Schistosomiasis is a parasitic disease caused by flatworms that live in freshwater. The parasites burrow through human skin during contact with contaminated lakes, rivers, or streams, then settle in blood vessels where they can cause damage to organs over months and years. It is one of the most widespread parasitic infections in the world: at least 253.7 million people required preventive treatment in 2024, and the disease kills an estimated 14,353 people each year.
How You Get Infected
Schistosomiasis spreads through a cycle between humans and freshwater snails. When an infected person urinates or defecates in or near fresh water, parasite eggs in the waste hatch into tiny larvae. Those larvae infect specific species of freshwater snails, where they multiply and develop into a free-swimming form called cercariae. The snails then release thousands of cercariae into the water.
When a person wades, swims, bathes, or washes clothes in that water, the cercariae penetrate the skin in minutes. Once inside, the parasites travel through the bloodstream and mature into adult worms that pair up and settle in the blood vessels around the intestines or bladder, depending on the species. A single pair of worms can produce hundreds to thousands of eggs per day. Many of those eggs get trapped in body tissues, and the immune reaction to those trapped eggs is what causes the disease’s damage.
Where It Occurs
Three main species cause the vast majority of human cases. One affects the urinary tract and is found across Africa and parts of the Middle East. A second infects the intestines and is the most geographically widespread, occurring in sub-Saharan Africa, Brazil, Venezuela, Suriname, the Caribbean, and parts of the Arabian Peninsula. A third intestinal species is found in China, the Philippines, and the Indonesian island of Sulawesi. Despite its name (it was first described in Japanese patients), it was eliminated from Japan long ago.
Three additional species exist in more limited pockets: parts of Cambodia and Laos, the Democratic Republic of the Congo, and West Africa. Overall, schistosomiasis is concentrated in tropical and subtropical regions where sanitation infrastructure is limited and people regularly contact untreated fresh water.
Early Symptoms
Many people notice nothing at first. Some develop a brief itchy rash at the spot where the parasites entered the skin, sometimes called “swimmer’s itch.” Weeks later, usually between 14 and 84 days after exposure, a small percentage of people develop an acute reaction known as Katayama syndrome. This can include fever, headache, muscle pain, cough, and a general feeling of illness. The liver and spleen may become enlarged. Blood tests at this stage often show elevated levels of a type of white blood cell involved in fighting parasites.
Katayama syndrome is more common in travelers or first-time visitors to endemic areas than in people who have lived with repeated low-level exposures since childhood. It typically resolves on its own but can be severe enough to require treatment.
Long-Term Damage
The real danger of schistosomiasis lies in chronic infection. Over months and years, the immune system’s constant reaction to trapped eggs causes scarring and inflammation in whatever organs the eggs accumulate. The specific complications depend on which species is involved.
For the species that targets the urinary tract, eggs lodge in the bladder and ureters. This leads to blood in the urine, scarring of the bladder wall, and progressive kidney damage. Bladder cancer is a recognized complication in later stages. In women, up to 75% of those living in areas where this species is common develop female genital schistosomiasis, a condition affecting an estimated 45 million women in sub-Saharan Africa. Parasite eggs cause lesions on the cervix and vaginal walls, leading to pain during intercourse, abnormal bleeding, and sometimes infertility. These lesions thin the tissue lining, which researchers believe makes women more vulnerable to sexually transmitted infections, including HIV.
For intestinal species, eggs become trapped in the walls of the intestines and the liver. Over time, this produces liver enlargement, significant scarring of liver tissue, and a buildup of pressure in the blood vessels of the abdomen. Advanced cases can develop fluid accumulation in the belly and dangerous bleeding from swollen veins in the esophagus. Infertility is another possible long-term consequence regardless of species.
How It Is Diagnosed
The standard method is microscopic examination of stool or urine samples, looking for the distinctive eggs of the parasite. This approach is highly specific, meaning a positive result is almost always correct, and it remains the gold standard recommended by the WHO. The limitation is sensitivity: in people with light infections, the number of eggs shed can be very low, making them easy to miss on a single sample.
A rapid urine test that detects a protein released by the worms is more sensitive than microscopy and can be done without lab equipment. However, it works best for one species and has notable rates of false positives, particularly in pregnant women and young children. Blood tests that detect antibodies are useful for travelers who may have been exposed but haven’t yet developed a heavy egg burden. These tests confirm exposure but can’t distinguish between a past infection that has cleared and an active one.
Treatment
Schistosomiasis is treatable with a single oral medication called praziquantel, which has been used successfully for over 30 years. It is given as a weight-based dose and is effective against all species. The drug works by damaging the adult worms, allowing the immune system to clear them. Recent formulations have been developed that are palatable and safe for young children.
In communities where the disease is common, mass treatment programs distribute praziquantel to entire populations, especially school-age children, without requiring individual diagnosis first. The frequency and scale of these campaigns depend on local infection rates. In 2024, over 100.5 million people received treatment through these programs globally. The long-term strategy moves through stages: first reducing the severe complications, then lowering infection rates, then interrupting transmission entirely.
Prevention for Travelers
If you’re traveling to an area where schistosomiasis occurs, the most effective prevention is avoiding contact with fresh water in lakes, rivers, streams, and canals. Swimming pools that are chlorinated and properly maintained are safe. Ocean water is not a risk.
If you need to use local water for bathing, boil it at a vigorous boil for at least one minute, then let it cool. Alternatively, you can treat water with chlorine at a concentration of 1 milligram per liter and let it sit for 30 minutes. Water stored in a tank for one to two days is generally safe for bathing, as the parasites’ free-swimming stage does not survive long outside a host. Iodine treatment alone does not reliably kill the parasites.
If you do have brief, accidental contact with potentially contaminated water, towel off vigorously and thoroughly. This may reduce the chance of parasites penetrating your skin, though it is not a reliable safeguard on its own. If you develop unexplained fever, rash, or bloody urine in the weeks after freshwater exposure in an endemic area, mention the exposure to your healthcare provider, as the infection is easily missed when it isn’t on a clinician’s radar.