Acanthamoeba

Acanthamoeba species are ubiquitous free-living amoebae found in soil, dust, freshwater, and even tap water and contact lens solutions. They have two life cycle stages: a motile, feeding trophozoite and a highly resistant double-walled cyst that can survive standard chlorination and even some contact lens disinfectants, making it a persistent threat in ophthalmologic infections.

Acanthamoeba keratitis is the more clinically common presentation, occurring predominantly in contact lens wearers—especially those who use homemade saline, swim or shower while wearing lenses, or have poor lens hygiene. Presentation includes severe eye pain (often out of proportion to clinical findings), photophobia, ring-shaped corneal infiltrate, and can progress to vision loss if untreated. Diagnosis involves corneal scraping with calcofluor white stain, culture on non-nutrient agar with E. coli overlay, or confocal microscopy. Treatment requires prolonged topical biguanides (e.g., polyhexamethylene biguonide) and/or chlorhexidine, sometimes combined with propamidine.

Granulomatous amoebic encephalitis (GAE) is a rare, subacute-to-chronic CNS infection typically seen in immunocompromised hosts (HIV/AIDS, chemotherapy, malnutrition). The organism is thought to enter via hematogenous spread from skin or lung lesions or the olfactory neuroepithelium. Pathologically, it causes granulomatous inflammation with necrosis, distinguishing it from the rapidly fatal primary amoebic meningoencephalitis caused by Naegleria fowleri, which occurs in immunocompetent hosts after nasal exposure to warm freshwater and progresses rapidly (days). GAE has an indolent course (weeks) with headache, seizures, focal neurologic deficits, and altered mental status, and is usually diagnosed post-mortem or via brain biopsy showing trophozoites/cysts. Mortality is high, and treatment is difficult, often involving combination antimicrobial and antifungal therapy.

Key USMLE distinctions: Naegleria fowleri (immunocompetent, rapid, freshwater swimming, primary amoebic meningoencephalitis) vs Acanthamoeba (immunocompromised, GAE, indolent, or keratitis in contact lens wearers, not linked to swimming in warm water directly but soil/water/dust exposure and improper contact lens care).