Intestinal Parasites

What they are, how they’re really tested, and what to know about “cleanses”

Last reviewed: October 2026 · 27 sources

Educational only, not medical advice. This page shares general information and community experience. It isn’t a diagnosis or a treatment plan. Please talk with your doctor or another licensed healthcare provider before starting, stopping, or changing any test, supplement, medication, or treatment.

At a glance

  • What they are: a parasite is an organism that lives on or in a host and gets its food from or at the expense of its host. Three main classes cause disease in people: protozoa, helminths (worms) and ectoparasites [1].
  • Most common in the U.S.: more people in the U.S. get sick from Giardia than from any other gut parasite, more than 1 million a year [2].
  • How they’re diagnosed: usually with a stool “ova and parasite” (O&P) exam; the CDC recommends three or more samples collected on separate days [3]. Antigen and DNA (PCR) tests are also used [4, 5].
  • “Parasite cleanses”: no scientific evidence shows they work [6], and none of the herbs or supplements in them are FDA-approved to treat parasitic infections [7].
  • If an infection is confirmed: a healthcare provider can prescribe medicine [2].

Social media is full of claims that many people carry hidden worms that only a do-it-yourself “detox” can remove [6]. If you live with long-lasting gut or whole-body symptoms, it’s natural to wonder whether a parasite could be part of it. This guide explains what intestinal parasites are, how doctors actually test for them, and how to weigh the claims.

What intestinal parasites are

The CDC describes two main groups that live in the gut [1]:

  • Protozoa are microscopic, one-celled organisms [1]. They can multiply inside the body, and intestinal protozoa usually spread by the fecal-oral route, for example through contaminated food or water or person-to-person contact [1].
  • Helminths are worms that are generally visible to the naked eye as adults [1]. Adult worms can’t multiply inside the human body [1].

Examples you may hear about in the U.S.:

  • Giardia causes diarrhea and spreads easily from person to person or through water, food, surfaces and objects [2].
  • Cryptosporidium (“Crypto”) causes watery diarrhea, though some people have no symptoms at all [8].
  • Pinworm is a small roundworm that causes itching around the anus [9]. It mainly affects children, people in long-term care and their households [9].
  • Blastocystis is common, but the CDC says whether it causes disease in humans is unclear, and many people who carry it have no symptoms [10].
  • Soil-transmitted worms (roundworm, whipworm and hookworm) are found mainly in warm, moist areas with poor sanitation [11].

Symptoms

Symptoms depend on the parasite:

  • Giardia: diarrhea, gas, greasy stools that can float, stomach cramps, nausea and dehydration [2]. Symptoms usually start 1 to 2 weeks after infection and last 2 to 6 weeks; occasionally they last for years [2].
  • Crypto: watery diarrhea that usually begins 2 to 10 days after infection and lasts 1 to 2 weeks [8].
  • Pinworm: usually mild, mainly itching around the anus at night; some people have trouble sleeping, and some have no symptoms [9].
  • Blastocystis: some people who carry it report loose stools, abdominal pain, gas, constipation or anal itching; some studies have linked it with rashes and hives [10].

These symptoms are not specific to parasites [6]. Plenty of people have digestive problems that aren’t related to a parasitic infection [6], which is why testing matters.

Who’s at risk

  • Close contact and childcare: being in close contact with someone sick from Giardia, especially in childcare settings, raises the risk [2].
  • Untreated water: drinking from springs, lakes, rivers or shallow wells, or swallowing water while swimming [2].
  • Travel: giardiasis, cryptosporidiosis and cyclosporiasis are among the more common parasitic infections travelers pick up from contaminated food and drink [12].
  • Animals and food: pets can carry parasites that pass to people, and undercooked meat can transmit Trichinella [13].
  • A weakened immune system raises the risk of getting sick from Giardia [2].

Intestinal parasites are less common in North America than in many other parts of the world [6].

How parasites are actually diagnosed

The CDC says your clinician will choose tests based on your symptoms, other medical conditions and travel history [3]. Diagnosis can be difficult, so more than one kind of test may be needed [3].

