Blastocystis hominis


Blastocystis hominis is a microscopic organism that sometimes is found in the stools of people who have ingested contaminated food or water. It can be found in healthy people who aren't having digestive symptoms, and it's also sometimes found in the stools of people who have diarrhea, abdominal pain or other gastrointestinal problems.

Researchers don't fully understand the role Blastocystis hominis plays, if any, in causing disease. Certain forms of the organism might be more likely to be linked to an infection with symptoms. Most commonly, blastocystis simply lives in a person's digestive tract without causing harm.

Blastocystis hominis, also known as blastocystis spp or Blastocystis hominis infection, usually clears on its own. There are no proven treatments for these infections.


Signs and symptoms possibly associated with Blastocystis hominis include:

  • Watery diarrhea
  • Nausea
  • Abdominal pain
  • Bloating
  • Excessive gas (flatulence)
  • Loss of appetite
  • Fatigue

When to see a doctor

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


Blastocystis is a parasite — a microscopic single-celled organism (protozoan). Many protozoans normally live in your gastrointestinal 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. Blastocystis often appears with other organisms, so it's not known whether it causes disease.

Experts suspect that blastocystis gets into the digestive system when people eat contaminated food or are exposed to the stool of a contaminated person, such as when changing a diaper in a child care setting. Rates of the organism in stool increase where there's inadequate sanitation and poor personal hygiene.

Blastocystis hominis Blastocystis hominis

Risk factors

Blastocystis hominis is common, and anyone can have the organism in his or her stools. You might be at higher risk if you travel or live where sanitation is inadequate or where the water might not be safe or if you handle contaminated animals, such as pigs and poultry.


If you have diarrhea associated with Blastocystis hominis, 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.


You might be able to prevent Blastocystis hominis or other gastrointestinal infection by taking precautions, especially while traveling in high-risk countries.

Watch what you eat

The general rule of thumb is this: If you can't boil it, cook it or peel it — forget it.

  • Avoid food from street vendors.
  • Don't eat soft-cooked eggs.
  • Avoid unpasteurized milk and dairy products, including ice cream.
  • Avoid raw or undercooked meat, fish and shellfish.
  • Steer clear of moist food at room temperature, such as sauces and buffet offerings.
  • Eat foods that are well-cooked and served hot.
  • Stick to fruits and vegetables that you can peel yourself, such as bananas, oranges and avocados. Stay away from salads and fruits you can't easily peel, such as grapes and berries.
  • Avoid frozen pops and flavored ice.
  • Skip salsa and other condiments made with fresh ingredients.

Don't drink the water

When visiting high-risk countries, keep the following tips in mind:

  • Avoid unsterilized water — from tap, well or stream. If you need to drink or wash fruits or vegetables in local water, boil it for at least three minutes and let it cool to room temperature.
  • Avoid ice cubes or fruit juices 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.

Feel free to drink canned or bottled drinks in their original containers — including water, carbonated beverages, beer or wine — as long as you break the seals on the containers yourself. Wipe off any can or bottle before drinking or pouring.

You can chemically disinfect water with iodine or chlorine. Iodine tends to be more effective, but limit its use, because too much iodine can be harmful to your body.

Take precautions against passing a parasite to others

If you have Blastocystis hominis or another gastrointestinal infection, good personal hygiene can help keep you from spreading the infection to others:

  • Wash hands with soap and water frequently, especially after using the toilet and before, during and after handling food. Rub soapy, wet hands together for at least 20 seconds before rinsing. Lather the backs of your hands and between your fingers. Dry your hands well with a clean towel.

    If soap and water aren't available, use an alcohol-based hand sanitizer that contains at least 60 percent alcohol.

  • Wash hands well after changing a diaper, especially if you work in a child care center, even if you wear gloves.


The cause of your diarrhea might be difficult to diagnose. Even if Blastocystis hominis is found in your stool, it might not be causing your symptoms. More commonly, it suggests you've been exposed to contaminated food or water that contains other organisms that can cause gastrointestinal symptoms.

Your doctor likely 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 or their eggs. Your doctor might give you a special container with preservative fluid for your stool samples. 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. After you're sedated, a doctor, usually a gastroenterologist, inserts a tube into your mouth or rectum to look for the cause of your symptoms. You'll need to fast beginning the night before the test.
  • 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 Blastocystis hominis 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 treating Blastocystis hominis 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)

Response to medication for Blastocystis hominis varies greatly from person to person. And 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 an 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 hominis, 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:
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