Can a Colonoscopy Detect Parasites?

Sometimes. A colonoscopy can show some parasites, such as whipworms or pinworms in the large intestine, but it isn't a first test for them. Most intestinal parasites are found with stool tests, and the CDC describes endoscopy and colonoscopy as tests used when stool exams don't explain your symptoms.

This page explains what a colonoscopy can and can't show when it comes to parasites, why doctors usually start with simpler tests, and where a colonoscopy fits. It is general education, not medical advice.

What a colonoscopy looks at

A colonoscopy is a procedure in which a doctor, usually a gastroenterologist, passes a thin, flexible tube with a camera through the anus to look at the inside of the large intestine (the colon and rectum). MedlinePlus explains that the scope is moved through the large intestine and as far as the lowest part of the small intestine, and that tiny tools passed through the scope can take tissue samples, called biopsies.

That reach matters for parasites. A colonoscopy gives a direct view of the colon, but it sees only the very end of the small intestine, which is where several common worms live.

The CDC's page on diagnosing parasitic diseases describes two related procedures:

• Colonoscopy: the tube goes in through the rectum to examine the large intestine.
• Upper endoscopy: the tube goes in through the mouth so the doctor can examine the intestine from above.

The CDC says these procedures are used to find parasites that cause diarrhea, loose or watery stools, cramping, gas and other belly problems, and that they are used when stool exams do not reveal the cause of the diarrhea. They look for the parasite itself or for other problems that may be causing your symptoms.

Which parasites a colonoscopy can show

Whether a colonoscopy can find a parasite depends mainly on where in the gut that parasite lives:

• Whipworm (Trichuris). The CDC says whipworm lives in the large intestine, the part of the gut a colonoscopy examines. Even so, the CDC says a stool sample checked for whipworm eggs under a microscope is the only way to know you have a whipworm infection. See whipworm in humans.
• Pinworm (Enterobius). The CDC notes that adult pinworms are occasionally found by chance during a colonoscopy or in intestinal biopsies. That is not how pinworm is normally diagnosed: the usual test is the tape test, done at home first thing in the morning. See our guide to the pinworm tape test.
• Hookworm and roundworm. The CDC says both live in the small intestine, mostly beyond the part a colonoscopy reaches. They are usually found by checking a stool sample for eggs. See hookworm in humans and roundworm in humans.
• Parasites in the blood or other organs. Some parasites are not in the gut at all. The CDC lists blood tests, such as a blood smear for malaria, and imaging scans such as X-ray, MRI or CT for parasites that cause lesions in organs. A colonoscopy is not the test for these.

A colonoscopy can also show other things that might explain gut symptoms. MedlinePlus lists findings such as inflammation (colitis), including colitis due to infection, areas of bleeding, polyps and cancer. That is one reason a clinician might choose it when symptoms go on without an answer.

Why a colonoscopy isn't usually the first test

For most suspected worm infections, doctors start with simpler tests:

• A stool test comes first. The CDC lists the stool exam, also called an ova and parasite test (O&P), as the test for parasites that cause diarrhea, cramping, gas and other belly problems. It looks for eggs or the parasite itself. The CDC recommends checking three or more samples collected on separate days, because one sample can miss an infection. Our guide to the stool test for parasites explains how it's done.
• The tape test is the usual pinworm test. The CDC says pinworm is usually diagnosed with the tape test, and that stool samples usually don't contain enough eggs to find it.
• Some infections are checked with blood tests. The CDC explains that some, but not all, parasitic infections can be found with a blood test, and that each blood test looks for a specific parasite. See can a blood test detect parasites.
• A colonoscopy is a bigger procedure. MedlinePlus explains that it needs a full bowel clean-out beforehand, medicine to help you relax during the test, and someone to take you home afterwards.

So a colonoscopy is usually not ordered just to look for worms. It tends to come up when stool tests are negative but symptoms such as ongoing diarrhea or belly pain continue, or when there is another reason for one.

Other reasons people have a colonoscopy

Many people who ask this question have a colonoscopy booked for a different reason and wonder whether it will also show parasites. MedlinePlus lists common reasons for a colonoscopy, including:

• Belly pain, changes in bowel habits or weight loss
• Blood in the stool, or black, tarry stools
• Iron-deficiency anemia, usually when no other cause has been found
• Inflammatory bowel disease, such as ulcerative colitis and Crohn's disease
• Screening for colorectal cancer, or follow-up of earlier polyps

If you think you may have worms and have a colonoscopy coming up, tell the doctor doing it about your symptoms and any travel or exposure, so they know what to look for and whether to take samples. A stool test is still often the simpler way to check. Our guide to can worms cause anemia explains how worms fit into an anemia workup.

