Question
Acanthamoeba.
Answer
Acanthamoeba is a free-living amoeba, widely distributed in soil, freshwater, and dust, existing in two forms — a motile, feeding trophozoite and a highly resistant, dormant cyst (double-walled, resistant to desiccation, many disinfectants, and standard chlorination levels used in water treatment/contact lens solutions) — capable of causing serious opportunistic human disease.
Two major clinical syndromes:
- Acanthamoeba keratitis — a serious, sight-threatening corneal infection, strongly and classically associated with contact lens wear (particularly with poor lens hygiene, use of non-sterile water for lens cleaning/storage, or swimming/showering while wearing lenses) or with minor corneal trauma in non-lens-wearers; presents with severe eye pain (often disproportionate to clinical signs, an important diagnostic clue), photophobia, redness, and blurred vision, potentially progressing to corneal ulceration/scarring and vision loss if not promptly treated. Often initially misdiagnosed as herpetic or bacterial/fungal keratitis due to non-specific early features, causing diagnostic delay.
- Granulomatous Amoebic Encephalitis (GAE) — a rare but almost universally fatal chronic CNS infection, occurring predominantly in immunocompromised individuals (unlike the acute primary amoebic meningoencephalitis caused by Naegleria fowleri, which occurs in healthy individuals); presents with a subacute-to-chronic course of headache, altered mental status, focal neurological deficits, and seizures, evolving over weeks.
Laboratory diagnosis: for keratitis — corneal scraping for microscopy (wet mount or calcofluor white staining showing characteristic cysts) and culture on non-nutrient agar seeded with a bacterial lawn (e.g., E. coli), on which the amoeba feeds and multiplies; confocal microscopy of the cornea can also directly visualize cysts in vivo. For GAE — brain biopsy or, less reliably, CSF examination.
Treatment: Acanthamoeba keratitis is treated with prolonged topical biguanide (polyhexamethylene biguanide, PHMB) and/or chlorhexidine, often combined with a diamidine (propamidine); treatment is prolonged and challenging given the organism’s cyst-form resistance. GAE has a very poor prognosis despite combination antimicrobial/antifungal therapy.
Prevention: proper contact lens hygiene — using sterile lens-care solutions (never tap water), avoiding swimming/showering with lenses in, and regular lens case cleaning/replacement.