Schistosomiasis
Schistosomiasis (bilharziasis) is infection caused by blood flatworms (flukes) that often produces symptoms in the intestine, liver, or urinary tract.
Schistosomiasis affects over 200 million people in tropical and subtropical regions of South America, Africa, and Asia. Three species cause most of the cases of schistosomiasis in people: Schistosoma hematobium infects the urinary tract and bladder, and Schistosoma mansoni and Schistosoma japonicum infect the intestine. The infection is acquired by swimming or bathing in fresh water that is contaminated with the parasites. 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 migrate through the bloodstream to the lungs, where the schistosomes mature into adult flukes. The adults pass through the bloodstream to their final home in small veins in the bladder or intestines, where they may remain for years. The adult flukes lay large numbers of eggs in the walls of the intestines or bladder, some of which flow through the bloodstream to the liver. These eggs elicit an inflammatory response that blocks veins in the intestines, bladder, and liver--resulting in ulcers, local bleeding, and scar tissue formation. Eggs produce enzymes that allow them to pass into the stool and urine. When people with the disease urinate or defecate in fresh water, these eggs are passed, and the cycle begins again.
The eggs of Schistosoma mansoni and Schistosoma japonicum typically lodge in the intestine and liver, whereas those of Schistosoma hematobium typically lodge in the bladder. The resulting inflammation can lead to scarring, and increased pressure in the veins that carry blood between the digestive tract and the liver (portal vessels). This high portal pressure can cause enlargement of the spleen and bleeding from veins in the esophagus. Other organs (such as the lungs, spinal cord, and brain) (see Section 6, Chapter 89) can also be involved.
Symptoms and Diagnosis
When schistosomes first penetrate the skin, an itchy rash (swimmer's itch) may develop. Fever, chills, body aches, headache, and cough may develop when the adult flukes begin laying eggs (4 to 8 weeks after entering the body). The liver, spleen, and lymph nodes may temporarily enlarge and then return to normal. A person may develop abdominal cramps and pass blood in the stool or urine. Blood loss may result in anemia. Chronic infection of the urinary tract can produce obstruction and is associated with later development of cancer of the bladder.
A doctor diagnoses schistosomiasis by examining samples of stool or urine for the presence of eggs. Blood tests are also available. Ultrasound scans can be used to assess the severity of infections in the urinary tract or liver.
Prevention and Treatment
Prevention of schistosomiasis is best achieved by avoiding swimming, bathing, or wading in fresh water in areas known to contain schistosomes. To treat the disease, a doctor prescribes praziquantel to be taken by mouth in 2 or 3 doses over 1 day.
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