Blastocystis hominis (B. hominis) is a microscopic parasite sometimes found in the stools of healthy people as well as in the stool of those who have diarrhea, abdominal pain or other gastrointestinal problems. However, it's not clear whether Blastocystis hominis actually causes an infection or if it simply resides in the digestive tract without causing harm.

If you have Blastocystis hominis in your stool without signs or symptoms of an infection, you don't need treatment. Even if you have symptoms, the Blastocystis hominis infection usually clears up on its own. If you have signs and symptoms that don't improve, your doctor may prescribe medications, although the drugs used to treat Blastocystis hominis infections aren't always effective.

Signs and symptoms that might be associated with a blastocystis infection include:

• Diarrhea
• Nausea
• Abdominal cramps
• Bloating
• Excessive gas (flatulence)
• Anal itching
• Fatigue

When to see a doctor
See your doctor if you have signs and symptoms associated with a blastocystis infection, such as diarrhea, cramps and fatigue that last for longer than three days.

Once thought to be a harmless yeast, B. hominis is a parasite, a microscopic single-celled organism (protozoan). It behaves like a tiny animal — hunting and gathering other microbes for food. Many protozoa inhabit your gastrointestinal tract and are harmless; others cause disease.

Whether B. hominis is the type of protozoa that causes disease is controversial. Many people who carry Blastocystis hominis have no signs or symptoms. Some people who have this parasite have diarrhea and other symptoms, which some experts believe is caused by an infection with B. hominis. However, Blastocystis hominis often appears with other organisms that may be the actual cause of the signs and symptoms commonly associated with blastocystis infection. So experts aren't sure whether Blastocystis hominis causes disease or merely serves as an indicator of other agents that might cause diarrheal symptoms. It's also possible that some people may be carriers of B. hominis and don't exhibit any signs or symptoms of infection, while other people are more susceptible to infection.

Many types of protozoa get into the intestinal tract through oral-fecal contact, such as occurs when a person who doesn't wash his or her hands thoroughly after using the toilet prepares food. No one knows for certain how B. hominis is transmitted, but experts suspect it's through oral-fecal contact. Experts do know that the incidence of infection associated with Blastocystis hominis increases in places with inadequate sanitation and poor personal hygiene.

Blastocystosis is common, and anyone can get the infection. You may be at higher risk if you travel where sanitation is inadequate or where the water may not be safe.

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're likely to start by first seeing your family doctor or a general practitioner. However, in some cases when you call to set up an appointment, you may be referred immediately to an infectious disease specialist or a gastroenterologist.

Because appointments can be brief, it's a good idea to be well prepared for your appointment. Here's some information to help you get ready for your appointment, and what to expect from your doctor.

What you can do

• Be aware of any pre-appointment restrictions. At the time you make the appointment, be sure to ask if there's anything you need to do in advance, such as restrict your diet.
• Write down any symptoms you're experiencing, including any that may seem unrelated to the reason for which you scheduled the appointment.
• Write down key personal information, including any major stresses or recent life changes. Let your doctor know if you've recently traveled out of the country, especially if you traveled to a developing country.
• Bring a list of all medications, as well as any vitamins or supplements, that you're taking.
• Write down questions to ask your doctor.

Your time with your doctor is limited, so preparing a list of questions ahead of time will help you make the most of your time together. List your questions from most important to least important in case time runs out. For a Blastocystis hominis infection, some basic questions to ask your doctor include:

• What is likely causing my symptoms?
• Are there other possible causes for my symptoms?
• Do I need any tests?
• What treatment do you recommend?
• Are there alternatives to the approach you're suggesting?
• Are there any dietary restrictions I need to follow?
• Do I need to see a specialist?
• Is there a generic alternative to the medicine you're prescribing me?
• Are there any brochures or other printed material that I can take home with me? What Web sites do you recommend visiting?

In addition to the questions that you've prepared to ask your doctor, don't hesitate to ask questions during your appointment at any time that you don't understand something.

What to expect from your doctor
Your doctor is likely to ask you a number of questions. Being ready to answer them may reserve time to go over any points you want to spend more time on. Your doctor may ask:

• When did you first begin experiencing symptoms?
• Have your symptoms been continuous, or occasional?
• How severe are your symptoms?
• What, if anything, seems to improve your symptoms?
• What, if anything, appears to worsen your symptoms?
• Have you traveled out of the country recently?
• Do you have any other health conditions?

