Medical quality assurance by Dr. Albrecht Nonnenmacher, MD at August 18, 2016

Blastocystosis is a gastrointestinal disease caused by the parasite, Blastocystis hominis. This particular parasite is widely regarded as a common source for waterborne disease and is found in all regions of the U.S. and around the world. Certain forms of the organism are more likely to cause symptomatic illness than others. The reason for this is not clear, but it may be due to variations in the genetic makeup of the microbe that cause it to become more virulent.


Definition & Facts

Blastocystis hominis lives in the intestines of infected animals and humans. Millions of the organisms can be released in the feces during a bowel movement where they can survive in the environment for a considerable time because of a hard protective outer shell.

Blastocystis can survive on surfaces and can also be found on soil and in contaminated food or contaminated water that has fecal matter. The organism is primarily transmitted through the fecal-oral route, and blastocystis cannot be spread through contact with blood. It is sometimes called 'the hippie disease' because of its prevalence in crowded and unsanitary areas.

Symptoms & Complaints

Some people infected with the Blastocystis hominis organism never develop symptoms, and the parasite is often found in the stools of both healthy and sick individuals. In animals, the symptoms often occur within two weeks following exposure; however, the incubation period in humans is less clear. Acute symptoms are primarily gastrointestinal in nature, including:

In some cases, patients may also experience joint pain, skin rashes, and fatigue. Symptoms may be mild during the initial infection and gradually worsen with time. Symptoms may last anywhere from a few weeks to indefinitely in the form of a chronic infection. Researchers are still trying to determine why some infections resolve and others become chronic. In cases of chronic infections, the symptoms will typically resurface during times of stress or when the immune system is weakened.


Blastocystis is most often spread through ingesting water, ice, or uncooked food contaminated with the organism. It can also be accidentally swallowed while swimming in contaminated water, after touching contaminated surfaces and bathroom fixtures, or while caring for someone who is infected. Those most at risk for blastocystosis include:

  • International travelers
  • Caregivers
  • Campers, hikers, and others who drink or use ice from untreated water sources
  • Children in child care facilities that include non-potty trained children
  • Individuals exposed to fecal matter during sexual contact
  • People who do not practice good hand hygiene habits
  • Individuals who swim in untreated water
  • Family members of infected individuals

Diagnosis & Tests

Anyone experiencing symptoms of blastocystosis should see their doctor as soon as possible. Diagnosis of blastocystosis can be challenging since the symptoms are the same as a number of other gastrointestinal conditions. A complete medical history will reveal if the patient has engaged in activities that would make them susceptible to infection.

A stool sample is usually obtained and submitted for laboratory analysis. The test looks for signs of the parasites or their eggs in the stool. The patient will typically collect the sample themselves at home and place it in a special container containing preservatives that was provided by the doctor. The patient then returns the sample to the doctor being careful to keep the sample cool but not frozen.

If the fecal exam is negative, the doctor may order an endoscopy. This involves inserting a thin, flexible tube that is outfitted with a small camera up the rectum to allow the doctor to visualize any potential problems with the lower GI tract. Blood tests are also available to detect Blastocystis hominis, but they are not commonly used.

Treatment & Therapy

It is not necessary to treat blastocystosis if the patient is asymptomatic. Mild symptoms typically resolve on their own within a few days even without treatment; however, the patient should be sure to drink plenty of fluids to prevent diarrhea-related dehydration.

Patients with severe or persistent symptoms are normally treated with antibiotic or antiprotozoal medications. The most commonly used medications for blastocystosis include metronidazole, tinidazole, sulfamethoxazole and trimethoprim, paromomycin, and nitazoxanide. The response to these treatments often varies from patient to patient since the symptoms may be due to something other than blastocystosis.

Antibiotics do carry the risk of causing stomach upset, increased diarrhea, and overgrowth infections, including C. difficile. Individuals with chronic blastocystis infections requiring prolonged treatment may need to rotate medications to reduce the risk of antimicrobial resistance.

Prevention & Prophylaxis

The best way to prevent gastrointestinal infections, including blastocystosis, is to practice good hand hygiene. This includes washing the hands thoroughly with soap and water for at least 20 seconds after visiting the toilet, changing diapers or providing personal care, and before handling food and beverages.

Parents of infected children should take extra care in helping them wash their hands after toileting and before eating. Although public health officials typically recommend that children with diarrhea be kept out of child care settings, this may not be feasible in cases of blastocystosis since the diarrhea can continue for a prolonged period.

Infected individuals are also advised to avoid swimming until the diarrhea has been resolved for at least a week. Extra precautions should be taken when traveling to areas without adequate sanitation, such as:

  • Avoiding raw or undercooked meats or seafood
  • Avoiding raw condiments, including salsa
  • Avoiding eating or drinking unpasteurized dairy products
  • Only eating fruits and vegetables that you can cook, peel, or boil yourself
  • Avoiding eating foods from street vendors
  • Only drinking bottled water and avoiding drinks with ice cubes
  • Being careful to avoid swallowing water while showering and only using bottled water when brushing teeth
  • Avoiding swimming in untreated or potentially contaminated water.

Water filters bearing the official Environmental Protection Agency designation filter out hundreds of different organisms and may offer some protection against blastocystosis. Chemical treatments, such as iodine, can be used on a short-term basis; however, excessive exposure to iodine can be harmful.