  • Stool O&P exam. A lab looks for eggs (ova) or parasites [3]. The CDC recommends three or more stool samples collected on separate days, often in containers with preservative [3]. Several samples help because people with Giardia don’t pass the parasite every time [14].
  • Antigen tests. These detect parasite proteins in stool and are commercially available for Cryptosporidium, Entamoeba histolytica and Giardia [4]. For giardiasis, the CDC calls microscopy with a fluorescent antibody test the gold standard [15].
  • DNA (PCR) tests. CDC laboratory guidance says microscopy is still the “gold standard,” with PCR used when a parasite can’t be clearly identified [5]. Hospital multiplex stool panels include only some parasites, such as Giardia, Cryptosporidium and E. histolytica [16].
  • Pinworm “tape test.” Clear tape pressed near the anus first thing in the morning, three mornings in a row, collects eggs [17]. Stool samples usually don’t contain enough eggs, and there is no blood test for pinworm [18].
  • Blood tests. Some parasitic infections can be found in blood, but there is no single blood test that looks for all of them [3].

Timing matters. Antacids, mineral oil, barium, bismuth and antibiotics can make stool samples unsatisfactory for testing, so tell your clinician what you’ve taken recently [19].

Mainstream view and treatment

When a test confirms an infection, a healthcare provider can prescribe medicine to treat it [2]. Most people with healthy immune systems recover from Crypto without treatment [8]. For Giardia, the CDC recommends retesting only if symptoms continue after treatment is complete [15]. Prescription drugs exist for people with symptoms who carry Blastocystis, but its role in disease is still unclear [10].

“Parasite cleanses”: what the evidence says

“Parasite cleanses” usually combine herbs, oils or supplements with a special diet [7].

  • No evidence they work. Cleveland Clinic says “no scientific evidence” shows parasite cleanse diets work, and most people with a real infection need prescription treatment [6]. Evidence: no trials show cleanses clear infections.
  • Not approved. None of the vitamins, oils, herbs and other supplements in these products are FDA-approved as treatments for parasitic infections [7].
  • Possible harms. Cleanses can cause nutrient deficiencies and diarrhea that leads to dehydration [6]. The FDA and FTC have acted against some companies selling “detox” and cleansing products for hidden ingredients or false claims [20].
  • Missed diagnoses. Self-treating a suspected parasite with herbs and spices “is just not a good idea,” a Cleveland Clinic dietitian says [6].

Functional & integrative perspective · evidence varies

The functional and integrative perspective

Some functional and integrative practitioners think parasites play a bigger role in chronic illness than mainstream medicine recognizes [21, 22]. These are their views, described in their terms. Read each one as a hypothesis.

Why some practitioners suspect parasites

  • Gut infections as a “core pathology.” Chris Kresser, a functional-medicine practitioner, lists gut infections, including parasites, among eight “core pathologies” he believes underlie chronic disease [21]. Evidence: a practitioner’s model.
  • Links with IBS. Kresser writes that parasites such as Blastocystis and Giardia “have also been correlated with progression of IBS, although causation has not yet been confirmed” [23]. Evidence: associations; his own caveat applies.
  • “Low-grade” infections. In a webinar hosted by a stool-testing lab, chiropractor Dan Kalish describes chronic low-grade gut infections as “way more common than you would imagine” and “very difficult to find on labs” [22]. Evidence: clinical experience, published by a company that sells testing.

What to know about their testing and treatment

  • Broad DNA stool panels. Some practitioners use commercial DNA-based (PCR) stool panels [22]. In a 2020 study with stool samples spiked with known organisms, one widely marketed panel had a specificity of only 26% because of many false positives, which the authors warned could lead to antiparasitic treatment “in the absence of any true pathogens” [24]. Evidence: one independent lab study.
  • Uncertain findings. A positive result for an organism like Blastocystis is hard to interpret, because the CDC says its role in disease is unclear [10]. Evidence: mainstream uncertainty.
  • Herbal “cleanses.” These are not FDA-approved for parasitic infections [7]. Evidence: no controlled trials showing benefit.