What you can do in the meantime
If your symptoms are related to a blastocystis infection, they may go away on their own before you even see your doctor. Be sure to stay well hydrated in the meantime. An oral rehydration solution (ORS) is a good way to replace lost fluids. These solutions contain water and salts in specific proportions to replenish both fluids and electrolytes. They also contain glucose or another carbohydrate, such as rice powder, to enhance absorption in the intestinal tract.

Bottled oral rehydration products are available in drugstores in developed areas, and many pharmacies carry their own brands. Packets of powdered oral rehydration solution are available at stores, pharmacies and health agencies in most countries. Reconstitute the powder in bottled or boiled water according to the directions on the package.

If no oral rehydration solutions are available, you can make your own by combining  about 1 quart (about 1 liter) of bottled or boiled water with 6 level teaspoons (about 30 milliliters) of sugar and 1 level teaspoon (about 5 milliliters) of table salt. Anti-diarrheal medications aren't generally recommended, because they can make some diarrheal illnesses worse.

The cause of your diarrhea may be difficult to diagnose. 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:

• Stool (fecal) exam. Also called an ova and parasite test, this test looks for parasites or their eggs (ova) that cause signs and symptoms, such as diarrhea and abdominal cramping and bloating. Your doctor may ask you for several stool samples, each from different days.

  Your doctor may give you a special container with preservative fluid for your stool samples. If not, refrigerate your samples until you take them to your doctor's office or the lab your doctor designates. Don't freeze the samples.

• Endoscopy. If you have diarrhea, but the fecal exam doesn't reveal the cause, your doctor may request this test. A doctor, usually a gastroenterologist, inserts a tube into your mouth or rectum to look for the cause of your symptoms. You'll be sedated for this test, and you'll need to fast beginning at midnight the night before the test.
• Blood tests. Although there are no blood tests that can detect Blastocystis hominis, your doctor may order blood tests to look for other causes of your signs and symptoms. That's because when your body is infected with a parasite, your immune system tries to fight the infection, which leaves antibodies or parasite antigens or both in your system. A lab can detect some parasites by examining a sample of your blood for the antibodies or antigens.
• Scans. Some parasitic diseases may cause swelling or scarring of internal organs. Your doctor may request an X-ray, magnetic resonance imaging (MRI) or a computerized tomography (CT) scan to detect the swelling or scarring.

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

Potential medications for treating blastocystosis include the antibiotic metronidazole (Flagyl), the combination medication sulfamethoxazole and trimethoprim (Bactrim, Septra, others) and the antiprotozoal medication iodoquinol (Yodoxin, others). However, keep in mind that response to medication for blastocystis varies greatly from person to person. And, because the symptoms you're having might be unrelated to blastocystis, it's also possible that any improvement may be due to the medication's effect on another organism.

You may be able to prevent blastocystis or other gastrointestinal infection by taking a number of precautions while traveling in high-risk countries.

Watch what you eat
The general rule of thumb is this: Boil it, cook it, peel it or forget it. Unfortunately, most travelers don't stick to these guidelines all of the time. Remember these tips:

• Don't buy food from street vendors.
• 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.
• Munch on dry foods — like breads — and high-sugar-content foods, such as jellies and syrups.
• Stick to fruits and vegetables that you can peel yourself, such as bananas, oranges and avocados. Stay away from salads and unpeelable fruits, such as grapes and berries.

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 consume local water, boil it for five to 10 minutes.
• Avoid ice cubes or fruit juices made with tap water.
• Beware of sliced fruit that may have been washed in contaminated water.
• Don't swim in water that may be contaminated.
• Keep your mouth closed while showering.
• 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.
• Use bottled water to brush your teeth.
• Use bottled or boiled water to mix baby formula.
• Make sure hot beverages, such as coffee or tea, are steaming hot.

If it's not possible to buy bottled water or boil your water, bring some means to purify water: Consider a water-filter pump with a microstrainer filter that can filter out small microorganisms. Look in camping stores for a filter that is certified by the National Science Foundation.

Another approach is to chemically disinfect water with iodine or chlorine. Iodine tends to be more effective, but reserve it for short trips, because too much iodine can be harmful to your system. You can purchase iodine tablets or crystals at camping stores and pharmacies. Be sure to carefully follow the directions.

Take precautions against passing a parasite to others
If you have a Blastocystis hominis infection or another gastrointestinal infection, good personal hygiene will help keep you from spreading the infection to others:

• Wash hands with soap and water frequently, especially after using the toilet and before handling food. Rub soapy, wet hands together for at least 15 seconds before rinsing. If soap and water aren't available, use an alcohol-based hand sanitizer.
• Wash hands well after changing a diaper, especially if you work in a child care center, even if you wear gloves.