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Schistosomiasis +shis-tu-sO-!mI-u-sus


by Richard D. Pearson, MD

Schistosomiasis is infection caused by flatworms (flukes), called schistosomes.

  • People acquire the infection by swimming or bathing in fresh water that is contaminated with the flatworms.

  • The infection may cause an itchy rash, then after several weeks, fever, chills, muscle aches, fatigue, nausea, abdominal pain, and, later, other symptoms depending on which organ is affected.

  • Doctors confirm the diagnosis by identifying eggs in a sample of stool or urine.

  • The infection is treated with praziquantel.

Schistosomiasis is the most common type of fluke infection. It 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:

  • Schistosoma hematobium infects the urinary tract (including the bladder).

  • Schistosoma mansoni , Schistosoma japonicum , Schistosoma mekongi , and Schistosoma intercalatum infect the intestine and liver. Schistosoma mansoni is widespread in Africa and is the only schistosome in the Western Hemisphere. Schistosoma japonicum and Schistosoma mekongi occur in Asia and Southeast Asia.

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 liver, where they mature into adult flukes. The adults travel to their final home in small veins in the bladder or intestine (depending on the species), 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 may result 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.

Life Cycle of Schistosoma

  • 1. In people, the schistosome eggs are eliminated in stool or urine into water.

  • 2. In water, the eggs hatch and release immature schistosome larvae (called miracidia).

  • 3. The miracidia swim and enter a snail.

  • 4–5. Within the snail, the miracidia develop into sporocysts and then into a form (called cercariae) that has a forked tail and can swim in water. The cercariae are released from the snail into the water and penetrate the skin of people who enter the water.

  • 6. When cercariae penetrate the skin, they lose their tail and become schistosomula. The schistosomula then travel to the liver, where they mature into adults.

  • 7. Male and female worms pair up and migrate to veins in the intestine or bladder (depending on their species). There, where they remain, and the females begin to lay eggs.

The eggs of Schistosoma hematobium typically lodge in the bladder, sometimes causing ulcers, bleeding into the urine, and scarring. 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), which can result in a type of heart failure called cor pulmonale.

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