Question
Naegleria fowleri
Answer
Naegleria fowleri (“the brain-eating amoeba”) is a free-living, thermophilic amoeba found in warm freshwater (lakes, ponds, poorly chlorinated swimming pools, hot springs) and soil, notorious as the cause of Primary Amoebic Meningoencephalitis (PAM) — a rare but almost universally fatal, rapidly progressive CNS infection.
Transmission: infection occurs when contaminated warm freshwater enters the nose (typically during swimming, diving, or other water-related nasal exposure — notably NOT through drinking contaminated water, since ingestion does not lead to infection); the amoeba (in its motile trophozoite form) then migrates along the olfactory nerve, crossing the cribriform plate to directly invade the brain, particularly the olfactory bulbs and frontal lobes.
Pathogenesis and clinical course: causes a fulminant, purulent, haemorrhagic meningoencephalitis, with rapid destruction of brain tissue; onset is typically abrupt in a previously healthy individual (notably unlike Acanthamoeba, which typically causes chronic granulomatous encephalitis predominantly in the immunocompromised), with severe headache, fever, neck stiffness, and rapid progression to altered consciousness, seizures, and death — typically within 3–7 days of symptom onset, with a case fatality rate exceeding 95%.
Laboratory diagnosis: CSF examination shows features resembling bacterial meningitis (neutrophilic pleocytosis, raised protein, low glucose) but with motile amoebae visible on wet-mount microscopy of fresh, unrefrigerated CSF (refrigeration can slow/stop amoebal motility, hindering detection) — direct microscopic visualization of motile trophozoites is a key, rapid diagnostic clue; PCR for confirmation where available; CSF Gram stain and bacterial culture are negative, an important clue prompting consideration of this diagnosis when bacterial meningitis workup is unexpectedly negative in a rapidly deteriorating patient with a relevant water-exposure history.
Treatment: extremely difficult given the rapid, fulminant course; amphotericin B (intravenous and intrathecal) combined with other agents (miltefosine, azithromycin, rifampicin, fluconazole) has been used in the rare reported survivors, but overall prognosis remains very poor.
Prevention: avoiding nasal water exposure in warm freshwater bodies during high-risk conditions (warm weather, stagnant water), use of nose clips while swimming in such environments, and proper chlorination/maintenance of swimming pools.