Schistosomiasis is a tropical disease caused by a parasitic infection with worms from the Schistosomatidae family. About 200 million people are infected with schistosomiasis worldwide. Schistosomiasis can be 'acute' or 'chronic'. Many people do not develop symptoms of acute schistosomiasis, or their symptoms may be mild and go unrecognised. Chronic schistosomiasis can produce symptoms months or years after infection. Diagnosis is usually made by testing your urine or poo (faeces) or through a blood test. Treatment is with a medicine called praziquantel. In those who have not been infected for long, or those without complications, there can be complete recovery after treatment.
What is schistosomiasis and what causes it?
Schistosomiasis is also known as Bilharzia. It is a tropical disease caused by a parasitic infection with worms from the Schistosomatidae family. A parasite is an animal that lives on or in another animal and feeds from it. There are three main worm species that cause schistosomiasis in humans:
- Schistosoma haematobium
- Schistosoma mansoni
- Schistosoma japonicum
Eggs from the mature worms are passed out (excreted) in the poo (faeces) or urine of an infected person. These eggs can get into fresh water if it is contaminated with human sewage. Once in the water, the eggs can infect snails living in the water and they then start to develop into larvae inside the snails. These snails are known as 'intermediate hosts' because they are needed for the worms to complete their life cycle.
The mature larvae are then released from the snails into the water. The larvae can swim around and can penetrate the skin of someone who bathes, wades or swims in the water. Once they have got through the skin, larvae then travel to the blood vessels where they mature into adult worms. The worms settle in the blood vessels around the intestine or the bladder (depending on the species). Here, the male and female worms mate and eggs are produced. These eggs can pass from the blood vessels through the wall of the bladder or the bowel so that they can be excreted in the urine or faeces. The life cycle of the worm is then able to start again. The worms can live inside your body for three to five years, producing eggs.
Note: not all eggs are excreted. Some stay in your body and are involved in the development of symptoms (see below).
How common is schistosomiasis and who gets it?
The World Health Organization (WHO) reports that about 200 million people are infected with schistosomiasis worldwide.
You are at risk of schistosomiasis if you live in or travel to an area where schistosomiasis occurs and you come into contact with fresh water that contains the snails that act as the intermediate host. This contact can be through, for example:
- Water skiing.
- Working on irrigation equipment.
- Drinking from streams or irrigation ditches.
Children and teenagers are most often affected. In some areas of the world nearly all those aged 10-19 may be, or have been, infected. Re-infection is also common.
In which areas of the world does schistosomiasis occur?
There are certain areas of the world where schistosomiasis occurs. If you travel to, or live in, the following areas you are at risk if you come into contact with contaminated fresh water:
- Africa: all fresh water in southern and sub-Saharan Africa, including the great lakes and rivers as well as smaller bodies of water. Transmission also occurs in the Nile River valley in Egypt.
- South America: including Brazil, Suriname, Venezuela.
- Caribbean: Antigua, Dominican Republic, Guadalupe, Martinique, Montserrat, Saint Lucia (risk is low).
- The Middle East: Iran, Iraq, Saudi Arabia, Yemen.
- Southern China
- Southeast Asia: Philippines, the Lao People's Democratic Republic, Cambodia, central Indonesia, Mekong Delta.
Note: chlorinated swimming pools and seawater are generally thought to be safe.
What are the symptoms of schistosomiasis?
Schistosomiasis can be 'acute' or 'chronic'. Also, within days of becoming infected, some people can develop a rash or itchy skin. This is known as 'swimmer's itch'.
This is also known as Katayama fever. Acute means 'sudden onset' or 'short duration'. Symptoms first develop some weeks after you have been in contact with the infested water. Symptoms can include:
- Joint and muscle pains.
- Bloody diarrhoea.
- Tummy (abdominal) pain.
Note: many people who are infected do not develop symptoms of acute schistosomiasis, or their symptoms may be mild and go unrecognised. However, they can go on to develop chronic schistosomiasis.
This is more common than acute schistosomiasis. Chronic means 'persistent' or 'ongoing'. If you have chronic schistosomiasis, you can first develop symptoms many months or years after you were first infected. It can cause long-term ill health.
