Schistosomiasis (bilharzia)
Schistosomiasis is caused by tiny worms that live in fresh water in warm countries. There is only one way to catch it: by being in that water.
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Schistosomiasis is caused by tiny worms that live in fresh water in warm countries. There is only one way to catch it: by being in that water. The larva makes its own way through the skin, so a swim, a load of washing done in a river, a stream crossed on foot or simply wet feet are enough. Here it is almost always something people bring back from a trip, and the unpleasant part is that they bring it back without noticing: you come home feeling perfectly well, go years without a single complaint, and all that time the worms are laying eggs in the wall of the bladder or the bowel. So this page is mostly about travel: where not to go into the water, and what to do once you are home.
Where you pick it up
The worms do not live in the water itself but in freshwater snails that stay near the bank. Larvae leave the snail, swim for a few hours and pass through the skin of anyone who happens to be there. You cannot see them because they are microscopic, and the water may look completely clear and clean.
There is a risk wherever it is warm all year round and the water is untreated:
- most of sub-Saharan Africa, including Lake Malawi and Lake Victoria, the Nile and the Zambezi;
- the Middle East and parts of North Africa;
- southern and south-east Asia, the Mekong basin, southern China, the Philippines and Indonesia;
- parts of South America, above all north-east Brazil, Venezuela and Suriname, and several Caribbean islands.
And here is what you do not need to worry about. Sea water is safe: the worms do not survive in salt water. A chlorinated pool is safe. Tap water, the hotel shower, rain and ice cubes have nothing to do with this illness. What is risky is natural fresh water: a lake, a river, a pond, an irrigation channel, a paddy field, a waterfall. Depth and duration make no difference — a few minutes ankle-deep is enough.
Schistosomiasis does not pass from person to person: someone who has it is no risk to their family or travelling companions.
How not to bring it home
There is no vaccine against schistosomiasis, and taking tablets "just in case" before you go does not work either. Only one thing works: not going into fresh water where there is a risk.
- Do not swim, dive, raft or wade across rivers, lakes and canals in areas where the infection occurs. If local people say everyone swims here and nobody gets ill, that means nothing: in the local population the same infection runs for years without obvious symptoms.
- For swimming, use the sea or a chlorinated pool.
- If you do have to go into the water for work or out of necessity, you need high rubber boots or waders — they protect you as long as the water stays off your skin.
- Water taken from a river or lake for washing yourself or your clothes should be boiled and cooled, filtered, or simply left to stand for two days in a closed container: the larvae are short-lived and will have died by then.
- Drink only bottled, boiled or filtered water. A larva can also be swallowed with water, though that route is less common.
One more thing, because the advice still turns up in guidebooks: rubbing yourself down briskly with a towel as soon as you leave the water does not help. The larva needs seconds, and by the time you reach the bank it is already under the skin. It is not something to rely on.
What appears, and when
Schistosomiasis is recognised by its timing: symptoms come in waves, and in between them people feel fine.
The first hours and days. An itchy rash comes up on the skin that was in the water — small bumps and red patches, sometimes with a prickling or stinging feeling. It is known as swimmer's itch. It lasts a few days and clears on its own, so it is usually put down to an allergy, mosquitoes or nettles. It is the only early sign, and not everyone gets it.
Two to eight weeks later. A fever may start, sometimes high and with shivering. With it come aching muscles and joints, headache, a dry cough, hives, tummy ache, diarrhoea and marked tiredness. This is the body reacting to the first batch of eggs, and in travellers it tends to be considerably heavier than in local residents. The picture is easily mistaken for flu, for malaria or for something eaten on the trip — and that is the main trap: a fever a month after returning from the tropics always means telling the doctor where you have been.
Months and years later. Signs appear that depend on where the worms have settled:
- blood in the urine — sometimes visible, more often picked up only on a test; stinging and needing to pass urine often, pain low in the abdomen;
- blood in the stools, long-lasting diarrhoea or diarrhoea alternating with constipation, tummy pain, an enlarged liver and spleen;
- in women, bleeding after sex, pain during sex and sores on the neck of the womb; in men, blood in the semen.
Most often, though, nothing appears at all. A silent course is the rule here, not the exception.
