Question
Primary amoebic meningoencephalitis – etiology, mode of transmission, laboratory diagnosis
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 bodies (lakes, ponds, poorly chlorinated swimming pools, hot springs) and soil.
Mode of transmission: infection occurs when contaminated warm freshwater enters the nasal cavity, typically during swimming or diving in warm freshwater bodies. The amoeba (in its 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 contaminated water, nor from person to person.
Clinical course: causes a rapidly progressive, almost universally fatal (>95–97% mortality) fulminant meningoencephalitis in otherwise healthy individuals (often children/young adults with a recent history of freshwater swimming), with onset of symptoms (headache, fever, vomiting, altered mental status, seizures) typically within 1–7 days of exposure, progressing to death within about a week.
Laboratory diagnosis:
- CSF examination — shows a picture resembling acute bacterial meningitis: markedly elevated opening pressure, neutrophilic pleocytosis, elevated protein, and low glucose — but with a negative bacterial Gram stain and culture, which should raise suspicion for PAM in the appropriate epidemiological context (recent freshwater exposure).
- Wet mount microscopy of CSF — direct visualization of motile trophozoites in a fresh, warm CSF sample examined promptly (motility is lost on cooling/delay), the key rapid diagnostic clue.
- Giemsa/trichrome stain of CSF sediment — for morphological identification of trophozoites.
- Culture — on non-nutrient agar seeded with a lawn of E. coli (allowing the amoeba to feed and multiply, forming visible trails), incubated at 37–44°C (favouring the thermophilic Naegleria).
- PCR — for 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.