Question
Free living amoebae (PTO)
Answer
Free-living amoebae: amoebae capable of independent existence in the environment (soil, freshwater), distinct from obligate parasitic amoebae like Entamoeba histolytica; a small number are opportunistic human pathogens, causing severe, often fatal CNS/ocular disease.
1. Naegleria fowleri: a thermophilic amoeba found in warm freshwater (lakes, ponds, poorly chlorinated pools, hot springs). Causes Primary Amoebic Meningoencephalitis (PAM) — infection occurs when contaminated water enters the nasal cavity (typically during swimming/diving), the amoeba penetrates the nasal mucosa, migrates along the olfactory nerve, crosses the cribriform plate, and directly invades the brain, causing a rapidly progressive, almost universally fatal (>95% mortality) fulminant meningoencephalitis, typically in previously healthy children/young adults, with death occurring within about a week of symptom onset. Diagnosis: wet mount of fresh, warm CSF showing motile trophozoites; culture on non-nutrient agar seeded with E. coli; PCR.
2. Acanthamoeba species: ubiquitous in soil, water, and dust. Causes:
- Granulomatous Amoebic Encephalitis (GAE) — a subacute-to-chronic CNS infection, typically in immunocompromised individuals, unlike the acute PAM of Naegleria.
- Acanthamoeba keratitis — a severe corneal infection, classically associated with contact lens wear (particularly with poor lens hygiene or exposure to contaminated water while wearing lenses), presenting with severe pain disproportionate to clinical signs, and characteristic ring-shaped corneal infiltrate. Diagnosis: demonstration of trophozoites/double-walled cysts on corneal scraping (for keratitis) or brain biopsy (for GAE); culture on non-nutrient agar with E. coli overlay; confocal microscopy for keratitis; PCR.
3. Balamuthia mandrillaris: another cause of Granulomatous Amoebic Encephalitis, affecting both immunocompetent and immunocompromised individuals, with a subacute-to-chronic course; diagnosis relies on brain biopsy/immunofluorescence and PCR, given its difficulty to culture on standard media (requires mammalian cell co-culture).
General principles: free-living amoeboid infections are rare but carry high mortality/morbidity given delayed diagnosis (non-specific initial presentation) and limited effective treatment options; prevention centres on avoiding nasal exposure to warm untreated freshwater (for Naegleria) and proper contact lens hygiene, avoiding water contact while wearing lenses (for Acanthamoeba keratitis).