Blastocystis is a microscopic parasite that can live in your digestive tract. Researchers don't fully understand the role blastocystis plays, if any, in causing disease. Some people experiencing diarrhea, abdominal pain or other gastrointestinal problems have blastocystis organisms in their stool.

Most commonly, however, blastocystis organisms simply live in a person's digestive tract without causing harm.

Blastocystis may be transmitted through food or water or by contact with human or animal feces. Blastocystis infection is generally more common among people who live in or travel to developing countries and among people who work with animals.

Blastocystis in humans was once identified as a single species, Blastocystis hominis. Researchers have found several variations — either different species or different strains within a species. The scientific name used now is Blastocystis spp, an abbreviation that means "multiple species." A blastocystis infection is called blastocystosis.


Signs and symptoms possibly associated with blastocystis include:

  • Watery diarrhea
  • Nausea
  • Abdominal pain
  • Bloating
  • Excessive gas
  • Loss of appetite
  • Weight loss
  • Anal itching
  • Fatigue

When to see a doctor

See your doctor if you have signs and symptoms, such as diarrhea or abdominal pain, that lasts longer than three days.


Blastocystis is a microscopic single-celled organism (protozoan). Many parasitic protozoans normally live in your digestive tract and are harmless or even helpful; others cause disease.

Blastocystis hominis

Blastocystis hominis

Blastocystis is a microscopic single-cell organism (protozoan) that lives in the digestive tract. Many protozoans normally live in the digestive tract and are harmless or even helpful, but some cause disease.

It's not clear whether blastocystis causes disease. Most people who carry the organism have no signs or symptoms, but it's also found in people who have diarrhea and other digestive problems.

Explanations for this variability include:

  • Some types of blastocystis may be more likely to cause disease than others.
  • Some people may be more vulnerable to blastocystis infection.
  • Disease may be caused by other organisms that coexist with Blastocystis.

Blastocystis can be passed between people and from animals to people. Transmission may be from:

  • Contaminated food or water
  • Exposure to human or animal feces

Risk factors

Blastocystis is common, but you may have a greater risk of exposure if you:

  • Work with animals
  • Are exposed to human feces at work, such as a children's day care
  • Travel to a country with poor water sanitation


If you have diarrhea associated with blastocystis, it's likely to be self-limiting. However, anytime you have diarrhea, you lose vital fluids, salts and minerals, which can lead to dehydration. Children are especially vulnerable to dehydration.


The best practice for preventing blastocystis infection is practicing good hygiene:

  • Wash fruits and vegetables before eating
  • Keep cooking surfaces clean
  • Wash your hands frequently

Handwashing tips

Wash your hands often with soap and water for at least 20 seconds. When you can't use soap and water, use a hand sanitizer that contains at least 60% alcohol.

Wash your hands frequently, particularly:

  • Before, during and after meal preparation
  • After using the toilet
  • After changing a diaper or helping a child use the toilet
  • After helping a person who is sick with vomiting or diarrhea
  • After touching animals or handling animal food or feces
  • After handling garbage
  • Before and after cleaning a cut or wound

Travel tips

When you travel, you can take steps to lower your risk of exposure to blastocystis. A general guideline is to avoid eating what you can't boil, cook or peel. More specifically, avoid:

  • Food from street vendors
  • Unpasteurized milk and dairy products, including ice cream
  • Raw or undercooked meat, fish, shellfish or eggs
  • Food at room temperature, such as sauces and buffet offerings
  • Fresh greens; foods that can't be peeled, such as berries; fruits or vegetables that you did not peel yourself
  • Frozen pops and flavored ice
  • Dishes or condiments made with uncooked fruits or vegetables

Safe drinking-water tips

If you're visiting a country with poor sanitation or possible unsafe drinking water, use the following tips:

  • Avoid unsterilized water — from tap, well or stream.
  • If you need to use unsterilized water for drinking or washing food items, boil the water for at least three minutes and let it cool.
  • Use a chemical purifier for water — usually a combination of bleach and iodine — that is often available at a sporting goods store.
  • Avoid ice cubes or beverages made with tap water.
  • Keep your mouth closed while showering.
  • Use bottled water to brush your teeth.
  • Make sure hot beverages, such as coffee or tea, are steaming hot.
  • Drink bottled beverages from original, unopened containers after cleaning them.

Feb 09, 2023

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  2. Blastocystis spp. FAQs. Centers for Disease Control and Prevention. https://www.cdc.gov/parasites/blastocystis/faqs.html. Accessed Dec. 2, 2020.
  3. Leder K, et al. Blastocystis species. https://www.uptodate.com/contents/search. Accessed Dec. 2, 2020.
  4. Diarrhea. Merck Manual Professional Version. https://www.merckmanuals.com/professional/gastrointestinal-disorders/symptoms-of-gastrointestinal-disorders/diarrhea. Accessed Dec. 3, 2020.
  5. When and how to wash your hands. Centers for Disease Control and Prevention. https://www.cdc.gov/handwashing/when-how-handwashing.html. Accessed Dec. 2, 2020.
  6. Food and water safety. Centers for Disease Control and Prevention. https://wwwnc.cdc.gov/travel/page/food-water-safety. Accessed Dec. 2, 2020.
  7. Water disinfection. Centers for Disease Control and Prevention. https://wwwnc.cdc.gov/travel/page/water-disinfection. Accessed Dec. 3, 2020.
  8. Freedman S. Oral rehydration therapy. https://www.uptodate.com/contents/search. Accessed Dec. 2, 2020.


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