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Schistosomiasis (Bilharzia) - Snail Fever

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Bilharzia, also known as snail fever, is caused by a parasitic worm that mainly affects the urinary system.

Medically reviewed by

Dr. N. Ashok Viswanath

Published At June 13, 2022
Reviewed AtMay 21, 2024

What Is Schistosomiasis?

WHO (World Health Organization) describes bilharzia mostly as schistosomiasis. It is both an acute and chronic disease. The symptoms appear when the body reacts to the parasite’s presence, but the complications persist for a long period of time. The disease affects different parts of our body (the lungs, the brain, and the nervous system), and the area of damage will depend on the parasite species. Schistosomiasis is not usually fatal, but it is a chronic disease that seriously damages the internal organs. Also, some types of bilharzia affect mammals and birds, such as water buffalo.

How Does Schistosomiasis Get Transmitted?

The infection starts when a person comes into contact with freshwater because certain types of water carry the worm. The parasites enter the body while washing, swimming, drinking, paddling, or eating food. The infective form of the worm is known as cercariae. As the infective form emerges from the snails, it passes into the person's skin through contaminated water and develops into adult worms in the individual’s blood. Depending on the type of worm, schistosomiasis affects different body parts.

  • Urinary system.

  • Intestines.

  • Liver.

  • Lungs.

  • Spleen.

  • Spinal cord.

  • Brain.

The infection cycle begins when the worm’s eggs enter the water through the urine and feces of humans who already have the infection. Over time, the eggs hatch in the water and release tiny larvae, which reproduce inside the snails. When the water snails get infected, the cercariae worms are released. These worms can survive for up to two days.

The cercariae worms enter the bloodstream by penetrating the human skin. Then, they travel through the lungs, liver, bowel, and bladder. After a few weeks, the worms mature, mate, and start producing eggs. These eggs pass through the bladder walls and the intestine and eventually leave the body through feces or urine. A person cannot pass schistosomiasis on to another person. Humans become infected only through contaminated water where the snails live.

What Is the Epidemiology of Schistosomiasis?

Schistosomiasis is a parasitic disease caused by a flatworm species belonging to the genus Schistosoma. It is most common in tropical and subtropical areas, particularly in communities with poor access to clean water and sanitation. The parasite is transmitted through contact with fresh water contaminated with parasite larvae.

How Common Is Schistosomiasis?

Schistosomiasis is one of the most prevalent parasitic infections worldwide, affecting over 250 million people in tropical and subtropical regions. Schistosomiasis occurs in places where these parasites survive.

The places where the parasite lives are,

  • Egypt.

  • Nile Valley.

  • South America.

  • The Caribbean.

  • Southeast Asia.

  • Middle East Yemen.

What Are the Symptoms of Schistosomiasis?

The impact of the disease will depend on the worm type and the stage of infection. Symptoms occur when the body starts to react to the worm’s eggs.

Acute Stage:

The symptoms start to appear after 14 to 84 days. Approximately after three to eight weeks of infection, the person may experience:

  • Fever.

  • Rash.

  • Headache.

  • Breathing difficulties.

  • Body aches.

Chronic Stage:

Many people do not show symptoms early, but they may develop signs and symptoms as the disease progresses. These later-stage symptoms also depend on the type of parasite involved. When the parasites affect the liver or intestines, the following symptoms occur.

They are,

  • Constipation.

  • Blood in the feces.

  • Diarrhea.

  • Liver fibrosis.

  • Intestinal ulcers.

  • Portal hypertension.

When the parasites affect the urinary system, blood may be in the urine; there is a high risk of bladder cancer, painful urination, and anemia that may also develop over time. In rare cases, the parasite affects the central nervous system. At the same time, children may have stunted growth and reduced capability to learn.

How Is Schistosomiasis Diagnosed?

When a person experiences symptoms or thinks they may have been in contact with contaminated water, they must visit a doctor, such as an infectious disease specialist or a tropical medicine specialist. The doctor will ask the following of the patient to come to a diagnosis.

They are,

  • Where has the patient traveled?

  • How long they had been there?

