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Schistosomiasis ˌshis-tə-sō-ˈmī-ə-səs, ˌskis-


By Richard D. Pearson, MD, Emeritus Professor of Medicine, University of Virginia School of Medicine

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.

Swimmer's itch

Certain Schistosoma species normally infect birds and mammals rather than people. However, sometimes cercariae of these species penetrate the skin of people. Because these schistosomes do not normally reside and develop in people, they cannot travel from the skin to other organs and mature. Thus, they affect only the skin, causing intense itching (sometimes called swimmer's itch).

Symptoms of Schistosomiasis

When schistosomes first penetrate the skin, an itchy rash may develop at the site of penetration. About 2 to 4 weeks later (when the adult flukes begin laying eggs), some people develop fever, chills, cough, muscle aches, fatigue, vague discomfort (malaise), nausea, and abdominal pain. Lymph nodes may temporarily enlarge, then return to normal. This group of symptoms is called Katayama fever.

If the infection lasts a long time, the body has an inflammatory response to the eggs, which causes other symptoms and scarring. Symptoms depend on the organs affected:

  • If blood vessels of the intestine are chronically infected: Abdominal discomfort, pain, and bleeding (seen in the stool), which may result in anemia

  • If the liver is affected and pressure in the portal vein is high: An enlarged liver and spleen or vomiting large amounts of blood

  • If the bladder is chronically infected: Painful, frequent urination, bloody urine, and an increased risk of bladder cancer

  • If the urinary tract is chronically infected: Inflammation and eventual scarring that can block the tube from the kidney to the bladder (ureters), sometimes causing urine to back up and damage kidneys

  • If the brain or spinal cord is chronically infected (rare): Seizures, muscle weakness, or paralysis

  • If the genitals are infected (in men and women): Infertility

Diagnosis of Schistosomiasis

  • Examination of samples of stool, urine, or sometimes tissue from the intestine or bladder

  • Sometimes blood tests

A doctor suspects schistosomiasis if travelers and immigrants from areas where schistosomiasis is common report typical symptoms and they have swum or waded in fresh water.

A doctor can confirm the diagnosis of schistosomiasis by examining samples of stool or urine for eggs. Usually, several samples are needed. If no eggs are found in the stool or urine, the doctor sometimes takes a sample of tissue from the intestine or bladder to be examined under a microscope for eggs. Eggs cannot be seen in stool or urine early in the infection—that is, soon after the parasites penetrate the skin or during Katayama fever.

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, how long the person has had it, or whether live adult worms are present.

Ultrasonography is frequently used to assess the severity of schistosomiasis in the urinary tract or liver. Alternatively, computed tomography (CT) or magnetic resonance imaging (MRI) may be done.

Prevention of Schistosomiasis

Schistosomiasis is best prevented by

  • Avoiding swimming, bathing, or wading in fresh water in areas known to contain schistosomes

  • Using latrines or toilets for urination and defecation

Freshwater used for bathing should be boiled for at least 1 minute and then cooled before bathing. However, water that has been held in a storage tank for at least 1 to 2 days should be safe without boiling.

People who are accidentally exposed to possibly contaminated water (for example, by falling into a river) should vigorously dry off with a towel to attempt to remove any parasites before they penetrate the skin.

Treatment of Schistosomiasis

  • Praziquantel (an antiparasitic drug)

For schistosomiasis treatment, 2 or 3 doses of praziquantel are taken by mouth over 1 day.

If the number of eggs being excreted has not greatly decreased after 3 months, people are treated again. They are checked again in another 3 months and treated again if needed. If symptoms of Katayama fever are severe, corticosteroids may help.

People with swimmer's itch do not need to take drugs to kill the schistosomes. If needed, cool compresses and/or corticosteroid creams or ointments may be used to help relieve the intense itching.

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