Schistosomiasis (also known as bilharzia or snail fever) is a chronic disease caused by parasitic worms that live in certain types of freshwater snails. Schistosomiasis is second only to malaria as the most devastating parasitic disease in tropical countries and is considered one of the most debilitating helminthic diseases among rural populations.
In sub-Saharan Africa, the disease accounts for more than 200,000 deaths a year. Both prevalence and intensity of infection increase with age, peaking in the five to 14 year age group. In children the disease contributes to stunted growth, impaired cognitive development, malnutrition, anemia and disrupts school attendance. Depending on the species of parasite, long-term infection can damage either the bladder and kidneys, or the liver and intestines, and can hinder people’s ability to work and contribute to society.
Worldwide, there are more than 230 million people at risk of schistosomiasis infection. The estimated 166 million cases in sub-Saharan Africa represent 90% of the world’s cases. In Africa two species that cause the most morbidity in humans are Schistosoma haematobium (urinary schistosomiasis) and Schistosoma mansoni (intestinal schistosomiasis).
Did you know?
Schistosomiasis is a disease of poverty that leads to chronic ill-health and is common in both tropical and sub-tropical climates where there is inadequate sanitation. A common sign of severe infection is blood in either the urine or in feces. Because infection intensity peaks in adolescents, the bloody urine that is a sign of severe infection is often misinterpreted as a sign of adulthood in communities where the disease is endemic.
Transmission cycle and symptoms:
When a person infected with the schistosomiasis parasite urinates or defecates in or near freshwater, parasite eggs pass from the body into the water. Once in the water the eggs hatch and infect the intermediate host, a freshwater snail. In the snail, the parasite develops and reemerges into the water as free-swimming larvae and can penetrate a person’s skin. Inside the human body, the larvae develop into adult worms which live and multiply in blood vessels for as long as seven years.
Female worms release thousands of eggs, some of which pass through in the urine (urinary schistosomiasis) or in feces (intestinal schistosomiasis), and others remain trapped in body tissue leading to long-term morbidity. The severity of infection depends on the amount of worms present and how long the person has been infected. Symptoms of urinary schistosomiasis infection, due to the damage eggs cause to the bladder and kidneys, include abdominal pain, painful urination, and blood in the urine. Symptoms of intestinal schistosomiasis, due to the damage caused by eggs to the intestine and liver, include abdominal pain, fever and blood in the stool.
Depending on the parasite, schistosomiasis infection can lead to chronic and life-threatening diseases or cancers of the bladder, kidney, liver or intestines. Schistosomiasis causes the highest mortality among the NTDs.
Diagnosis and treatment:
Schistosomiasis is diagnosed through detection of parasite eggs in either urine or stool samples. Children with urinary schistosomiasis almost always have blood in their urine that can either be detected by the naked eye if severely infected, or by the use of chemical reagent strips if only a microscopic amount of blood is present.
Once diagnosed, the fast, safe and effective treatment for schistosomiasis is a drug called praziquantel. Ideally, the provision of safe water and sanitation would accompany treatment with praziquantel. The disease can be prevented and transmission controlled by a single annual dose of praziquantel. Praziquantel donation programs are being stepped up. For instances where praziquantel donations are not yet available, the price per tablet has significantly decreased and efforts are being made to assist countries to purchase the necessary praziquantel for their control efforts. The WHO goal for eliminating schistosomiasis is 2020.