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.

It is possible to develop schistosomiasis if you have contact with infected water – for example, when paddling, swimming, washing or showering.

Read about preventing schistosomiasis.

Life cycle of schistosomes parasites

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 that then swim and look for freshwater snails. When they find a snail, the larva enters its tissue where it matures into the next stage of development, known as a cercaria. These are just visible without a microscope. 

After four to six weeks, the cercariae leave the snail and can survive in the water for up to 72 hours. If during that time the cercariae come into contact with human skin, they burrow in and develop into the next stage, known as a schistosomule. These are able to move around the body through the blood vessels, eventually reaching the organs of the abdomen. 

About four to six weeks after infection, the schistosome is mature, the males and females mate, and the female worm starts to lay eggs. Some of these eggs remain in the body and move through the blood to other organs, some remain in organs close to the adult worms, where the body mounts an immune reaction, and some pass out in urine and faeces, allowing the life cycle to begin again if suitable snails are present. Direct infection from human to human does not occur.

Adult worms can remain active for five years or more, and 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 lifespan.

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:

• flu-like symptoms, such as a high temperature (fever) above 38ºC (100.4ºF) and muscle aches
• a skin rash
• a cough

Alternatively, more serious symptoms can develop months – and possibly years – after infection, and include:

• cystitis 
• blood in the urine
• bloody diarrhoea
• abdominal pain or cramps
• vomiting blood
• paralysis of the legs

Doctors refer to this as chronic schistosomiasis.

Read more information about the symptoms of schistosomiasis.

Seeing your GP

Contact your GP if you develop any of the above symptoms after travelling to parts of the world where schistosomiasis is widespread.

They will ask you about your recent travel history and whether you may have been exposed to contaminated water.

If your GP suspects you have schistosomiasis, you will probably be referred to an expert in tropical diseases. The diagnosis is usually made by testing a sample of your blood. In some cases, eggs may be seen in a urine or stool (faeces) sample.

In cases of chronic schistosomiasis, ultrasound scans are often used to check whether there is any damage to the urinary system.

How do you get schistosomiasis?

The parasites that cause schistosomiasis are called schistosomes. At the stage of infection, they are just barely visible to the naked eye and known as a cercaria.

They are capable of burrowing into human skin, where they develop into later stages and move closer to the abdomen to lay eggs. The disease is caused by the immune system reacting to these eggs.

Schistosomes live in lakes, rivers, reservoirs and canals, and can infect anyone who comes into contact with contaminated water.

An infected person will pass out parasite 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, poor sanitation, and a lack of clean water and medical services all increase the risk of getting schistosomiasis.

Read more about the causes of schistosomiasis.

Treating schistosomiasis

If schistosomiasis is diagnosed and treated promptly, it can usually be successfully treated with a medication called praziquantel. This kills the adult worms, which will be broken down by 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.

Who is affected?

All cases of schistosomiasis that have been reported in England developed in people who travelled abroad. There were 77 reported cases in England in 2011-12.

Popular tourist destinations that are known to have high levels of parasites include:

• Lake Malawi, Lake Victoria and the White Nile in Africa
• the Mekong River in Cambodia and Laos

Schistosomiasis is also present in Egypt, many sub-Saharan African countries, the Philippines and China.

Globally, schistosomiasis is the second most common disease caused by parasites after malaria. An estimated 243 million people are infected worldwide, and around 90% of cases occur in sub-Saharan Africa.

Prevention

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.

There is some evidence that applying the insect repellent DEET (diethyltoluamide) at 50% concentration immediately after exposure kills the parasite in the skin.

Read more about preventing schistosomiasis.

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.

If you are unsure whether the country you are visiting is known to have a problem with schistosomiasis, check the National Travel Health Network and Centre (NaTHNaC) website.

While you are away

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 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 wear waterproof shoes or boots if possible.

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.

What to avoid

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, substandard, or not given at the correct dosage. In addition, they are only effective once the worms have matured, so taking medication a few days after possible exposure to the schistosomes parasites will not be effective.

What you can do

There is some evidence that applying 50% DEET (diethyltoluamide) insect repellent immediately after contact with infected water kills the invading cercaria in the skin. Tests to exclude infection should still be undertaken after your return to this country.

When to seek medical advice

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. 

It is important to get the diagnosis confirmed (or ruled out) by a doctor with experience of treating the condition.

Read more about healthcare abroad.

If you are concerned that you may have been exposed to the parasites when travelling, contact your GP. They should be able to refer you for a stool sample. This would normally be carried out at a hospital or somewhere where doctors have experience of diagnosing and treating tropical diseases.

The symptoms of schistosomiasis depend on the type of infection and where the parasites are in the body.

The symptoms can follow one of two patterns:

• acute schistosomiasis – symptoms develop a few weeks after infection with the parasites
• chronic schistosomiasis – symptoms develop months, or possibly years, after infection with the parasites

Chronic schistosomiasis is the most common type of infection. Some people have acute schistosomiasis followed by chronic schistosomiasis, but most people will only have one or the other.

Acute schistosomiasis

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.

Symptoms include:

• a high temperature (fever) above 38ºC (100.4ºF)
• headache
• joint and muscle pain
• cough
• bloody diarrhoea
• a dark red, blotchy, raised skin rash
• pain in the abdomen
• a general sense of feeling unwell

In many cases, the symptoms usually get better by themselves within a few weeks. However, it is still important to seek treatment as the parasites will stay in your body and chronic schistosomiasis will develop. This does not always lead to symptoms, but the infection will remain and problems may develop at a later date.

Chronic schistosomiasis

If schistosomiasis is not treated, the parasites remain in your body and will go on to cause further symptoms. The immune system reacting to the eggs may damage your organs, but fails to kill the parasites.

The symptoms of chronic schistosomiasis depend on where in the body the parasites have travelled to.

If the parasites travel to the digestive system, they can cause the following symptoms:

• feeling tired all the time (fatigue)
• abdominal pain
• bowel problems – such as mild or severe watery diarrhoea that contains blood and mucus

If the parasites travel to the urinary system, they can cause the following symptoms:

• symptoms of cystitis – such as pain when urinating
• frequent need to urinate
• blood in your urine

If the parasites travel to the heart or lungs, they can cause the following symptoms:

• persistent cough – in some cases, people cough up blood
• wheezing
• feeling breathless and very tired after physical activity

If the parasites travel to the central nervous system or brain, they can cause the following symptoms:

• seizures (fits)
• headache
• back pain
• urinary incontinence
• weakness and numbness in your legs
• dizziness
• feeling sick

The parasites can also sometimes travel to the female genitals, where they can cause the following symptoms:

• bleeding after sex
• genital ulcers
• irregular menstruation
• pelvic pain

When to seek medical advice

Contact your GP 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 East Africa, Malawi or South Africa.

If you are diagnosed with schistosomiasis, you will probably be seen in a hospital outpatient clinic where your health will be carefully assessed.

This will help ensure that any serious complications are identified. It is not usually necessary to be admitted as an inpatient.

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, but sometimes a longer course is needed.

The side effects of praziquantel are usually mild and include:

• nausea
• vomiting
• abdominal pain

Steroid medication (corticosteroids) may be used to relieve the symptoms of acute schistosomiasis, as they help control the allergic reaction to the eggs responsible for the symptoms.

Treatment with praziquantel is usually delayed until three months after infection as this is when it is most effective.

If eggs were identified in your urine or stools, a further sample may be taken after four to six weeks to check whether there are still any eggs. If eggs are present, further treatment with praziquantel may be given.