Schistosomiasis is a type of infection caused by parasites that live in fresh water, such as rivers or lakes, in subtropical and tropical regions worldwide.
Schistosomiasis is also known as bilharzia.
Symptoms can develop a few weeks after someone is infected by the parasite and include:
Alternatively, more serious symptoms can develop months and possibly years after infection and include:
Doctors refer to this as chronic schistosomiasis.
Contact your doctor if you develop any of the symptoms of schistosomiasis and you have travelled in parts of the world where schistosomiasis is widespread.
The parasites that cause schistosomiasis are called schistosomes. Schistosomes are small, white-grey worms that are 7–20mm long. They are capable of burying into human skin and can then move into other organs of the body, such as the liver or bladder, where they lay eggs.
They live in lakes, rivers, reservoirs and canals and can infect anyone who comes into contact with contaminated water.
Once a person is infected, the parasites then pass out eggs in their urine or faeces (stools), which can survive in water for up to seven days.
Schistosomiasis is a disease associated with poverty. Poor living conditions, overcrowding, and lack of sanitation, clean water and medical services increase the risk of schistosomiasis.
Read more about the causes of schistosomiasis.
If schistosomiasis is diagnosed and treated promptly, it can usually be successfully treated with a medication called praziquantel. This kills the parasites, which are then passed out of the body.
Schistosomiasis is usually only a serious health problem for people who do not have access to effective medical treatment, particularly people who are already vulnerable because of malnutrition or dehydration.
Read more about treating schistosomiasis.
Popular tourist destinations that are known to have high levels of parasites include Lake Malawi in Africa and the Mekong River in Southeast Asia.
Globally, schistosomiasis is the second most common disease caused by parasites after malaria. An estimated 200 million people are infected worldwide, and 85% of cases occur in sub-Saharan Africa.
There is currently no vaccine available for schistosomiasis so it is important to take precautions when travelling in parts of the world where the infection is widespread. This includes not swimming or wading in fresh water.
Read more about preventing schistosomiasis.
Symptoms of schistosomiasis can follow one of two patterns:
Some people have acute schistosomiasis followed by chronic schistosomiasis. But most people will only have one or the other.
The symptoms of acute schistosomiasis are not directly caused by the parasites but by your immune system (the body’s defence against infection) reacting to the parasites.
In many cases, the symptoms usually get better by themselves within a few weeks. However, it is still important to seek treatment as there is a risk that the parasites will stay in your body and you will progress to chronic schistosomiasis.
If schistosomiasis is not treated, the parasites remain in your body and will go on to cause further symptoms. It may damage your organs or the eggs can cause an allergic reaction as your immune system continues, but fails, to kill the parasites.
The symptoms of chronic schistosomiasis depend on where in the body the parasites have travelled too.
If the parasites travel to the digestive system, they can cause the following symptoms:
If the parasites travel to the urinary system, they can cause the following symptoms:
If the parasites travel to the heart or lungs, they can cause the following symptoms:
If the parasites travel to the central nervous system or brain, they can cause the following symptoms:
The parasites can also sometimes travel to the female genitals where they can cause the following symptoms:
Contact your doctor if you develop any of the symptoms above and you have travelled in parts of the world where schistosomiasis is widespread, particularly countries in sub-Saharan Africa, such as Kenya or South Africa.
Schistosomiasis is caused by tiny parasitic worms called schistosomes. The worms live in fresh water in tropical and subtropical countries.
As well as rivers and lakes, the worms can also live in man-made structures such as reservoirs, irrigation ditches and canals.
Schistosomiasis is most common in rural environments that have high levels of poverty and poor sanitation, particularly where fresh water is used for irrigation.
Schistosomes are an unusual type of parasite in that they use a two-stage system to infect humans. An infected human can pass out eggs in their urine or faeces (stools), which can survive in water for up to seven days.
Once the eggs are in the water, they hatch, releasing larvae (small, maggot-like creatures). The larvae can swim and they begin to seek out freshwater snails. When they find a snail, the larva enter its tissue where it matures into the next stage of development, known as cercariae.
