Blastocystis hominis

Date Updated: 09/12/2026

By Mayo Clinic Staff

Overview

Blastocystis is a group of tiny parasites that can live in the digestive tract. Researchers don't fully understand the role blastocystis plays, if any, in causing disease. Some people with diarrhea, stomach pain or other gastrointestinal issues have blastocystis organisms in their stool.

Blastocystis organisms often live in the digestive tract without causing harm.

Blastocystis may spread through food or water. Contact with human or animal feces also may spread blastocystis. Blastocystis infection tends to be more common among people who live in or visit developing countries. People who work with animals also may be more likely to have the infection.

People with blastocystis infection also may be infected with other pathogens from contaminated food or water. These other pathogens may be the cause of the symptoms.

When it came to blastocystis, researchers used to label all infectious parasites Blastocystis hominis. Now it's clear there are different types or subtypes of the organism. To cover all the options, the parasite may be labeled blastocystis species or blastocystis spp. Spp. is an abbreviation that means multiple species. A blastocystis infection is called blastocystosis.

Symptoms

Symptoms that may be due to blastocystis infection include:

Watery diarrhea.

Feeling sick to the stomach.
Belly pain.
Bloating.
Lots of gas.
Loss of appetite.
Weight loss.
Skin rashes or hives.
Anal itching.
Fatigue.

When to see a doctor

See a healthcare professional if you have diarrhea, belly pain or other symptoms that don't go away after a few days.

Causes

Blastocystis spp are single-celled organisms that you can see only with a microscope. Many single-celled organisms that live in the digestive tract are harmless. They may even be helpful. Others cause disease.

It's not clear whether blastocystis spp cause disease. Most people who carry the organisms have no symptoms. But they're also found in people who have diarrhea and other digestive issues.

Reasons why the effects might differ include:

Some types of blastocystis may be more likely to cause disease than others.
Some people may be more vulnerable to blastocystis infection.
Other organisms that coexist with blastocystis may cause disease.

Blastocystis may be spread by contact with infected stool. The parasite may be passed between people and from animals to people. Transmission may come from:

Consuming contaminated food or water.
Being exposed to human or animal feces.
Having close contact with animals.

Risk factors

Blastocystis is common. You may have a higher risk of exposure if you:

Work with animals.
Are exposed to human feces at work, such as at a child care facility.
Travel to areas with poor water sanitation.

Contaminated food or water is a frequent cause of blastocystis infection. But blastocystis may not be the cause of diarrhea and other digestive issues. Infection with other pathogens in the contaminated food or water may cause the symptoms.

Complications

If you have diarrhea associated with blastocystis infection, it's likely to go away on its own without treatment. But anytime you have diarrhea, you lose vital fluids, salts and minerals. This can lead to dehydration. Children are especially vulnerable to dehydration.

Prevention

Good hygiene is the best way to prevent blastocystis infection. Taking the following measures can help lessen the chance that you'll become infected or spread the organism to others:

Wash your hands with soap and water often. Hand-washing is key after you use the toilet and before you handle food. Rub soapy, wet hands together for at least 20 seconds before rinsing. If soap and water aren't available, use an alcohol-based hand sanitizer with at least 60% alcohol.
Wash fruits and vegetables before eating. Produce can carry organisms from contaminated water or soil or introduced during transport and handling. Washing helps remove any contaminants along with dirt and pesticides.
Keep cooking surfaces clean. To remove any substances on surfaces after use, wash them thoroughly with water and soap and rinse with clean water.

When visiting or living in an area with poor sanitation, take extra steps to help lower the risk of blastocystis infection:

Don't use water that isn't safe. If tap water might be unsafe, use bottled water or water you've boiled or disinfected yourself. Most often, hot drinks are safe. Canned or bottled drinks also are safe, but wipe the outside before you open them. Ask for drinks without ice. Use bottled water to brush your teeth and try not to swallow water in the shower.
Choose hot foods. Don't eat food that's stored or served at room temperature. Cooked, steaming hot foods may be less risky than other foods.
Only eat fruits and vegetables that you can peel yourself. Some examples are bananas, oranges and avocados. Don't eat raw foods that can't be peeled, such as lettuce, grapes and berries.

Diagnosis

Blastocystis is a group of parasites that can live in the digestive tract. Often they live there without causing harm, but sometimes they may cause symptoms.

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 waterborne organism is the cause of illness.

A healthcare professional may take your medical history and do a physical exam. You likely will be asked about recent travel and other activities. Lab tests can help diagnose parasitic diseases and other causes of gastrointestinal symptoms:

Stool exam, also called a fecal exam. This test looks for parasites. You might receive a container with preservative fluid for your stool sample. Refrigerate the samples until you take them to the medical office or lab. Do not freeze the samples.
Endoscopy. If the fecal exam doesn't show the cause of your symptoms, a healthcare professional might order this test. It uses a tiny camera on the end of a long, flexible tube to see into the digestive system.
Blood tests. A blood test to check for blastocystis is available but rarely used. A healthcare professional might order blood tests to look for other causes of your symptoms.

Treatment

If you have a blastocystis infection without symptoms, you don't need treatment. Mild symptoms might go away on their own within a few days.

Medicines to get rid of a blastocystis infection and help symptoms go away might include:

Antibiotics, such as metronidazole (Flagyl) or tinidazole (Tindamax).
Combination medicines, such as sulfamethoxazole-trimethoprim (Bactrim).
Antiprotozoal medicines, such as paromomycin or nitazoxanide (Alinia).

Response to these medicines can vary greatly. People with blastocystis infection also may be infected with other pathogens from contaminated food or water. These other pathogens may be the cause of the symptoms. (3p5)

Because blastocystis infection might not be the cause of your symptoms, the treatment might help symptoms go away by getting rid of another organism.

Preparing for your appointment

You may start by seeing a member of your healthcare team. Depending on your symptoms, you may see a healthcare professional who specializes in digestive system conditions, called a gastroenterologist. Or you may see a healthcare professional who specializes in infectious diseases.

What you can do

To get ready for your appointment:

Ask if there are things you need to do in advance, such as change your diet.
Take note of your symptoms, even if you're not sure they're related to blastocystis infection. Does anything seem to make your symptoms better or worse? Do you notice your symptoms at a certain time of day?
Make a list of all the medicines, vitamins and other supplements that you take and the doses.
Write down key personal information, including any major stresses or recent life changes.
Write down questions to ask your healthcare professional.

What to expect from your doctor

Your healthcare team is likely to ask you questions, such as:

When did you first notice your symptoms?
How bad are your symptoms?
Do you have symptoms all the time, or do they come and go?
What, if anything, seems 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 infection, they'll likely go away on their own before you even see a healthcare professional. Stay well hydrated. Water or drinks with minerals, called rehydration solutions, may help replace fluids.