[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Free-living amoebae](https://durgical.com/en/microbiology/groups/free-living-amoebae/) › Acanthamoeba spp.

[Microbiology](https://durgical.com/en/microbiology/) · [Free-living amoebae](https://durgical.com/en/microbiology/groups/free-living-amoebae/)

# Acanthamoeba spp.

- Parasite
- Exam priority: medium

A water and soil amoeba that infects the cornea of contact-lens wearers and, rarely, the brain of immunocompromised people.

Updated 2026-11-01

## Key points

- Contact lenses + severe eye pain + ring infiltrate: Acanthamoeba.
- Double-walled, polygonal, very hardy cyst.
- Subacute granulomatous encephalitis in immunocompromised people (unlike acute Naegleria meningoencephalitis).
- Culture on non-nutrient agar with E. coli; topical biguanide treatment.

## At a glance

**Shape**
Trophozoite with spiny pseudopods (acanthopodia) and double-walled cyst

**Family**
Acanthamoebidae

**Type**
Free-living amoeba

**Stain**
Calcofluor white, Giemsa or trichrome on corneal scrapings or biopsy

**Size**
Cyst about 10 to 25 µm

## Morphology

- Trophozoite with fine, thorn-like projections (acanthopodia).
- Very hardy cyst with a double wall with pores and a polygonal or star-shaped outline.
- No flagellate form, unlike Naegleria.

## Transmission

- Eye contact with contaminated water through contact lenses (rinsing them with tap water, swimming or showering in them).
- Entry through broken skin or the airways in encephalitis.
- Not spread from person to person.

**Reservoir:** Tap water, swimming pools, hot tubs, soil and dust

### Isolation precautions

What the care team adds to standard precautions.

- Standard

## Life cycle

- It lives in water and soil as a bacteria-eating trophozoite.
- Under harsh conditions it forms cysts that resist chlorine and drying.
- It reaches the cornea on contaminated lenses or through tiny epithelial defects.
- In immunocompromised people it enters via skin or lung and spreads by blood to the brain.
- Both trophozoites and cysts can be found in infected tissue.

## Virulence factors

- Adhesion to the cornea through mannose-binding proteins.
- Proteases that destroy the corneal stroma and corneal nerves.
- Cysts highly resistant to disinfectants and drugs, which explains relapses.

## Diseases it causes

- Acanthamoeba keratitis: severe eye pain out of proportion to findings, ring infiltrate and risk of blindness. [Keratitis and corneal ulcer →](https://durgical.com/en/conditions/keratitis-and-corneal-ulcer/)
- Granulomatous amoebic encephalitis: subacute or chronic, in immunocompromised people, almost always fatal.

## Clinical picture

- Keratitis: intense pain, photophobia, red eye and blurred vision; ring infiltrate and radial keratoneuritis.
- Often first mistaken for herpes or fungal keratitis.
- Granulomatous encephalitis: headache, behavior change, focal deficits and seizures over weeks to months.
- Nodular or ulcerated skin lesions in immunocompromised people.

## Diagnosis

- Corneal scraping or biopsy with stains (calcofluor white, Giemsa) and culture on non-nutrient agar with E. coli.
- Confocal microscopy of the cornea to see cysts.
- PCR at reference labs.
- Encephalitis: brain biopsy with granulomatous inflammation, cysts and trophozoites.

## Treatment

Keratitis: intensive, prolonged topical antiseptics (biguanides such as polyhexanide or chlorhexidine, sometimes with diamidines); severe cases may need corneal transplant. Encephalitis: multidrug combinations that may include miltefosine, with a very poor prognosis.

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

In Pharmacology:

- [Miltefosine](https://durgical.com/en/pharmacology/miltefosine/)

## Prevention

- Never rinsing or storing contact lenses in tap water.
- Removing lenses before swimming, showering or using a hot tub.
- Cleaning and replacing the case and solution as the manufacturer directs.

**Vaccine:** There is no vaccine.

## In Colombia

Acanthamoeba keratitis cases are described in Colombia, mostly linked to poor contact-lens care; granulomatous encephalitis is exceptional.

## Memory aid

«Acantho = thorn»: its spiny pseudopods dig into the cornea of whoever rinses lenses in tap water.

## Test yourself

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

## What is the main risk factor for Acanthamoeba keratitis?

Contact-lens use, especially rinsing lenses with tap water or wearing them while swimming or showering.

## How does Acanthamoeba encephalitis differ from Naegleria infection?

Acanthamoeba causes subacute granulomatous encephalitis in immunocompromised people; Naegleria causes acute fulminant meningoencephalitis in healthy people after swimming.

## Which corneal finding is typical of Acanthamoeba keratitis?

A ring infiltrate with intense pain out of proportion to the exam findings.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/microbiology/acanthamoeba-spp/)

## Related

- [Naegleria fowleri](https://durgical.com/en/microbiology/naegleria-fowleri/)

## Related conditions

- [Keratitis and corneal ulcer](https://durgical.com/en/conditions/keratitis-and-corneal-ulcer/)

## 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: Free-living amoebae](https://durgical.com/en/microbiology/groups/free-living-amoebae/)

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

Sources: [CDC - DPDx - Free Living Amebic Infections](https://www.cdc.gov/dpdx/freelivingamebic/index.html) (CDC, 2024); [Acanthamoeba Infections | Acanthamoeba Infections | CDC](https://www.cdc.gov/acanthamoeba/about/index.html) (CDC, 2024).

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Representative model, for educational purposes only.

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