Bilharzia, or “snail fever,” is a disease caused by a parasitic worm. The worm, or fluke, has several different species. It affects the intestines and the urinary system preferentially, but because it lives in the blood vessels, it can harm other systems in the body too.

The World Health Organization (WHO) describe bilharzia — now mostly known as schistosomiasis — as both an acute and chronic disease. Symptoms appear as the body reacts to the parasite’s presence, but complications can persist long-term.

The disease can affect different parts of the body, including the lungs, the nervous system, and the brain. The area of damage will depend on the species of parasite.

Bilharzia is not usually fatal immediately, but it is a chronic illness that can seriously damage the internal organs. In children, it can lead to reduced growth and problems with cognitive development.

Some types of bilharzia can affect birds and mammals, such as water buffalo.

Schistosomiasis or bilharziaShare on Pinterest
Schistosomiasis or bilharzia is passed on by a parasite that is present in fresh water in some places.

According to the WHO, the infection starts when a person comes into direct contact with fresh water where certain types of water snail carry the worm.

The parasites enter the body when a person is swimming, washing, or paddling in contaminated water. They can also become infected by drinking the water or eating food that a person has washed in untreated water.

The infective form of the fluke is known as cercariae. The cercariae emerge from the snails, pass through a person’s skin when they are in the water, and develop into adult worms that live in the individual’s blood.

Depending on the type of worm, bilharzia can affect:

  • the intestines
  • the urinary system, increasing the risk of bladder cancer
  • the liver
  • the spleen
  • the lungs
  • the spinal cord
  • the brain

The infection cycle of the parasite begins when the worm’s eggs enter fresh water through the feces and urine of humans who already have the infection.

The eggs hatch in the water, releasing tiny larvae, and the larvae reproduce inside the water snails.

After they have infected water snails, the worm’s cercariae, are released. The cercariae can survive for up to 48 hours.

The cercariae can penetrate human skin and enter the bloodstream. There, they travel through the blood vessels of the lungs and liver, and then to the veins around the bowel and bladder.

After some weeks, the worms are mature. They mate and start producing eggs. These eggs pass through the walls of the bladder, the intestine, or both. Eventually, they leave the body through urine or feces. At this point, the cycle starts again.

A person with schistosomiasis cannot pass it on to another person. Humans only become infected through contaminated water where the snails are living.

More than 200 million people have bilharzia worldwide, according to the Centers for Disease Control and Prevention (CDC), although the parasite is not present in the United States.

Places where the parasite occurs include:

  • Africa, including Egypt and the Nile Valley
  • South America and parts of the Caribbean
  • Southeast Asia
  • Yemen, in the Middle East

Bilharzia can affect people of any age in an affected area, but those who are most at risk include:

  • children
  • people who swim, work, or have other contacts with freshwater rivers, canals, lakes, and streams

Bilharzia does not occur in the U.S., but people have developed the rash known as swimmer’s itch, or cercarial dermatitis, after exposure to a related species of schistosomes, the parasite that causes bilharzia.

Health authorities have investigated outbreaks of cercarial dermatitis in Stubblefield Lake in northern New Mexico, and one in Prospect Lake in the heart of Colorado Springs, Colorado.

Americans are at risk of infection if they travel to areas where the disease exists. Anyone who is visiting these regions should check with a doctor about any precautions they may need to take.

The impact of an infection depends on the type of worm and the stage of infection.

Symptoms occur when the body reacts to the worm’s eggs.

Acute stage

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A fever and a rash are symptoms of bilharzia.

Symptoms can take between 14 and 84 days to appear, according to the CDC.

Approximately 3 to 8 weeks after infection, the person may experience:

Chronic stage

Many people do not show symptoms at the early stage, but they may develop symptoms as the disease progresses. These later symptoms again depend on the type of parasite.

If the parasites affect the liver or intestines, symptoms may include:

If the parasites affect the urinary system, there may also be:

  • blood in the urine
  • painful urination
  • higher risk of bladder cancer

Over time, anemia can develop. In rare cases, the parasite may affect the central nervous system.

Children may have stunted growth and a reduced capability to learn, according to the WHO.

If a person has symptoms, or they think they may have had contact with contaminated water, they should see a doctor. The doctor may refer them to an infectious disease or tropical medicine specialist.

They should be ready to tell the doctor:

  • where they have traveled
  • how long they were there
  • whether they were in contact with contaminated water
  • any symptoms and when these first appeared
  • whether they have had an itchy rash or blood in the urine

A stool or urine sample will indicate whether any eggs are present. The doctor may request a blood test.

The worm takes about 40 days to mature. A blood sample may not show reliable results until at least 6 to 8 weeks after exposure.

If there are intestinal symptoms, the person may need a biopsy of the rectum, even if urine and blood tests are negative. They may also have a bladder biopsy.

It may be a good idea for a person to have a checkup 3 months after returning home, even if they have no symptoms, because the symptoms may not show up until later.

No vaccine is available for schistosomiasis, but treatment can help reduce the impact of infection.

If a person’s test result is positive, a short course of a medication called praziquantel is usually effective as long as the individual has not experienced significant damage or complications.

Praziquantel can help, even at an advanced stage, but it does not prevent re-infection.

People who live in a high-risk area can take a single oral dose of praziquantel to reduce the chance of infection and complications. People may need this treatment yearly for several years.

Researchers are working to develop a vaccine that will stop the parasite’s lifecycle from continuing in humans.

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Tourists should take care about coming into contact with fresh water in some parts of Asia, Africa, and Central and South America.

The CDC advise people to avoid all contact with fresh water in areas where contamination is likely.

People should take care when:

  • swimming unless in seawater or a chlorinated pool
  • drinking water
  • eating food washed in water
  • bathing

Treating water with iodine does not kill parasites. Anyone who is living or traveling where the disease is present should drink only bottled water or boil the water for at least 1 minute beforehand.

Any contact with contaminated water, however slight, can lead to infection. For this reason, people should also boil and then cool their bathing water before using it. You can store the water safely for 1 to 2 days before using it to wash.

Ways that authorities can reduce the chance of people becoming infected in high-risk areas include:

  • Reducing the levels of infection: Providing drug treatment to the population can help achieve this.
  • Snail control: This may involve the use of chemicals and redesigning or clearing irrigation schemes to make it harder for snails to proliferate. Another option is to introduce predators, such as crayfish.

Anyone who travels to or spends time in an area where bilharzia is prevalent should seek medical advice if symptoms appear or if they think they may have been exposed to fresh water or the parasite.


If I get swimmer’s itch when I go to the lake, does this mean I have bilharzia?


No, it is not the same condition.

Bilharzia, also known as schistosomiasis, is an infection that happens when a person has contact with a specific type of fluke from the Schistosoma family. It can lead to long-term consequences in the urinary, gastrointestinal, or hepatobiliary tract.

By contrast, swimmer’s itch, or cercarial dermatitis, is caused by other species of fluke. These only cause a local itching rash that goes away without treatment. A topical antihistamine can help treat the reaction.

Daniel Murrell, MD Answers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.
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