What happens if a parasite is found

If a colonoscopy, biopsy or stool test shows a worm, the result goes to the clinician who ordered the test, who explains it and talks about treatment. The CDC says these worm infections can be treated with medication from a healthcare provider. The choice of medicine, the dose and how long to take it are the prescriber's decision. We don't publish doses or schedules.

Get in-person care promptly if you have blood in your stool or black, tarry stools, severe or worsening belly pain, fainting, or a fever after travel abroad. In-person care is also the right route if you are pregnant or breastfeeding, or if the person with symptoms is a child. Our guide to what kind of doctor treats parasites explains who to see.

Getting a prescription through Get It Prescribed

Get It Prescribed is a telehealth service for adults. It does not perform colonoscopies or other procedures. A licensed provider reviews a written health assessment, including any test results you already have, and decides whether mebendazole or ivermectin is appropriate for a labeled use. Mebendazole's labeled uses are pinworm, whipworm, roundworm and hookworm; ivermectin's are intestinal strongyloidiasis and onchocerciasis. No prescription is guaranteed, and the provider may recommend testing or an in-person visit instead. How it works shows each step.

The medicines dispensed through Get It Prescribed are compounded preparations made by a licensed compounding pharmacy. They are not FDA-approved, the FDA has not reviewed them for safety, effectiveness or quality, and they are not generics of, or substitutes for, the brand-name products.

The cost is a one-time $50 provider consultation fee, plus $3.00 per ivermectin tablet or $3.50 per mebendazole capsule if a prescription is issued, with the quantity decided by the provider. The service is available in all 50 states and the District of Columbia; some states require a live phone or video visit.

This article is general education, not medical advice. It does not replace an evaluation by a licensed clinician who knows your history.

Related questions

Can a doctor see worms during a colonoscopy?

It can show worms that live in the large intestine. Whipworms live there, and the CDC notes that adult pinworms are occasionally seen by chance during a colonoscopy. Worms that live higher up in the small intestine, such as hookworm and roundworm, are usually found with a stool test instead.

Can a colonoscopy miss parasites?

Yes. A colonoscopy sees the large intestine and only the very end of the small intestine, so it can miss worms that live higher up. Small parasites and eggs are found under a microscope, which is why the CDC lists stool exams first and recommends several samples.

Is a stool test or a colonoscopy better for parasites?

They answer different questions. A stool test is the usual first test for intestinal worms and their eggs. The CDC describes endoscopy and colonoscopy as tests used when stool exams don't explain diarrhea or other belly symptoms. Your clinician decides which you need.

Can an upper endoscopy find parasites?

Sometimes. The CDC lists endoscopy, where a tube goes in through the mouth, alongside colonoscopy as a way to look for parasites and other causes of symptoms when stool exams don't find the cause. A gastroenterologist usually does it.

Should I ask for a colonoscopy if I think I have worms?

Start by talking to a clinician about your symptoms, travel and exposures. For most suspected worm infections they will order a stool test or, for pinworm, a tape test first. A colonoscopy is usually considered when those tests are negative and symptoms continue, or for another reason.

Sources

• CDC: Diagnosis of Parasitic Diseases — stool exams (O&P) with three or more samples, endoscopy and colonoscopy when stool exams don't find the cause, blood tests and imaging.
• CDC: About Whipworms — whipworm lives in the large intestine; stool exam for eggs is the way to diagnose it.
• CDC: Clinical Overview of Pinworm Infection — the tape test; adult worms occasionally found incidentally on colonoscopy or biopsy.
• CDC: About Hookworm — hookworms live in the small intestine; stool test for diagnosis.
• CDC: About Ascariasis — roundworm lives in the small intestine.
• MedlinePlus: Colonoscopy — how the test is done, preparation, reasons it's performed and what abnormal results can mean.
• MedlinePlus: Ova and Parasite Test — the stool test used for most intestinal parasites.
• DailyMed: mebendazole chewable tablet prescribing information — labeled uses: pinworm, whipworm, roundworm and hookworm.
• DailyMed: ivermectin tablet prescribing information — labeled uses: intestinal strongyloidiasis and onchocerciasis.
• FDA: Compounding and the FDA — Questions and Answers — what compounded preparations are and how they are regulated.