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Microbiology · 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

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

  1. It lives in water and soil as a bacteria-eating trophozoite.
  2. Under harsh conditions it forms cysts that resist chlorine and drying.
  3. It reaches the cornea on contaminated lenses or through tiny epithelial defects.
  4. In immunocompromised people it enters via skin or lung and spreads by blood to the brain.
  5. 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 →
  • 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:

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.

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.

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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 - Free Living Amebic Infections (CDC, 2024); Acanthamoeba Infections | Acanthamoeba Infections | CDC (CDC, 2024).

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