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Schistosomiasis

(Bilharziasis)

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 schistosomiasis by swimming or bathing in fresh water that is contaminated with the flukes.

  • 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 of schistosomiasis 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 218 million people in tropical and subtropical regions of South America, Africa, and Asia.

Only five Schistosoma species develop into adult flukes in people. These five species cause most of the cases of schistosomiasis in people:

  • Schistosoma hematobium infects the urinary tract (including the bladder). This species is widely distributed over the African continent and occurs in some parts of the Middle East, Turkey, and India.

  • 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 (in parts of South America and the Caribbean). Schistosoma japonicum and Schistosoma mekongi occur in Asia and Southeast Asia. Schistosoma intercalatum occurs in Central and West Africa.

None of these five Schistosoma species cause schistosomiasis in people who live in Canada or the United States, including Puerto Rico, where schistosomiasis used to be endemic.

Schistosomiasis is acquired by swimming, wading, or bathing in fresh water that is contaminated with the free-swimming stage of the parasite.

An infected person passes schistosome eggs in stool or urine. In water, the eggs release immature larvae (called miracidia), which enter specific types of water-dwelling snails, multiply, and mature into a form called cercariae, which can swim. The cercariae 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, releasing immature larvae, which enter snails to begin the cycle again.

Life Cycle of Schistosoma

  • 1. In people, the schistosome eggs are passed 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 cercaria) 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.

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. 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.

Adult schistosomes live 3 to 7 years. Females are about 1/4 to 3/4 inch long. Males are slightly smaller.

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 into adult flukes. Thus, they affect only the skin, causing intense itching (sometimes called swimmer's itch). Some of these Schistosoma species, unlike the five species that cause schistosomiasis, are present in the United States and Canada.

Symptoms

Most people have no symptoms. But when schistosomes first penetrate the skin, an itchy rash may develop at the site of penetration.

After about 2 to 4 weeks but as long as 12 weeks later (when the adult flukes begin laying eggs), some people develop acute schistosomiasis with 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 (called chronic schistosomiasis), 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 (portal hypertension): An enlarged liver and spleen

  • 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 (ureter), sometimes causing urine to back up and damage the kidney

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

  • If the lungs are chronically infected: High blood pressure in the arteries of the lungs and shortness of breath, light-headedness, and chest pain due to a type of heart failure called cor pulmonale

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

Diagnosis

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

  • Sometimes blood tests

A doctor suspects schistosomiasis in people who have traveled to, immigrated from, or live in areas where schistosomiasis occurs if they 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 but symptoms and circumstances suggest schistosomiasis, 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 acute schistosomiasis (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.

After schistosomiasis is diagnosed, 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

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

  • Using chemicals that kill snails (molluscicides) in bodies of fresh water known to contain schistosomes

Fresh water 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.

Using molluscicides in bodies of fresh water that contain schistosomes can be effective in preventing schistosomiasis, but it can also be difficult to do and expensive, and raises environmental concerns.

Treatment

  • Praziquantel (an antiparasitic drug)

For schistosomiasis treatment, 2 or 3 doses of praziquantel are taken by mouth over 1 day, depending on the Schistosoma species that is causing the infection. If the stool or urine initially contained living eggs, doctors may check samples again in 1 to 2 months to determine whether treatment was successful. If living eggs are still present, treatment with praziquantel is repeated.

Praziquantel effectively kills adult schistosomes, but not the immature forms, which are present early in the infection. Thus, for travelers, treatment with praziquantel is delayed for 6 to 8 weeks after people were last exposed to the parasite—to give the immature forms time to become adults.

If symptoms of acute schistosomiasis (Katayama fever) are severe, corticosteroids may help.

People with swimmer's itch do not need to take drugs to kill the schistosomes. Cool compresses, baking soda, anti-itch lotions, and/or corticosteroid creams or ointments may be used to help relieve intense itching.

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