Schistosomiasis (bilharziasis) is infection caused by flatworms (flukes), called schistosomes.
Schistosomiasis affects over 200 million people in tropical and subtropical regions of South America, Africa, and Asia. Five Schistosoma species cause most of the cases of schistosomiasis in people:
Schistosomiasis is acquired by swimming, wading, or bathing in fresh water that is contaminated with the free-swimming stage of the parasite. Schistosomes multiply inside specific types of water-dwelling snails, from which they are released to swim free in the water. If they encounter a person's skin, they burrow in and move through the bloodstream to the lungs, where they mature into adult flukes. The adults pass through the bloodstream to their final home in small veins in the bladder or intestine, where they may remain for years. The adult flukes lay large numbers of eggs in the walls of the intestine or bladder. The eggs cause local tissue damage and inflammation, which results in ulcers, bleeding, and scar tissue formation. Some eggs pass into the stool (feces) or urine. If urine or stool of infected people enters fresh water, the eggs hatch, and the parasite enters snails to begin the cycle again.
Schistosoma mansoni and Schistosoma japonicum typically lodge in small veins of the intestine. Some eggs flow from there through the bloodstream to the liver. The resulting liver inflammation can lead to scarring and increased pressure in the vein that carries blood between the intestinal tract and the liver (the portal vein). High blood pressure in the portal vein (portal hypertension) can cause enlargement of the spleen and bleeding from veins in the esophagus.
The eggs of Schistosoma hematobium typically lodge in the bladder, sometimes causing ulcers, bleeding into the urine, and scarring. Chronic Schistosoma hematobium infection increases the risk of bladder cancer.
All types of schistosomiasis can affect other organs (such as the lungs, spinal cord, and brain). Eggs that reach the lungs can result in inflammation and increased blood pressure in the arteries of the lungs (pulmonary hypertension).
Symptoms and Diagnosis
When schistosomes first penetrate the skin, an itchy rash (swimmer's itch) may develop. About 4 to 8 weeks later (when the adult flukes begin laying eggs), fever, chills, muscle aches, fatigue, vague discomfort (malaise), nausea, and abdominal pain may develop. Lymph nodes may temporarily enlarge, then return to normal. This group of late symptoms is called Katayama fever.
Other symptoms depend on the organs affected:
Travelers and immigrants from areas where schistosomiasis is common should be asked whether they have swum or waded in fresh water. A doctor can confirm the diagnosis by examining samples of stool or urine for eggs. Usually, several samples are needed. Blood tests can be done to determine whether someone has been infected with Schistosoma mansoni or another species, but the tests do not indicate how severe the infection is or how long the person has had it. Occasionally, a doctor takes a sample of intestinal or bladder tissue to be examined under a microscope for eggs. Ultrasonography can be used to assess the severity of schistosomiasis in the urinary tract or liver.
Prevention and Treatment
Schistosomiasis is best prevented by avoiding swimming, bathing, or wading in fresh water in areas known to contain schistosomes. For treatment, 2 or 3 doses of praziquantel are taken by mouth over 1 day.
Last full review/revision March 2007 by Richard D. Pearson, MD