After four to six weeks, the cercariae leave the snail. After they have left, they can survive in the water for up to 72 hours. If, during that time, the cercariae come into contact with a human, they can burrow through their skin. Once inside, they are able to move around the body through the veins and can get into the large vein that supplies blood to the liver (the portal vein).
The cercariae will mature into adult worms after four to six weeks. After mating with a male worm, the female worm will lay eggs, which may be passed out in urine and faeces, allowing the life cycle to begin again.
Adult worms can live in the human body for an average of three to five years, although there may have been cases where worms have lived in a human host for up to 30 years. The female worm will continue to lay eggs throughout her life span.
If you go to your doctor with symptoms of schistosomiasis, they will ask you about your recent travel history and whether you may have been exposed to contaminated water.
If your doctor suspects that you have schistosomiasis, you will probably be referred to an expert in tropical diseases.
A diagnosis of schistosomiasis can be confirmed by taking a stool (faeces) sample and checking it for the presence of eggs.
In cases of chronic schistosomiasis, ultrasound scans are often used to check whether there is any damage to your organs.
If you are diagnosed with schistosomiasis, you will probably be admitted to hospital so that your health can be carefully monitored. This will help ensure that any serious complications you have, such as dehydration, are picked up.
The medication used to treat schistosomiasis is called praziquantel. Praziquantel works by first paralysing the worms and then dissolving their outer casing, killing them. A single dose of praziquantel is usually required.
The side effects of praziquantel are usually mild and include:
Steroid medication (corticosteroids) can also be used to relieve the symptoms of acute schistosomiasis as they help control the allergic reaction to the eggs that is responsible for the symptoms.
A further stool sample may be taken after four to six weeks to check whether there are still any eggs in your stools. If eggs are present, a further dose of praziquantel may be given.
There is no vaccine for schistosomiasis, but scientists are working on developing one that will prevent the parasite completing its life cycle in humans.
If you are planning to visit an infected area, it is a good idea to take waterproof trousers and boots with you just in case you have to cross a stream or river.
Avoid swimming in fresh water (ponds, lakes and rivers) when visiting areas where schistosomiasis is widespread. This includes popular holiday spots such as Lake Malawi. Only swim in chlorinated swimming pools or safe sea water.
Always boil or filter water using a travel kettle or a portable water filter before drinking it to kill any harmful parasites, bacteria and viruses.
If you accidentally swim or paddle in contaminated water, drying yourself vigorously with a towel may help stop the parasite from penetrating your skin. Although this may prevent infection, it should never be regarded as a preventative measure.
If you have to cross a river or go into a lake in contaminated areas, aim for clear patches of water with no vegetation and dry yourself as soon as you get out. Try to cross rivers upstream from villages and, if possible, wear waterproof shoes or boots.
You should not rely on assurances from hotels, tourist boards or similar that a particular stretch of water is safe. There have been reports of these sorts of organisations downplaying the risks of exposure when in fact the risk was very high.
Medication that is advertised to treat schistosomiasis is widely available over-the-counter in most African countries, but buying it is not usually recommended. These types of medication are often either fake or substandard. In addition, they are only effective once the worms have matured. So taking medication a few days after a possible exposure to the schistosomes parasites will not be particularly helpful.
If you develop symptoms that could be the result of schistosomiasis while on holiday or travelling, you should seek medical advice. While schistosomiasis is not especially serious in people who are otherwise healthy, it does share symptoms with more serious tropical conditions such as malaria. So it is important to get the diagnosis confirmed (or ruled out) by a doctor with experience in treating the condition.
Insect repellent cream provides some limited protection against infection, but it should not be used as a substitute for following the advice above.
If you are concerned that you may have been exposed to the parasites when travelling, contact your doctor. They should be able to refer you for a stool sample (read more about diagnosing schistosomiasis). This would normally be carried out at a hospital or somewhere where doctors have experience in diagnosing and treating tropical diseases.
Important: Our website provides useful information but is not a substitute for medical advice. You should always seek the advice of your doctor when making decisions about your health.