  • Were the patient in contact with contaminated water?

  • What were the symptoms when these first appeared?

  • Was there an itchy rash or blood in the urine?

The doctor may ask for the following tests to be taken:

  • Stool or Urine Sample - It indicates whether there are any eggs, and the worm takes about one and a half months to mature.

  • Blood Sample - It may not show reliable results until six to eight weeks after exposure.

  • Biopsy - A rectum biopsy is needed when there are intestinal symptoms, even when urine and blood tests are negative. They can also have a bladder biopsy.

A person returning home should have a checkup after three months, even when there are no symptoms, because the symptoms may appear later.

How Is Schistosomiasis Treated?

There is no vaccine for schistosomiasis, but the treatment can help reduce the infection. A short course of Praziquantel medication is usually effective when a person tests positive and if the individual has not experienced any significant damage or complications. Praziquantel also helps with advanced stages of schistosomiasis but does not prevent re-infection. So, people living in high-risk areas can take a single oral dose of Praziquantel to reduce the chance of infection and further complications. In addition, those people will need this treatment every year for several years.

How Can Schistosomiasis Be Prevented?

Avoid contact with fresh water in areas where there is more contamination. People should take care during,

  • Swimming unless in a chlorinated pool or seawater.

  • Eating food that is washed in contaminated water.

  • Drinking water.

  • Bathing.

Iodinated water does not kill parasites. People living or traveling to disease-preventive areas should drink only bottled or boiled water. Any slight contact with contaminated water can lead to infection; for this reason, people should bathe in boiled water. Store the water for two days before using it to wash.

Ways to reduce the chance of becoming infected in high-risk areas include:

  • Providing drug treatment to the population.

  • Snail Control: One option is redesigning the irrigation schemes to make it hard for snails to proliferate. Another is to introduce predators, like crayfish. People who travel to or spend time in an area where schistosomiasis is prevalent should seek medical advice when symptoms appear or if they think they might be exposed to freshwater or the parasite.


Schistosomiasis is a rare disease in developing nations but has been significantly ignored. It has limited healthcare access, and one particular affordable medicine has been used successfully to treat it. So, this disease has to be taken seriously and prioritized at local, national, and international levels. People should adopt preventive measures to prevent complications.

Frequently Asked Questions


What Is Schistosomiasis and How Is It Transmitted?

Schistosomiasis is a parasitic disease; about 200 million individuals are affected worldwide. The condition is also referred to as bilharzia and is mainly caused by parasitic worms in contaminated fresh water. Although the primary mode of transmission is from contaminated fresh water, the parasitic larvae may penetrate the skin of the individual’s skin who is bathing or swimming in that water. Once it has entered, it may stay in the human body for several weeks. Later it matures into adult forms and starts releasing eggs. Their lifecycle may continue once these eggs are passed out in urine and feces.


How to Know if I Have Schistosomiasis?

Schistosomiasis is a chronic parasitic disease. Once the person is infected with schistosomiasis, the following symptoms may occur within a few days:
- Rash.
- Fever.
- Headache.
- Breathing difficulties.
Prolonged infections damage the intestine, liver, bladder, and lungs.


Where Is Schistosoma Found in the Body?

Schistosoma is a parasite found in snails in contaminated fresh water. When a person swims or bathes in this contaminated water, the parasite enters the human body to cause infections. The parasitic larvae enter the blood vessels and mature into adult form after several days. Then it may affect the internal organs like:
- Intestines.
- Urinary system.
- Liver.
- Lungs.
- Spleen.
- Brain and spinal cord.


Who Gets Schistosomiasis?

The individuals at risk of developing schistosomiasis are:
Those who swim or bathe in contaminated freshwater lakes, ponds, etc.
Those who are exposed to fresh water in the following regions:
- Africa.
- Nile river in Egypt.
- South America.
- Southeast Asia.
- The Caribbean, etc.


How Is Schistosomiasis Caused?

Schistosomiasis is also known as snail fever and is mainly caused by the parasitic worms on the snails. The several species involved in causing the disease are listed down:
- Schistosoma mansoni.
- Schistosoma haematobium.
- Schistosoma japonicum.
- Schistosoma mekongi.
- Schistosoma intercalatum; these species may penetrate the human body to cause infection.


