Question
Free living amoebae
Answer
Free-living amoebae are amoeboid protozoa that exist independently in soil, freshwater, and other environmental sources (unlike obligate parasites requiring a host), but certain species can opportunistically cause serious, often fatal human infection.
Important pathogenic species:
- Naegleria fowleri — a thermophilic free-living amoeba found in warm freshwater; causes Primary Amoebic Meningoencephalitis (PAM), a rare but almost universally fatal, rapidly progressive infection in previously healthy individuals, acquired when contaminated warm water enters the nose (typically during swimming/diving) and the amoeba migrates via the olfactory nerve/cribriform plate directly into the brain; onset is abrupt with death typically within 3–7 days.
- Acanthamoeba species — cause Granulomatous Amoebic Encephalitis (GAE), a chronic, subacute CNS infection occurring predominantly in the immunocompromised (in contrast to Naegleria, which affects the healthy); also cause Acanthamoeba keratitis, a serious corneal infection strongly associated with contact lens wear and poor lens hygiene.
- Balamuthia mandrillaris — another free-living amoeba causing a GAE-like chronic encephalitis, can occur in both immunocompetent and immunocompromised hosts, and can also affect skin.
Distinguishing features (clinically important): Naegleria causes an acute, fulminant illness in healthy hosts via nasal water exposure; Acanthamoeba/Balamuthia cause a chronic, indolent illness, predominantly in immunocompromised hosts (for Acanthamoeba GAE), acquired via less clearly defined routes (possibly respiratory/skin entry).
Laboratory diagnosis: CSF wet-mount microscopy for motile trophozoites (best for Naegleria, using fresh, unrefrigerated CSF); corneal scraping microscopy/culture on a non-nutrient agar with a bacterial lawn (for Acanthamoeba keratitis); PCR and brain biopsy for confirmation in CNS disease.
Treatment: challenging given rarity and rapid progression; amphotericin B-based regimens (plus other agents) for Naegleria; prolonged topical biguanide/diamidine therapy for Acanthamoeba keratitis; overall prognosis remains poor for CNS disease from either organism.