This is not medical advice. If you’re working with a practitioner who suggests parasite testing or treatment, it’s reasonable to ask which test they’re using, how it was validated and how results will change your care.

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What to do next

General information, not a diagnosis or a treatment plan. A licensed clinician can tell you which of these steps fit you.

  1. Step 1Start this week

    • Make notes about your symptoms before your visit: when they started, how often they happen and whether they’re getting better or worse [25].
    • Write down any recent travel, untreated water, childcare contact or animal contact, since these raise the risk of some infections [2, 12].
    • Our symptom checklist can help you organize what you notice.

  2. Step 2Choose who to see

    • Start with your primary care doctor: AHRQ calls your doctor “the first person to ask” [26].
    • If tests or symptoms are hard to sort out, ask whether a referral to an infectious disease or gastroenterology specialist would help. Endoscopy is used when stool exams don’t find the cause of diarrhea [3].

  3. Step 3Tests to ask about

    • A stool O&P exam on three or more samples from separate days [3], or antigen or PCR testing where it fits [4, 5].
    • A tape test if pinworm is a concern [17].
    • Ask how a commercial stool panel was validated before relying on it; one study found many false positives [24].

  4. Step 4Everyday steps mainstream sources support

    • Wash your hands with soap and water at key times, such as after using the toilet and before preparing food [8].
    • Avoid untreated water from springs, lakes and rivers [2]. Evidence: public health guidance.

  5. Step 5Prepare for the appointment

  6. Step 6When to get urgent care

    • Dehydration is a symptom of Giardia [2]. Signs of severe dehydration include not urinating, dizziness, confusion and a rapid heartbeat [27].
    • See When to get urgent help.

Questions to ask your doctor

  • Could a parasite explain my symptoms, or are other causes more likely?
  • Which test fits my situation: stool O&P, antigen or PCR?
  • How many samples should I collect, and how?
  • Should I stop any medicines before collecting samples?
  • If a test finds Blastocystis, what does that mean for me?
  • If my results are negative and symptoms continue, what should we check next?

When to get urgent help

Get prompt medical care for signs of severe dehydration, such as not urinating or very dark urine, dizziness, a rapid heartbeat or rapid breathing [27]. Call 911 or your local emergency number if someone loses consciousness, becomes confused or has a seizure [27].

Sources

Numbers in brackets in the text link to these sources. Links were checked in October 2026. Some sources are written by people who support a particular approach; we've tried to say so in the text.

