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Microbiology · Intestinal and urogenital protozoa

Blastocystis spp. !

  • Parasite
  • Exam priority: medium

A very common organism in stool, including in healthy people. Whether it causes disease is unclear; it is usually an incidental finding.

Updated 2026-11-01

Also called: Blastocystis hominis (old name)

At a glance

Shape
Round vacuolar form with a large central vacuole and nuclei at the rim
Family
Blastocystidae
Type
Intestinal protist (stramenopile), neither an amoeba nor a flagellate
Stain
Permanent stains (trichrome); direct exam
Size
Highly variable; cysts about 3 to 5 µm

Morphology

  • Vacuolar form: the most common in stool, round, with a large central vacuole and a thin rim of cytoplasm.
  • Granular and amoeboid forms, occasionally.
  • Small cyst, the probable infective form.
  • Several genetic subtypes exist (ST1 to ST4 are the most frequent in humans).

Transmission

  • Probably fecal-oral, through contaminated water or food.
  • Possible spread from animals for some subtypes.

Reservoir: Humans; Many animals (pigs, birds, primates, dogs)

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Life cycle

  1. Cysts are swallowed with contaminated water or food.
  2. In the large intestine they turn into vacuolar forms.
  3. They multiply by binary fission.
  4. Some forms encyst and leave in the feces.

Virulence factors

  • Its role as a pathogen is debated: it is found as often in people with and without symptoms.
  • Some studies link it to gut symptoms depending on subtype and burden.

Diseases it causes

  • Blastocystosis: debated link with abdominal pain, bloating and diarrhea; a frequent finding in people without symptoms.

Clinical picture

  • Most people have no symptoms.
  • Linked to abdominal pain, bloating, diarrhea and irritable-bowel-like symptoms.
  • Other causes are sought before blaming it for symptoms.

Diagnosis

  • Stool exam: vacuolar forms on direct exam or trichrome stain.
  • Permanent stains avoid confusing it with debris or yeasts.
  • PCR and subtyping are mainly research tools.

Treatment

Usually no treatment is needed without symptoms. In symptomatic patients with no other cause, nitroimidazoles (metronidazole) or other antiparasitics have been used, with variable results.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Safe water and handwashing.
  • Washing fruit and vegetables.
  • Basic sanitation.

Vaccine: There is no vaccine.

Test yourself

Think of the answer, then tap the question to see it.

Which Blastocystis form is most often seen in stool?

The vacuolar form: round, with a large central vacuole and nuclei at the rim.

Does finding Blastocystis in a patient without symptoms call for treatment?

No. It is very common in healthy people and its role as a pathogen is still debated.

What does finding Blastocystis in a sample mean epidemiologically?

Exposure to water or food contaminated with feces, which calls for checking sanitation and looking for other parasites.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: CDC - DPDx - Blastocystis hominis (CDC, 2024); Parasitosis humanas (6.ª ed.), Botero D, Restrepo M (Corporación para Investigaciones Biológicas (CIB), 2019).

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Representative model, for educational purposes only.