Intestinal Parasites: Symptoms, Warning Signs and Stool Tests
The essentials
Intestinal parasites usually cause diarrhea that will not go away, gas, cramps and greasy stools, or night-time anal itching in children. In the United States, Giardia is the most frequent. Diarrhea lasting more than a week, travel, untreated water or a weak immune system justify a stool test, chosen according to the parasite suspected.
When are intestinal parasite symptoms an emergency?
Healthy people often recover without treatment[10]. Urgency comes from dehydration, blood in the stool or a fragile patient.
When to see a doctor
- Emergency: call 911 in case of confusion, fainting, little or no urine, severe belly pain, or bloody diarrhea with high fever, as in amebic dysentery[6]. See our guide to signs of dehydration.
- Within 24 to 48 hours for diarrhea with fever, blood or mucus, or severe cramps: national guidelines advise a stool test in these cases[13]. The same applies to babies, older adults and anyone with weakened immunity (HIV, cancer, immune-lowering drugs), in whom parasites can cause severe, long-lasting diarrhea[10].
- At your next appointment for diarrhea that has lasted more than seven days, greasy floating stools, weight loss, or anal itching at night[12][7].
What are the symptoms of intestinal parasites?
Typical symptoms are diarrhea, belly pain, gas, nausea, vomiting, fever, weight loss, blood or mucus in the stool, and foul-smelling greasy stools that float[10]. Some people have no symptoms at all[3]. The pattern and timing point to one parasite more than another.
- Common Short diarrhea that is not a parasite. A healthy person with diarrhea of less than seven days and no travel does not need a parasite exam[12].
- Common Giardia. Two to five loose stools a day and growing tiredness, then gas, greasy stools, cramps and nausea. Symptoms start one to two weeks after infection and last two to six weeks[1]. Childcare, untreated water and swallowed pool water raise the risk[1]. Chronic forms can be mistaken for irritable bowel syndrome[12].
- Common Pinworm. Itching around the anus, mainly at night, in children and the people who live with them. No worm infects more people in the United States[7]. If you actually see threads in the toilet, read worms in stool.
- Possible Cryptosporidium. Watery diarrhea starting 2 to 10 days after infection and lasting one to two weeks[3]. It is the leading cause of waterborne disease in the US[3] and spreads in swimming pools and hot tubs, because some parasites are not easily killed by chlorine[10].
- Possible Cyclospora. Watery diarrhea, weight loss, bloating and fatigue about a week after infection; it may stop and come back[5]. Details in our Cyclospora guide.
- Rare Amebiasis. Only 10% to 20% of people infected with Entamoeba histolytica become ill; in the US it is seen mostly after travel to tropical areas with poor sanitation[6].
- Rare Strongyloides and other worms. Strongyloides can become life-threatening in people who start corticosteroids or have immune disorders[8].
What tests check for intestinal parasites?
There is no single "parasite test": the doctor chooses according to the suspected parasite.
| Test | What it looks for | Good to know |
|---|---|---|
| Giardia or Cryptosporidium antigen test (stool) | Proteins of these two parasites | Faster than the microscope; sensitivity 91% to 100%[12] |
| Stool PCR or multiplex panel | DNA of many germs at once | Suggested as the first step for diarrhea of seven days or more[12]; see multiplex stool panels |
| Ova and parasites (O&P) exam | Eggs, cysts and parasites under the microscope | Done if the first tests are negative[12]; how it works: ova and parasites test |
| Specific Cryptosporidium test | Acid-fast stain, antigen or PCR | Not routine in most labs: must be requested[4] |
| Tape test | Pinworm eggs collected from the anal skin | Stool tests usually miss pinworm[7] |
| Blood count with eosinophils | Immune cells raised by worms | Eosinophil count, part of the complete blood count |
For an overview of every stool exam, from culture to calprotectin, see our guide to stool test types.
A stool panel or O&P report full of Latin names? BloodSense translates each line into everyday language, so you arrive at your appointment knowing what to ask.
Explain my stool test reportHow do I read my parasite test results?
| Test | Result | Reference range | Flag |
|---|---|---|---|
| Giardia antigen, EIA | Detected | Not detected | A |
| Cryptosporidium antigen, EIA | Not detected | Not detected | |
| Ova and parasites, concentrate and trichrome | Blastocystis hominis, few | No parasites seen | |
| Eosinophils, absolute | 210 cells/µL | 0-500 cells/µL |
- "Detected" with an A (abnormal) flag: Giardia was found.
- "Not detected" means not found in this sample, not ruled out.
