliable criterion to select communities for installing a clean water supply.
Modern diagnostic techniques are simple, easy to apply and cost very little. Although reinfection may occur after treatment, the risk of serious disease developing in the body organs has been greatly reduced, and it has been observed that there is a marked regression of lesions in young children following treatment of the infection. In the majority of localities where treatment is provided, the total number of cases is reduced within 18 to 24 months. In other localities, according to the local situation, the prevalence has been substantially reduced, and it is encouraging to note that no further intervention is required for intervals between 2 and 5 years.
To be effective, schistosomiasis control strategies should be adapted to the local epidemiological situation and caution must be taken when destroying freshwater snails using chemicals - particularly in terms of impact on the environment.
In 1983, the World Health Organization, in association with the health ministries of several endemic countries (Botswana, Egypt, Madagascar, Mauritius and Zanzibar), launched a massive programme to assess control methods. The findings of this programme are as follows:
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