Question
Primary Amoebic meningoencephalitis – discuss etiology, pathogenesis and laboratory diagnosis.(1+2+3)
Answer
Primary Amoebic Meningoencephalitis (PAM)
Etiology: caused by the free-living amoeba Naegleria fowleri (the “brain-eating amoeba”), a thermophilic organism found in warm freshwater (lakes, ponds, poorly chlorinated swimming pools, hot springs) and soil.
Pathogenesis: infection occurs when contaminated warm freshwater enters the nasal cavity, typically during swimming/diving. The amoeba (motile trophozoite form) penetrates the nasal mucosa, migrates along the olfactory nerve fibres, crosses the cribriform plate, and directly invades the brain (olfactory bulb and frontal lobes) — it is not transmitted via drinking water, nor person-to-person. This causes a rapidly progressive, almost universally fatal (>95% mortality) fulminant meningoencephalitis in otherwise healthy individuals (often children/young adults with recent freshwater swimming), with death typically occurring within about a week of symptom onset.
Laboratory diagnosis:
- CSF examination — resembles acute bacterial meningitis: markedly elevated opening pressure, neutrophilic pleocytosis, elevated protein, low glucose, but with a negative bacterial Gram stain and culture.
- Wet mount microscopy of CSF — direct visualization of motile trophozoites in a fresh, warm CSF sample (motility is lost on cooling/delay) — the key rapid diagnostic clue.
- Giemsa/trichrome stain of CSF sediment — for morphological identification.
- Culture — on non-nutrient agar seeded with a lawn of E. coli, incubated at 37–44°C (favouring the thermophilic Naegleria).
- PCR — rapid, specific molecular confirmation, increasingly the preferred confirmatory method.
- Neuroimaging (CT/MRI) — may show oedema/haemorrhagic changes in the olfactory bulb/frontal lobe region, supportive but non-specific.