How Does Schistosomiasis Affect the Body?

The parasites on the snails in the contaminated fresh water are the main reason for causing schistosomiasis. When an individual swims or bathes in this contaminated water, the parasitic larvae enter the human body through its forked head. After entering the human body, the larvae mature into adult forms within days or weeks. Over several months of infection, the parasite damages the internal organs. In addition, the infected human lets out the parasitic eggs in urine and feces. When it is done in freshwater containing snails, the parasite moves into snails, and their life cycle continues. However, the disease does not spread from person to person.


How Is Schistosomiasis Treated?

The parasitic disease called schistosomiasis is treated with anthelmintic drugs, mainly Praziquantel. The doctor may suggest it as a single large dose or as three smaller doses for a day. Praziquantel may reduce the complications and relieve the infected person from symptoms. However, it does not prevent the re-infection. In addition, discuss with the healthcare specialist any drug or food allergy before taking this medication.


Which Drugs Can Treat Schistosomiasis?

The parasite called Schistosoma is involved in causing schistosomiasis. Once the doctor confirms the diagnosis, they may suggest high-spectrum anthelmintic drugs to treat schistosomiasis. According to current research, the efficacious drugs for the treatment of schistosomiasis are:
- Praziquantel (PZQ).
- Oxaminiquine (OXA).


How Can We Prevent Schistosomiasis?

Schistosomiasis may severely affect the internal organs of the infected person. Therefore, it is essential to know the following preventive measures:
- Avoid swimming or bathing in contaminated freshwater lakes, ponds, etc.
- Do not expose yourselves to contaminated water for a long time.
- Drink boiled water.
- Filter the drinking water before use.
- Choose chlorinated swimming pools for bathing.


Are Metronidazole and Albendazole Used to Treat Schistosomiasis?

Metronidazole and Albendazole are widely used to treat parasitic infections like amebiasis, strongyloidiasis, giardiasis, and schistosomiasis. But according to research, Praziquantel, an anthelmintic drug, is a safe and effective medicine in managing schistosomiasis. In addition, it significantly prevents complications and reduces the severity of symptoms. Therefore, the doctor might prescribe a single large oral dose to treat the disease.


Does Schistosomiasis Cause Bladder Cancer?

Bladder cancer so far is known to be caused by exposure to carcinogenic agents (hydrocarbons, etc.). However, studies show that the development of bladder squamous cell carcinoma is significantly linked with schistosomal infection. These cancers develop among people in endemic areas of schistosomiasis—the schistosomal infection results in chronic cystitis and gradually developing bladder cancer.


What Causes Urinary Schistosomiasis?

Urinary schistosomiasis is a parasitic disease caused by Schistosoma haematobium species. The individual gets the disease from the parasite in the contaminated fresh water in endemic areas like Africa, North America, Southeast Asia, etc. The parasite enters the human body and causes infection around the urinary bladder. The eggs released by this organism pass out in the urine, and thus the parasite's life cycle continues when the infected urine is let out in freshwater lakes and ponds.


What Is Bladder Schistosomiasis?

The parasitic infection called schistosomiasis may cause severe complications in humans. The bladder schistosomiasis is mainly caused by the Schistosoma haematobium species found in snails present in freshwater lakes, ponds, and rivers. It is more commonly affected by individuals between 15 to 20 years of age. The occurrence of bladder schistosomiasis is high in areas like Africa, North America, Egypt, the Caribbean, etc. Therefore, the doctor may suggest high-spectrum anthelmintic drugs treat bladder schistosomiasis.


What Is the Characteristic Feature of Bladder Schistosomiasis?

Bladder schistosomiasis may cause the following signs and symptoms:
- Maculopapular rash.
- Fever.
- Cough.
- Malaise.
- Weight loss.
- Suprapubic pain.
- Blood in the urine.
- Difficulty in urination.
Dr. N. Ashok Viswanath
Dr. N. Ashok Viswanath

Infectious Diseases


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