Why a silent infection is dangerous
The damage is done not by the worms but by their eggs. They lodge in the wall of an organ, and the body builds a small scar around each one. A single scar is nothing, but eggs are laid for years and scar tissue gradually replaces normal tissue. Nothing is felt until the organ starts to fail.
What gets damaged over those years:
- the bladder — the wall thickens and loses its stretch, ulcers and polyps appear, and passing urine becomes frequent and painful;
- the ureters and kidneys — scarring narrows the drainage of urine, the kidney swells and is slowly lost; if both sides are affected it leads to kidney failure;
- the liver — scarring around the vessels raises the pressure in the portal vein system. Hence an enlarged spleen, fluid in the abdomen and swollen veins in the gullet that can bleed heavily. The liver itself keeps working normally for a long time, and routine tests may show nothing;
- fertility — scarring in the female genital tract and in the sperm-carrying ducts in men makes conception harder;
- many years of bladder schistosomiasis appreciably raises the risk of bladder cancer.
That is why the infection deserves attention even in someone with no complaints: treatment takes a day, while a scar does not undo itself once the worms are dead.
Getting tested after the trip, even with no symptoms
If you had contact with fresh water on a trip to an area where schistosomiasis occurs, it is worth being tested regardless of how you feel. That is the most useful line on this page. Having had no rash and no fever rules nothing out.
The timing matters. Tests done straight after you get home are useless: the worms need time to grow and start laying, and the body needs time to make antibodies. So testing is planned for about three months after the last contact with the water. Do it earlier, get a negative, and it will have to be repeated.
Several things are usually done together:
- a blood test for antibodies — the most sensitive way of picking up the infection in a traveller, who is likely to carry few worms;
- a full blood count, looking at the eosinophil count, which is often raised;
- a urine test for eggs — you will be asked to collect the sample in the middle of the day, when egg numbers are highest;
- a stool test for eggs, sometimes on several samples from different days.
Do tell the doctor exactly where you were, when, and how you came into contact with the water, even if it was one short swim a year or five years ago. Without that sentence schistosomiasis simply does not come to mind, and people spend years being investigated for "blood in the urine of unclear cause".
Treatment
Here is the good news. Schistosomiasis is treatable, the treatment is short and it almost always works: praziquantel, dosed by body weight and taken over a single day. It kills the adult worms, and egg-laying stops there.
A few points worth knowing in advance:
- the drug has no effect on young, immature worms. So when the infection is recent the course is often repeated a few weeks later — not a sign of failure but a planned step;
- follow-up tests are not done immediately but after several months: antibodies stay in the blood for a long time and on their own do not mean the infection is still there;
- with a severe fever in the acute phase, and where the nervous system is involved, corticosteroids are added, because a large number of worms dying at once increases the inflammation;
- the medicine does not undo scarring that has already formed. If the kidneys, liver or bladder have suffered, the relevant specialists take over from there;
- another trip means another infection. Having had it once leaves no protection.
When to see a doctor urgently
In most cases schistosomiasis is not urgent: it is a planned set of tests and a planned course of treatment. But there are situations where days count, and they are worth knowing about.
Get help immediately if, after a trip to the tropics, you develop:
- weakness in the legs, numbness around the perineum, retention of urine or loss of control of urine or bowels — this is how a rare involvement of the spinal cord shows itself, and without quick treatment the damage is permanent;
- seizures, confusion, or a sudden severe headache with vomiting;
- vomiting blood or passing black, tarry stools — a sign of bleeding from veins in the gullet;
- a high fever with severe breathlessness and increasing weakness.
With these signs, call an ambulance — in European countries on the single number 112.
A same-day appointment, without an ambulance, is needed for a high fever after returning from the tropics, visible blood in the urine or stools, or severe abdominal pain.
Online consultation
A remote appointment covers precisely what usually brings people to this page. The doctor will go through your trip: whether the contact with fresh water was a real risk, whether schistosomiasis occurs in the area you visited, and what to do if the swim was a long time ago. They will tell you which tests to have and, more importantly, how many weeks need to pass for the result to mean anything. They can help you read a result you already have — positive antibodies alongside negative urine and stool tests frighten people more than they should, and it takes five minutes to explain. It is also a convenient way to go through an itinerary in advance and work out which parts are safe and which are not. The signs in the previous section are not assessed remotely: with those, go straight to a doctor or call an ambulance.
This material is for information only and does not replace medical advice.
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