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03 9331 3122 4/8 Eddy Street, Moonee Ponds VIC 3039

Bowel Parasites

Blastocystis hominis & Dientamoeba fragilis infection

Blastocystis hominis & Dientamoeba fragilis infection


Blastocystis hominis (B. hominis) and Dientamoeba fragilis (D. fragilis) are  microscopic parasites sometimes found in the stools of people who have diarrhea, abdominal pain or other gastrointestinal signs and symptoms. However, it’s not clear whether blastocystis infection, as it’s called, is actually an infection or that it causes any abdominal signs and symptoms.

Many people who carry blastocystis hominis have no signs or symptoms. Some people who have this parasite have diarrhea and other symptoms. However, blastocystis hominis often appears with other organisms that may be the actual cause of the signs and symptoms commonly associated with blastocystis infection, also known as blastocystosis. So experts aren’t sure whether blastocystis hominis causes disease or merely serves as an indicator of other agents that might cause diarrheal symptoms.

Travelers who have visited developing countries often pick up blastocystis hominis, one of the most common parasites found in stool samples. However, it’s not known whether the organism causes traveler’s diarrhea. If you have blastocystis hominis in your stool but no signs or symptoms, you don’t need treatment. Even if you have symptoms, the blastocystis infection may clear up on its own. If you have signs and symptoms that don’t improve, you can take medications that may help, but the medication isn’t always effective.

Signs and symptoms

The signs and symptoms that might be associated with blastocystosis include:

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


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 causes disease is controversial. Some experts believe that the blastocystis parasite does cause diarrhea and other signs and symptoms. Others believe that’s not the case — that other organisms that may exist with B. hominis may be the true culprits.

Many types of protozoa get into the intestinal tract through oral-fecal contact, such as occurs when a person who doesn’t wash 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 blastocystosis increases in places with inadequate sanitation and poor personal hygiene.

Screening and diagnosis

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 three or more 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.
  • Blood tests. 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 computerized tomography (CT) scan to detect the swelling or scarring.


If you have diarrhea associated with blastocystosis, 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.

An oral rehydration solution (ORS) is the best 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 chemists or drugstores in developed areas, and many pharmacies carry their own brands. You can find packets of powdered oral rehydration solution recommended by the World Health Organization 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 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 blastocystosis varies greatly from person to person, and that improvement may be due to the medication’s effect on another organism.

If oral therapy fails, then intra-colonic infusion of high dose triple therapy is used via a colonoscopy.


You may be able to prevent blastocystis or other gastrointestinal infection by taking the following 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 100 percent 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 blastocystosis, 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 nappy or diaper, especially if you work in a child care center, even if you wear gloves.