  1. Centers for Disease Control and Prevention (CDC). About Parasites. Web page, accessed October 18, 2026. https://www.cdc.gov/parasites/about/index.html
  2. CDC. About Giardia Infection. Web page, accessed October 18, 2026. https://www.cdc.gov/giardia/about/index.html
  3. CDC. Diagnosis of Parasitic Diseases. Web page, accessed October 18, 2026. https://www.cdc.gov/parasites/testing-diagnosis/index.html
  4. CDC DPDx – Laboratory Identification of Parasites of Public Health Concern. Detection of parasite antigens. Web page, accessed October 18, 2026. https://www.cdc.gov/dpdx/diagnosticprocedures/stool/antigendetection.html
  5. CDC DPDx. Molecular diagnosis. Web page, accessed October 18, 2026. https://www.cdc.gov/dpdx/diagnosticprocedures/stool/moleculardx.html
  6. Cleveland Clinic Health Essentials. Parasite Cleanse: Is It Safe? And Side Effects. Web page, accessed October 18, 2026. https://health.clevelandclinic.org/parasite-cleanse
  7. Wen LS (interviewed). Do you need to cleanse your body of parasites? A doctor explains the social media fad. CNN, August 6, 2025. Web page, accessed October 18, 2026. https://www.cnn.com/2025/08/06/health/parasite-worm-detox-cleanse-wellness
  8. CDC. About Crypto Infections. Web page, accessed October 18, 2026. https://www.cdc.gov/cryptosporidium/about/index.html
  9. CDC. About Pinworm Infection. Web page, accessed October 18, 2026. https://www.cdc.gov/pinworm/about/index.html
  10. CDC. About Blastocystis. Web page, accessed October 18, 2026. https://www.cdc.gov/blastocystis/about/index.html
  11. CDC. About Soil-Transmitted Helminths. Web page, accessed October 18, 2026. https://www.cdc.gov/sth/about/index.html
  12. CDC. Planning for Parasitic Diseases When Travelling. Web page, accessed October 18, 2026. https://www.cdc.gov/parasites/travelers/index.html
  13. CDC. What Causes Parasitic Diseases. Web page, accessed October 18, 2026. https://www.cdc.gov/parasites/causes/index.html
  14. CDC. Testing and Diagnosis for Giardia Infection. Web page, accessed October 18, 2026. https://www.cdc.gov/giardia/testing/index.html
  15. CDC. Clinical Testing and Diagnosis for Giardia Infection. Web page, accessed October 18, 2026. https://www.cdc.gov/giardia/hcp/diagnosis-testing/index.html
  16. Garcia LS, Arrowood M, Kokoskin E, et al. Practical guidance for clinical microbiology laboratories: laboratory diagnosis of parasites from the gastrointestinal tract. Clin Microbiol Rev. 2017;31(1):e00025-17. PMID: 29142079. https://pmc.ncbi.nlm.nih.gov/articles/PMC5740970/
  17. CDC. Diagnosing Pinworms. Web page, accessed October 18, 2026. https://www.cdc.gov/pinworm/diagnosing/index.html
  18. CDC. Clinical Overview of Pinworm Infection. Web page, accessed October 18, 2026. https://www.cdc.gov/pinworm/hcp/clinical-overview/index.html
  19. CDC DPDx. Specimen Collection (stool). Web page, accessed October 18, 2026. https://www.cdc.gov/dpdx/diagnosticprocedures/stool/specimencoll.html
  20. National Center for Complementary and Integrative Health (NCCIH). “Detoxes” and “Cleanses”: What You Need To Know. Web page, accessed October 18, 2026. https://www.nccih.nih.gov/health/detoxes-and-cleanses-what-you-need-to-know
  21. Kresser C. Why We Get Sick and How To Get Well. ChrisKresser.com. Web page, accessed October 18, 2026. https://chriskresser.com/why-we-get-sick-and-how-to-get-well/
  22. Kalish D. The Role of Parasitic Infections in Chronic Health Imbalance (webinar). Diagnostic Solutions Laboratory, March 29, 2021. Web page, accessed October 18, 2026. https://www.diagnosticsolutionslab.com/webinars/role-parasitic-infections-chronic-health-imbalance
  23. Kresser C. IBS – 5 Causes That Your Doctor May Overlook. ChrisKresser.com. Web page, accessed October 18, 2026. https://chriskresser.com/5-causes-of-ibs-your-doctor-may-not-be-looking-for/
  24. Gingras BA, Maggiore JA. Performance of a new molecular assay for the detection of gastrointestinal pathogens. Access Microbiol. 2020;2(10). PMID: 33195974. https://pmc.ncbi.nlm.nih.gov/articles/PMC7660239/
  25. National Institute on Aging (NIA). What Do I Need to Tell the Doctor? Accessed October 17, 2026. https://www.nia.nih.gov/health/medical-care-and-appointments/what-do-i-need-tell-doctor
  26. Agency for Healthcare Research and Quality (AHRQ). Next Steps After Your Diagnosis: Finding Information and Support (patient booklet, PDF). Accessed October 17, 2026. https://www.ahrq.gov/sites/default/files/wysiwyg/patients-consumers/diagnosis-treatment/diagnosis/diaginfo/diaginfo.pdf
  27. MedlinePlus Medical Encyclopedia. Dehydration. Web page, accessed October 18, 2026. https://medlineplus.gov/ency/article/000982.htm

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