Schistosomiasis

Schistosomiasis (shis-to-so-MY-a-sis), also known as bilharzia (bil-HAR-zee-a), is a disease caused by parasitic worms that affects more than 200 million people worldwide.

Schistosomiasis is caused by parasitic^* worms called blood flukes of the genus Schistosoma. Three species within the genus (Schistosoma mansoni, S. japonicum, and S. haematobium) are responsible for most infections. Blood flukes live in water in tropical areas of the world and are common in Africa, South America (Brazil, Venezuela, and Surinam), parts of the Caribbean (Saint Lucia, Antigua, Montserrat, Martinique, Guadeloupe, the Dominican Republic, and Puerto Rico), some Middle Eastern countries, parts of China, the Philippines, and Southeast Asia. Contact with contaminated water through bathing, swimming, or wading is the most common method of infection.

What Is the Life Cycle of Schistosoma?

Freshwater rivers, lakes, and streams become contaminated when people who have schistosomiasis urinate or defecate in the water, leaving behind

blood fluke eggs. After the eggs hatch, the Schistosoma larvae^* attack a certain species of snail (if the snail is not present, the larvae will die). After the snail is infected, the larvae grow and develop in the snail. When the larvae leave the snail, if they come in contact with a human within 48 hours they burrow into the skin and enter blood vessels. The parasites then grow in the bloodstream, where they produce eggs. Some of the eggs travel to the liver. Others enter the intestine or the urinary bladder, where they pass out of the body through urine or stool. If the eggs reach a freshwater supply, the cycle begins again.

What Are the Signs and Symptoms of Schistosomiasis?

Initially, skin may be itchy and a rash may appear where the Schistosoma burrow into the skin. As the worms develop in the liver, fever, chills, cough, and muscle aches may develop. There may be liver enlargement or malfunction, diarrhea, abdominal pain, and vomiting. The kidneys may be also affected. In rare cases, eggs can reach the brain or spinal cord and cause seizures. Even without treatment, most cases do not result in permanent damage to organs, though there may be significant long-term health effects.

Sometimes, however, the infection scars the liver so much that blood flow through the liver is partially blocked. This causes a condition called portal hypertension (POR-tal HY-per-ten-shun), which may cause sometimes fatal bleeding from swollen veins in the stomach and esophagus^*.

How Is Schistosomiasis Diagnosed and Treated?

Schistosomiasis is diagnosed by identifying eggs in stool or urine samples. Repeated samples may be required to identify the parasite. Blood tests may be used to identify the need to search for eggs but are not usually enough to make decisions about treatment. Treatment depends on the severity of the infection. A drug called praziquantel (pra-zi-KWAN-tel) is used to treat the infection.

How Is Schistosomiasis Prevented?

In many parts of the world, there is no way of knowing whether water is contaminated with blood fluke larvae. It is best to avoid any contact with fresh water in areas where Schistosoma are known to occur. Swimming in ocean water and chlorinated pools is generally considered safe.

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