The symptoms depend on the species of schistosoma that you have and where this species produces most of its eggs. Schistosoma mansoni and Schistosoma japonicum tend to produce eggs in the blood vessels around the bowel. Schistosoma haematobium tends to produce eggs in the blood vessels around the bladder. The symptoms are caused by your body's immune system reacting against the eggs that the worms produce, not by the worms themselves. This reaction can cause inflammation and scarring of the tissues and organs of the body.
If eggs are mainly produced in the blood vessels around the bowel, symptoms can include bloody diarrhoea and tummy (abdominal) pain which tends to be cramping. If they are produced in the blood vessels around the bladder, symptoms can include blood in your urine and pain on urinating.
Eggs that have not been excreted by the body can also travel to other parts of the body and cause symptoms. For example, your liver, lungs, heart, brain or nervous system may be affected. Symptoms depend on the affected area but can include:
- Chest pain.
- Liver failure.
- Paralysis (if the spinal cord is involved).
Children who are repeatedly infected with schistosomiasis can develop anaemia, malnutrition and learning difficulties.
How is schistosomiasis diagnosed?
There are a number of tests that can be used to help confirm the diagnosis, or that can be used to assess which parts of the body are affected by the infection. These include:
- Urine and poo (faeces) samples: schistosomiasis is usually diagnosed by testing urine and faeces samples. The test looks for schistosomal eggs in the urine or faeces, using a microscope.
- Blood tests: an antibody or antigen blood test can show if someone has, or has had, schistosomiasis. It can take some time for these tests to become positive (4-8 weeks). Other blood tests can check for anaemia and to see if your liver or kidneys are affected.
- Chest X-ray: this can sometimes show if the lungs are affected.
- Scans: an ultrasound scan of the liver or the heart may be useful to show if schistosomiasis has affected these organs. Sometimes CT scanning or MRI scanning is used, especially if the brain or spinal cord is affected.
- Colonoscopy or cystoscopy: sometimes schistosomiasis is diagnosed after taking samples during one of the following procedures:
What is the treatment for schistosomiasis?
Praziquantel is the common medicine used to treat schistosomiasis. It is a tablet taken by mouth and is usually effective. The treatment is usually just a single dose, or two doses taken on the same day. Steroid medication is also sometimes given in acute schistosomiasis. Any complications (see below) will also need to be treated.
Are there any complications of schistosomiasis?
In some people, if schistosomiasis is untreated, or if infection goes unrecognised, complications can sometimes occur. These can include:
- Chronic kidney disease.
- Chronic liver damage and an enlarged spleen.
- Colon (large intestine) inflammation.
- Kidney and bladder obstruction.
- Repeated blood infections can occur - germs (bacteria) can easily enter the bloodstream through an inflamed colon.
- Heart failure.
- Bladder cancer - your risk of bladder cancer is slightly increased if schistosomiasis affects your bladder.
What is the outlook (prognosis) for schistosomiasis?
Almost all people who receive treatment for schistosomiasis will improve. There can be complete recovery in those who have not been infected for long, or those without complications.
Even if someone has developed complications such as liver damage, these can sometimes improve after treatment. However, for some people, complications cannot be reversed and they can develop chronic ill health; sometimes, death can occur. If someone is also infected with malaria, HIV, or hepatitis, there is a worse prognosis.
Can schistosomiasis be prevented?
People who travel to areas affected by schistosomiasis should avoid exposure to fresh water that is likely to be contaminated. This means:
- Avoid swimming or wading in fresh water.
- Make sure that you only drink safe water. This means either drinking bottled water (ensuring that the bottle is sealed before opening), filtering water or boiling it for at least one minute.
- Bathe in safe water. Bath water should be heated for five minutes at 150°F (about 66°C). Water held in a storage tank for at least 48 hours should be safe for showering.
Various measures are being taken to help to reduce schistosomiasis infection in affected countries. These include:
- Improved sanitation to decrease sewage contamination of fresh water.
- Measures to help reduce occupational and recreational contact with contaminated water.
- Treatment of water to help reduce the numbers of the snail 'intermediate hosts'.
- Treatment of people in affected areas with medicines to 'kill off' the infection. The WHO has launched a strategy using medicines to help control schistosomiasis in a number of countries.
Dr Michelle Wright
Dr Colin Tidy
Dr Adrian Bonsall