Blastocystis hominis


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.

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
Blastocystis hominis

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.


If you have diarrhea and related symptoms, the cause may be difficult to diagnose. Even if blastocystis is found in your stool, it might not be causing your symptoms. Often another food- or water-borne organism is the likely cause of illness.

Your doctor will take your medical history, ask you about recent activities, such as traveling, and perform a physical exam. A number of lab tests help diagnose parasitic diseases and other noninfectious causes of gastrointestinal symptoms:

  • Stool (fecal) exam. This test looks for parasites. Your doctor might give you a container with preservative fluid for your stool sample. Refrigerate — don't freeze — your samples until you take them to your doctor's office or lab.
  • Endoscopy. If you have symptoms, but the fecal exam doesn't reveal the cause, your doctor might request this test. When you're under sedation, a special camera on a tube is used to exam your digestive tract.
  • Blood tests. A blood test that can detect blastocystis is available but not commonly used. However, your doctor might order blood tests to look for other causes of your signs and symptoms.


If you have a blastocystis infection without signs or symptoms, then you don't need treatment. Mild signs and symptoms might improve on their own within a few days.

Potential medications for eliminating a blastocystis infection and improving symptoms include:

  • Antibiotics, such as metronidazole (Flagyl) or tinidazole (Tindamax)
  • Combination medications, such as sulfamethoxazole and trimethoprim (Bactrim, Septra, others)
  • Anti-protozoal medications, such as paromomycin or nitazoxanide (Alinia)

Responses to these medications vary greatly. Also, because the organism might not be the cause of your symptoms, improvement might be due to the medication's effect on another organism.

Preparing for your appointment

You'll likely see your primary care doctor. However, in some cases, you might be referred to someone who specializes in either infectious disease or in digestive system disorders (gastroenterologist).

Here's some information to help you get ready for your appointment.

What you can do

Be aware of pre-appointment restrictions. When you make the appointment, ask if there's anything you need to do in advance, such as restrict your diet.

Make a list of:

  • Your symptoms, and when they began
  • Key personal information, including any major stresses or recent life changes and whether you've recently traveled to a developing country
  • All medications, vitamins or supplements you take, including doses
  • Questions to ask your doctor

Questions to ask your doctor include:

  • What's the most likely cause of my symptoms?
  • Are there other possible causes?
  • What tests do I need?
  • What treatments are available, and which one do you recommend for me?
  • Should I change my diet?
  • Are there brochures or other printed material that I can take home with me? What websites do you recommend?

Don't hesitate to ask other questions.

What to expect from your doctor

Your doctor is likely to ask you questions, including:

  • Do you have symptoms all the time, or do they come and go?
  • How severe are your symptoms?
  • Does anything seem to improve your symptoms?
  • What, if anything, appears to worsen your symptoms?
  • Do you have any other health conditions?

What you can do in the meantime

If your symptoms are related to blastocystis, they'll likely go away on their own before you even see your doctor. Stay well hydrated. Oral rehydration solutions — available through drugstores and health agencies worldwide — can replace lost fluids and electrolytes.

Content From Mayo Clinic Updated: 02/08/2023
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