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|Eggs from the parasite can cause liver damage, which can result in swelling of the abdomen, as seen here in this young boy in Uganda.
Source: Andrea Peterson/USAID
Schistosomiasis, also known as bilharzia, is a chronic disease caused by parasitic worms that live in certain types of freshwater snails. People who come into contact with water that contains these snails are at risk of infection. Schistosomiasis is considered second only to malaria as the most devastating parasitic disease in tropical countries. In sub-Saharan Africa, more than 200,000 deaths per year are due to schistosomiasis. Depending on the species of parasite, schistosomiasis causes renal and bladder dysfunction or liver and intestinal disease, and it contributes to anemia and growth retardation in children.
Individuals become infected with schistosomiasis through contact with water contaminated with schistosome parasites while bathing, swimming, or performing daily chores, such as washing laundry, fetching water, and herding animals. Thus, patterns of sanitation, water supply, and human water use are crucial elements in determining the risk of infection.
The geographic distribution of the various species of schistosomes is dependent on the distribution of the species of their intermediate freshwater snail hosts (see map below). Schistosomiasis can be found in 74 tropical countries in Africa, the Caribbean, South America, East Asia, and the Middle East, with 62 percent of the burden occurring in 10 countries in Africa. Worldwide, more than 700 million people are at risk of infection and more than 207 million people are infected.
Schistosomiasis infection is usually acquired in childhood when children tend to spend time swimming or bathing in water containing the larval form of the parasite. Prevalence and intensity of infection increase with age, peaking in the 5 to 14 year age group. Children also suffer the most side effects of the disease, especially poor growth and impaired cognitive development. The disease also contributes to malnutrition and disrupts school attendance. In older people, there is a drastic decline in intensity of infection but not in the prevalence of the disease.
|Credit: WHO Map Library|
For more information visit the CDC schistosomiasis epidemiology page.
The three main species infecting humans are Schistosoma haematobium, S. japonicum, and S. mansoni. Two other species, which are more localized geographically, are S. mekongi and S. intercalatum.
Steps 1–3: When people infected with schistosomiasis parasites urinate or defecate in freshwater, parasite eggs pass from the body. Once in freshwater, the eggs hatch and infect freshwater snails that serve as an intermediate host. The parasites develop and multiply inside the snails.
Steps 4–5: During its larval stage, the parasite emerges from infected snails back into freshwater, where they can survive for about 48 hours.
Step 6: Free-swimming larva penetrate a person’s skin.
Steps 7–10: Once in the body, the larvae develop into adult male and female parasites, which can live, mate, and multiply in blood vessels for as long as 7 years. Female parasites release thousands of eggs, some of which are passed out in the urine, in the case of urinary schistosomiasis, or feces in the case of intestinal schistosomiasis. Some eggs remain trapped in body tissues.
Most people have no symptoms when they are first infected. However, a person who becomes infected with schistosomiasis parasites may develop a rash or itchy skin within days of becoming infected. Within 1 to 2 months of infection, flu-like symptoms may develop.
Symptoms of chronic schistosomiasis infection are caused by the body’s reaction to the parasites’ eggs, which become lodged in the intestine or bladder, causing inflammation or scarring. In children, infection can cause anemia, malnutrition, and learning difficulties.
With urinary schistosomiasis, the parasites’ eggs damage the bladder and kidneys, which causes painful urination, blood in the urine, and abdominal pain. Intestinal schistosomiasis damages the intestines and liver, resulting in abdominal pain, fever, and bleeding. Damage to the liver can produce swelling of the abdomen, which is a classic sign of infection. Symptoms of chronic schistosomiasis include abdominal pain, enlarged liver, blood in the stool or in the urine, and problems passing urine; it also can increase the risk of bladder cancer.
In women, urogenital schistosomiasis may cause 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.
In rare cases, eggs are found in the brain or spinal cord and can cause seizures, paralysis, or spinal cord inflammation.
|In Nasarawa North in Nigeria, 12-year-old Dauda Usman holds a sample of his urine, which is red with blood, a sign of schistosomiasis.
Carter Center Photo: Emily Staub
The gold standard for schistosomiasis diagnostic is the examination of stool and urine specimens by microscopy to detect the presence of parasite eggs.
Urinary schistosomiasis also can be detected based on the presence of blood in the urine. Children with S. haematobium almost always have microscopic blood in their urine that can be detected by chemical reagent strips. Asking children about a history of blood in their urine can be used to identify communities at high risk of infection (refer to photo to the left), assisting in the mapping of priority areas for intervention.
Serologic testing for antibody is indicated for diagnosis of travelers or immigrants from endemic areas who have not been treated appropriately for schistosomiasis in the past.
The major intervention used to control the disease is treatment with praziquantel, accompanied by the provision of safe water, adequate sanitation, and, where possible, snail control. According to the World Health Organization (WHO), only 8 percent of people with schistosomiasis had access to praziquantel in 2008. WHO has developed guidelines for community treatment of schistosomiasis with praziquantel. Although schistosomiasis is not yet eradicable, the disease can be prevented and transmission controlled with a single annual dose of praziquantel. Additional control measures, including improved water and sanitation as well as reduction or elimination of intermediate host snails, could sustain and/or enhance control of transmission in endemic areas.