Schistosomiasis

Schistosomiasis, also known as Bilharzia, is a parasitic disease caused by flatworms and is widely found throughout the tropics. Following malaria, it is the most common parasitic disease in the world. The world health organization estimates that over 200 million people world wide are infected with the parasite and that of these, 120 million have clinical disease and approximately 20 million have severe disease.

Schistosomiasis is caused by five species of flatworms in the genus Schistosoma. The natural life cycle of the schistomoma parasite consist of a sexual stage in the human host and an asexual stage in a fresh water snail host. Schistosomes in contaminated water can penetrate the skin of individuals who are using the water to wade, bathe, swim, or clean. Once inside the human host, the schistosomes reach the blood vessels and travel to the portal veins where they mature and begin to produce eggs.

Clinical symptoms of schistosomiasis are caused by the immune response to the parasite's eggs rather than to the worms themselves. While approximately half of all eggs migrate through the bowel or bladder wall and are excreted in the feces or urine, half remain in the body and generate a granulomatous immune response. Early symptoms, within one to two months of infection, are non-specific and may include fatigue, muscle soreness, rash, itchy skin, fever, and chills. Eggs that are not shed return to the portal circulation and can ultimately reach the lungs, heart, central nervous system eyes, and muscle. Severe schistosomiasis is clinically variable depending on the location of egg deposition and may include pulmonary fibrosis, splenomegaly, central nervous system disease, and increased risk of bladder and liver cancer. While most schistosomiasis infections cause chronic disease, acute severe disease can have high levels of morbidity.

Schistosomiasis may be diagnosed through the identification of eggs in the feces or urine. Blood serology tests and tissue biopsy of bowel, bladder, liver, or muscle may also aid in diagnosis. Various imaging modalities including ultrasound and CT scan may be useful in identifying chronic disease.

Treatment of schistosomiasis aims to cure disease, decrease morbidity, and reduce transmission in endemic areas. Praziquantel is the drug of choice for all species of schistosomiasis. Treatment for a single day will cure approximately 85 percent of infected individuals and significantly decrease the number of eggs in those who are not completely cured. Future public health interventions aimed at decreasing the overall burden of disease in endemic areas include the development of an effective vaccine and control of the fresh water snail host.