Yes, several types of parasitic worms can cause blood in your stool. Hookworms, whipworms, and schistosomes are the most common culprits, each damaging the intestinal lining in a different way. The bleeding can range from invisible traces detectable only by lab testing to severe, visible blood mixed with mucus and diarrhea, depending on the type of worm and how heavy the infection is.
How Hookworms Cause Bleeding
Hookworms attach directly to the wall of your small intestine and feed on blood. Adult worms latch on with sharp mouthparts, creating small wounds that continue to bleed even after the worm moves to a new spot. In light infections, this blood loss is microscopic. You wouldn’t see anything unusual in the toilet, but a lab test could detect hidden (occult) blood in your stool.
Heavy hookworm infections are a different story. The cumulative blood loss from dozens or hundreds of worms drains your iron stores over time, leading to iron-deficiency anemia. In a study of school-aged children, hookworm-infected kids had roughly twice the odds of anemia compared to uninfected children. Symptoms of this chronic blood loss include fatigue, pale skin, weakness, and shortness of breath, sometimes before any visible bleeding appears in the stool.
Whipworms and Bloody Diarrhea
Whipworms (Trichuris trichiura) burrow into the lining of the large intestine and colon. Light infections often cause no symptoms at all. Heavy infections, however, produce a recognizable pattern: frequent, painful bowel movements containing a mixture of mucus, water, and blood. The diarrhea typically smells worse than usual. In severe cases, the constant straining and inflammation can cause rectal prolapse, where the rectum sags and protrudes from the anus.
Schistosomes and Intestinal Inflammation
Schistosomiasis works differently from hookworms or whipworms. The adult worms themselves live in the blood vessels around your intestines, not in the gut itself. The damage comes from their eggs. As eggs pass through the intestinal wall, some get trapped in the tissue. Your immune system recognizes these eggs as foreign and mounts an inflammatory response, forming small clusters of immune cells called granulomas around each trapped egg.
Over time, this cycle of egg deposition and inflammation causes chronic colitis, scarring, and the formation of small polyps in the colon. These inflamed, fragile areas bleed, producing blood in the stool. The longer the infection persists, the more extensive the damage becomes.
Strongyloides: A Dangerous Exception
Most worm infections cause gradual, low-level bleeding. Strongyloides stercoralis can cause something far more serious, particularly in people with weakened immune systems. This parasite has a unique ability to reinfect you from within your own body, a process called autoinfection. In healthy people, the immune system keeps this in check, and chronic infections can be completely asymptomatic for years.
The danger comes when the immune system is suppressed, especially by corticosteroids. Steroids allow the larvae to reproduce unchecked, and the parasite load explodes in what’s called hyperinfection syndrome. The larvae directly invade the intestinal lining, causing ulcerations and potentially massive gastrointestinal hemorrhage. One documented case involved a 57-year-old man on corticosteroid therapy whose Strongyloides hyperinfection caused fatal blood loss. Because chronic Strongyloides infection is often silent, diagnosis frequently comes too late.
How Parasitic Bleeding Gets Diagnosed
The standard first step is an ova and parasite (O&P) stool test. You’ll collect a stool sample in a provided container, making sure it doesn’t mix with urine, toilet paper, or toilet water. The key detail most people don’t expect: you’ll likely need to collect several samples over multiple days. Parasites don’t shed eggs consistently, so a single negative result doesn’t rule out infection. Testing several samples increases the chance of catching them.
Stool microscopy alone has limited sensitivity. In one review, stool microscopy detected the parasite in only 41% of confirmed amoebic colitis cases. Blood tests that look for antibodies against specific parasites are much more reliable, coming back positive in 95% of confirmed cases where they were used. A type of white blood cell called an eosinophil can also provide a clue. Elevated eosinophil counts showed up in 58% of Strongyloides cases and 67% of schistosomiasis cases, though a normal count doesn’t rule out infection.
Why It’s Sometimes Mistaken for IBD
Parasitic infections can look remarkably similar to inflammatory bowel disease on both symptoms and colonoscopy. In a review of parasitic colitis cases in high-income countries, 81% of patients had diarrhea, 52% had rectal bleeding, 65% had abdominal pain, and 37% had weight loss. That symptom profile overlaps almost entirely with Crohn’s disease or ulcerative colitis.
This matters because the treatments go in opposite directions. IBD is typically managed with immune-suppressing medications, including corticosteroids. If a parasitic infection is misdiagnosed as IBD and treated with steroids, the results can be catastrophic, particularly with Strongyloides. Patients who received a correct initial diagnosis were far more likely to have had stool studies or blood antibody tests performed early: 53% of correctly diagnosed patients had these tests done before diagnosis, compared to only 19% of those who were misdiagnosed.
The practical takeaway: if you have unexplained blood in your stool and any history of travel to tropical or subtropical regions, or exposure to contaminated soil or water, make sure your doctor considers parasitic infection alongside other causes. Early testing with both stool samples and blood antibody tests dramatically improves the chance of getting the right diagnosis.
How Worm Infections Are Treated
Most intestinal worm infections respond well to short courses of antiparasitic medication, typically taken by mouth for a few days. The specific drug and duration depend on the type of worm identified. Treatment is straightforward for most infections, and symptoms, including bleeding, generally resolve once the parasites are cleared. Heavy infections that have caused significant iron-deficiency anemia may also require iron supplementation to rebuild your body’s depleted stores, which can take several weeks to months even after the worms are gone.
Strongyloides hyperinfection is the exception and requires urgent, aggressive treatment in a hospital setting. For people who know they carry Strongyloides or who have lived in areas where it’s common, screening and treatment before starting any immunosuppressive therapy is critical to preventing this complication.