[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Other fungi of clinical interest](https://durgical.com/en/microbiology/groups/other-fungi-of-clinical-interest/) › Microsporidia (Enterocytozoon, Encephalitozoon)

[Microbiology](https://durgical.com/en/microbiology/) · [Other fungi of clinical interest](https://durgical.com/en/microbiology/groups/other-fungi-of-clinical-interest/)

# Microsporidia (Enterocytozoon, Encephalitozoon)

- Fungus
- For completeness

Tiny parasites that live inside cells and cause chronic diarrhoea in AIDS. Once taught as protozoa, they are now grouped with the fungi.

Updated 2026-11-01

Also called: microsporidiosis

## Key points

- Chronic diarrhoea in AIDS with very low CD4: think microsporidia along with Cryptosporidium and Cystoisospora.
- Spores with a polar tube; they need a chromotrope (modified trichrome) stain.
- Enterocytozoon bieneusi is the most frequent and responds poorly to albendazole.
- Now classed with the fungi, although many texts list them among the protozoa.

## At a glance

**Shape**
Oval spores with a coiled polar tube inside

**Type**
Obligate intracellular spore-forming single-celled organism; classed today with the fungi (formerly protozoa)

**Stain**
Chromotrope 2R (modified trichrome), Gram-chromotrope or calcofluor white; not seen with routine stains

**Size**
Spores of 1 to 4 µm; those of Enterocytozoon bieneusi about 1 µm

## Morphology

- A tough thick-walled spore with a coiled polar tube, its infection organ.
- Among the smallest eukaryotic cells infecting humans: in stool they pass for bacteria or yeasts without a special stain.

## Transmission

- Ingestion of spores in contaminated water or food (faecal-oral); animal contact and inhalation also seem possible.
- Spores survive in the environment; the exact routes are not fully established.

**Reservoir:** Humans (Enterocytozoon bieneusi); Mammals such as rabbits (Encephalitozoon cuniculi) and psittacine birds (E. hellem)

### Isolation precautions

What the care team adds to standard precautions.

- Standard

## Life cycle

- The resistant spore reaches the gut (or another tissue) with water or food.
- The polar tube everts, pierces the host cell and injects the sporoplasm.
- Inside the cell it multiplies by merogony.
- By sporogony it forms new mature spores.
- The cell bursts and releases spores that infect nearby cells or leave in stool or urine.

## Virulence factors

- The polar tube lets it inject itself into the cell without being phagocytosed.
- Intracellular multiplication destroys the intestinal epithelium and causes malabsorption.
- Encephalitozoon can spread to the biliary tree, kidney, eye and other organs.

## Diseases it causes

- Chronic diarrhoea and disseminated infection as opportunists of advanced HIV [HIV infection and AIDS →](https://durgical.com/en/conditions/hiv-infection-and-aids/)
- Keratoconjunctivitis, also in healthy people

## Clinical picture

- Chronic watery diarrhoea with weight loss in AIDS; usually self-limited in immunocompetent people.
- Cholangitis, sinusitis, nephritis or disseminated infection with Encephalitozoon.
- Keratoconjunctivitis, from mild punctate keratopathy to stromal keratitis.

## Diagnosis

- Chromotrope (modified trichrome) stain of stool or other samples: tiny pink spores.
- Calcofluor or other fluorochromes as a quick screen.
- Species-specific PCR; electron microscopy is the reference standard but impractical.

## Treatment

In people with HIV, starting or optimising antiretroviral therapy is the base. Albendazole works for Encephalitozoon but hardly acts on Enterocytozoon bieneusi, for which fumagillin is used where available; topical fumagillin is used for the eye.

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

In Pharmacology:

- [Albendazole](https://durgical.com/en/pharmacology/albendazole/)

## Prevention

- Safe water and handwashing.
- HIV control with antiretroviral therapy, which makes these infections much rarer.

## Memory aid

A microscopic syringe: the spore fires its polar tube like a needle and injects its contents into the cell.

## Test yourself

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

## Which structure do microsporidia use to enter the host cell?

The polar tube, a coiled filament inside the spore that everts, pierces the membrane and injects the sporoplasm.

## Which stain shows them in stool?

Chromotrope 2R (modified trichrome) or Gram-chromotrope; calcofluor works as a screen.

## Why is albendazole not enough against Enterocytozoon bieneusi?

It has very little activity against this species; Encephalitozoon does respond. For E. bieneusi, fumagillin is used where available, along with antiretrovirals.

## Are microsporidia protozoa or fungi?

They were long classed as protozoa; today they are considered fungi or their very close relatives.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/microsporidia/)

## Related

- [Cryptosporidium spp.](https://durgical.com/en/microbiology/cryptosporidium-spp/)
- [Cyclospora cayetanensis and Cystoisospora belli](https://durgical.com/en/microbiology/cyclospora-cayetanensis-and-cystoisospora-belli/)
- [Human immunodeficiency virus](https://durgical.com/en/microbiology/human-immunodeficiency-virus/)

## Related conditions

- [HIV infection and AIDS](https://durgical.com/en/conditions/hiv-infection-and-aids/)

## 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: Other fungi of clinical interest](https://durgical.com/en/microbiology/groups/other-fungi-of-clinical-interest/)

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

Sources: [CDC - DPDx - Microsporidiosis](https://www.cdc.gov/dpdx/microsporidiosis/index.html) (CDC, 2024); [Microsporidiosis - Infectious Disease - Merck Manual Professional Edition](https://www.merckmanuals.com/professional/infectious-diseases/intestinal-protozoa-and-microsporidia/microsporidiosis) (Merck Manual, 2024).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/microbiology/microsporidia/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/microsporidia/