DurgicalDurgical
EN
Open the app

Microbiology · 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

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

  1. The resistant spore reaches the gut (or another tissue) with water or food.
  2. The polar tube everts, pierces the host cell and injects the sporoplasm.
  3. Inside the cell it multiplies by merogony.
  4. By sporogony it forms new mature spores.
  5. 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 →
  • 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:

Prevention

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

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)

← Back to the group: 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 (CDC, 2024); Microsporidiosis - Infectious Disease - Merck Manual Professional Edition (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.