- Blastocystis is listed without a flag: many people carry it without symptoms, and whether it causes disease is unclear[9].
- A normal eosinophil count does not rule out Giardia: this blood test does not help for it.
Positive antigen or PCR: a clear answer
A positive result usually explains the symptoms. Giardia, Cryptosporidium and Cyclospora are notifiable diseases in the United States[12], so the lab or your doctor may report the result to the health department. Treatment is not always needed: Cryptosporidium often clears within two weeks in healthy people[12].
Negative result: one sample is not always enough
Parasites are not shed in stool every day[10]. The CDC advises three stool samples over several days for Giardia[2]. With microscopy alone, Giardia is seen in about half of cases with one sample and up to 90% with three or more[12]. If symptoms persist, ask whether another sample or test is needed.
Eosinophils: useful for worms, not for Giardia
A normal count is below 500 cells per microliter, and the test cannot tell an allergy from a parasite[11]. It does not help for Giardia[12]; a high count rather points to worms, and Strongyloides has its own blood test[8].
After treatment: retest only if symptoms persist
Once diarrhea has resolved, most people need no follow-up stool test[13]. Giardia antigen can stay positive after the parasite is gone, and 20% to 40% of people develop temporary lactose intolerance, so bloating after dairy is not always a relapse[12].
What should I write down before my appointment?
- When the diarrhea started, how many stools a day, and whether they are watery, greasy, bloody or contain mucus
- Weight lost, fever, signs of dehydration
- Travel, camping, well water or swimming in the weeks before
- Childcare, a sick person at home, or night-time anal itching in the family
- Medicines that weaken immunity (steroids, chemotherapy), and a possible pregnancy
Questions to ask your doctor
- Which parasites does this test cover, including Cryptosporidium?
- How many stool samples should I bring?
Frequently asked questions
Can you have intestinal parasites without any symptoms?
Yes. Some people with Cryptosporidium have no symptoms[3], and 80% to 90% of people carrying Entamoeba histolytica never become ill[6].
Are intestinal parasites contagious?
Many are. Giardia spreads through contaminated stool, water, food and surfaces, and a few germs are enough to make you sick[1]. Handwashing with soap is the main protection.
How long can I swim in a pool after Cryptosporidium?
Stay out of public pools for two weeks after the diarrhea stops[12], because some parasites are not easily killed by chlorine and can infect other swimmers[10].
Related tests and guides
- O&P stool examInside the microscope exam, and why labs want several samples.
- Worms in stoolWhat visible worms look like and which tests identify them.
- Cyclospora infectionA parasite that needs a specific stool test.
- Multiplex stool panelsHow one PCR test screens for many germs at once.
References
- Centers for Disease Control and Prevention. About Giardia. cdc.gov (accessed November 2, 2026)
- Centers for Disease Control and Prevention. Clinical testing and diagnosis for Giardia. cdc.gov (accessed November 2, 2026)
- Centers for Disease Control and Prevention. About Cryptosporidium. cdc.gov (accessed November 2, 2026)
- Centers for Disease Control and Prevention. Clinical testing and diagnosis for Cryptosporidium. cdc.gov (accessed November 2, 2026)
- Centers for Disease Control and Prevention. Symptoms of cyclosporiasis. cdc.gov (accessed November 2, 2026)
- Centers for Disease Control and Prevention. About amebiasis. cdc.gov (accessed November 2, 2026)
- Centers for Disease Control and Prevention. About pinworm infection. cdc.gov (accessed November 2, 2026)
- Centers for Disease Control and Prevention. About Strongyloides. cdc.gov (accessed November 2, 2026)
- Centers for Disease Control and Prevention. About Blastocystis. cdc.gov (accessed November 2, 2026)
- MedlinePlus Lab Tests. Ova and parasite test. medlineplus.gov (accessed November 2, 2026)
- MedlinePlus Medical Encyclopedia. Eosinophil count - absolute. medlineplus.gov (accessed November 2, 2026)
- Pyzocha N, Cuda A. Common intestinal parasites. Am Fam Physician. 2023;108(5):487-93. aafp.org (accessed November 2, 2026)
- Shane AL, Mody RK, Crump JA, et al. 2017 Infectious Diseases Society of America clinical practice guidelines for the diagnosis and management of infectious diarrhea. Clin Infect Dis. 2017;65(12):e45-e80. doi:10.1093/cid/cix669 (accessed November 2, 2026)
General health information only, not a substitute for a medical examination, diagnosis or treatment. Bloody diarrhea with fever, signs of dehydration or severe belly pain need prompt medical care.
