[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Intestinal and urogenital protozoa](https://durgical.com/en/microbiology/groups/intestinal-and-urogenital-protozoa/) › Blastocystis spp.

[Microbiology](https://durgical.com/en/microbiology/) · [Intestinal and urogenital protozoa](https://durgical.com/en/microbiology/groups/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)

## Key points

- The most common protozoan in Colombian stool exams.
- Vacuolar form with a large central vacuole.
- Debated pathogenicity: finding it without symptoms is not a reason to treat.
- A marker of fecal contamination, like the commensal amoebae.

## 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

- Cysts are swallowed with contaminated water or food.
- In the large intestine they turn into vacuolar forms.
- They multiply by binary fission.
- 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:

- [Metronidazole](https://durgical.com/en/pharmacology/metronidazole/)

## Prevention

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

**Vaccine:** There is no vaccine.

## In Colombia

It is the most frequent protozoan in intestinal parasite surveys of Colombian schoolchildren. Its presence signals fecal exposure and is a common reason for unnecessary treatment when there are no symptoms.

## Memory aid

«Blasto, the noisy lodger»: it shows up in nearly every stool exam but is rarely the culprit.

## 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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## Related

- [Commensal intestinal amoebae (Entamoeba coli, E. dispar, Endolimax nana and others)](https://durgical.com/en/microbiology/commensal-intestinal-amoebae/)
- [Giardia duodenalis](https://durgical.com/en/microbiology/giardia-duodenalis/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

[← Back to the group: Intestinal and urogenital protozoa](https://durgical.com/en/microbiology/groups/intestinal-and-urogenital-protozoa/)

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](https://www.cdc.gov/dpdx/blastocystis/index.html) (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.

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