Bilharzia is a parasitic infection caused by blood flukes, tiny flatworms that burrow through your skin when you wade, swim, or bathe in contaminated freshwater. Also called schistosomiasis, it affects an estimated 254 million people worldwide and remains one of the most significant parasitic diseases after malaria. The infection is most common in sub-Saharan Africa, parts of South America, the Caribbean, the Middle East, and Southeast Asia.
How You Get Infected
The parasites responsible for bilharzia belong to a group called Schistosoma, and three species cause the vast majority of human infections. One species targets the blood vessels around the bladder, while the other two settle into the blood vessels surrounding the intestines and liver. Which species you encounter depends on where in the world you’re exposed.
The life cycle starts when an infected person urinates or defecates in freshwater. Parasite eggs in the waste hatch into tiny larvae that infect specific types of freshwater snails. Inside the snail, the larvae multiply and eventually emerge into the water in a different form, one capable of penetrating human skin. This whole process means that any lake, river, stream, or canal in an affected region can harbor the parasite. Saltwater and chlorinated pools are safe.
The larvae can pierce your skin in a matter of minutes. Once inside, they travel through your bloodstream, mature into adult worms (ranging from about 7 to 28 millimeters long), and pair up in the blood vessels of your intestines or bladder. A mated pair can produce hundreds of eggs per day for years. It’s these eggs, not the worms themselves, that cause most of the damage.
Early Symptoms
Many people notice nothing at first. When early symptoms do appear, they typically show up between 2 and 12 weeks after exposure. This acute reaction, sometimes called Katayama syndrome, can include fever, headache, muscle aches, a rash, and respiratory symptoms like coughing. The liver and spleen may become temporarily enlarged. Because these symptoms overlap with many other infections, bilharzia is easy to miss in the early phase, especially if a doctor isn’t aware of freshwater exposure in an affected region.
There is no effective treatment for the earliest stage of infection. The immature worms are largely resistant to the standard antiparasitic drug, so management during this period focuses on controlling symptoms with anti-inflammatory medications.
What Chronic Infection Does to the Body
The real harm from bilharzia comes from years of untreated infection. The adult worms themselves cause relatively little inflammation. The problem is the eggs. Many eggs don’t make it out of the body. Instead, they become lodged in tissue, where the immune system walls them off in tiny nodules of scar tissue. Over time, this scarring accumulates.
The type of damage depends on which species is involved. The species that lives around the bladder causes scarring of the bladder wall, blood in the urine, and an increased risk of bladder cancer. The intestinal species cause scarring in the liver, which can lead to dangerous increases in blood pressure within the liver’s blood vessels, an enlarged spleen, and fluid buildup in the abdomen. Kidney damage can also occur. These complications develop over years or decades, which is why bilharzia is sometimes called a silent disease: people may carry the infection for a long time before serious problems emerge.
How Bilharzia Is Diagnosed
The standard way to diagnose bilharzia is by finding parasite eggs under a microscope. For intestinal species, this involves examining a stool sample using a technique called the Kato-Katz smear. For the bladder species, a urine sample is filtered and examined. These methods are highly specific, meaning a positive result is reliable, but they can miss light infections because egg output varies day to day.
Newer tests detect proteins the parasite releases into urine or blood. One widely used urine-based antigen test picks up intestinal bilharzia with about 95% sensitivity, though it produces more false positives (specificity around 74%). Blood-based antibody tests are also available and perform comparably, but they can’t distinguish between a current infection and one that was treated in the past. For travelers returning from affected areas, doctors often combine microscopy with antibody testing to improve accuracy.
Treatment
Bilharzia is treatable with a single day of oral medication. The drug, praziquantel, is effective against all species and is given as two or three doses in one day depending on the species involved. The WHO recommends treating children as young as one or two years old and pregnant women in areas where the disease is common, based on extensive safety data.
Praziquantel works well overall, with typical cure rates between 60% and 90% after a single treatment round. Some people need a second course. However, there are growing concerns about reduced effectiveness in areas where the drug has been used heavily for years. The first notable case of apparent resistance appeared in Senegal in 1994, where cure rates dropped to just 18 to 36%. Similar patterns have since been reported in Egypt, Kenya, and Uganda, particularly in communities that have undergone many rounds of mass treatment. In one case in Kenya, a patient was treated 18 times without being fully cured, and worms isolated from that patient showed significantly reduced sensitivity to the drug in laboratory testing.
These cases remain relatively uncommon, and praziquantel is still the backbone of global treatment efforts. But with no alternative drug currently available and over 100 million people treated in 2024 alone, the possibility of widespread resistance is a serious concern.
Where Bilharzia Is Most Common
Sub-Saharan Africa carries the heaviest burden by far, accounting for the vast majority of the estimated 254 million people who needed preventive treatment in 2024. The disease also occurs in parts of Brazil, the Middle East (particularly Yemen), and Southeast Asia (Cambodia, Laos, parts of China and the Philippines). The WHO has set a target of eliminating bilharzia as a public health problem globally by 2030, using a combination of mass drug treatment, improved sanitation, snail control, and access to clean water.
Prevention for Travelers
No vaccine exists for bilharzia. If you’re traveling to an area where the disease is common, the most reliable precaution is straightforward: avoid freshwater. Don’t swim, wade, or bathe in lakes, rivers, streams, or canals. Saltwater and chlorinated pools carry no risk.
If you need to use freshwater for drinking, bring it to a vigorous boil for at least one minute. Iodine treatment alone does not reliably kill the parasite. For bathing, you can boil the water and let it cool, or add chlorine (1 milligram per liter) and let it sit for 30 minutes. Water stored in a tank for at least one to two days is generally considered safe, since the free-swimming larvae survive only about 48 hours without a human host.
If you’ve had a brief, accidental exposure, towel off vigorously and quickly. This may reduce the number of larvae that penetrate your skin, though it’s not a reliable method of prevention on its own. If you develop a fever, rash, or other unexplained symptoms in the weeks following freshwater contact in an endemic area, let your doctor know about the exposure so they can test appropriately.