Question
Primary amoebic meningoencephalitis
Answer
Primary Amoebic Meningoencephalitis (PAM) is a rare but almost universally fatal, rapidly progressive CNS infection caused by Naegleria fowleri (“the brain-eating amoeba”), a thermophilic free-living amoeba found in warm freshwater (lakes, ponds, poorly chlorinated pools, hot springs) and soil.
Transmission: contaminated warm freshwater enters the nose (typically during swimming, diving, or other water-related nasal exposure — NOT via drinking contaminated water), and the amoeba (motile trophozoite form) 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; occurs in previously healthy individuals (notably unlike Acanthamoeba, which typically causes chronic granulomatous encephalitis predominantly in the immunocompromised); onset is abrupt — severe headache, fever, neck stiffness, rapid progression to altered consciousness, seizures, and death — typically within 3–7 days of symptom onset, with case fatality 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 slows/stops amoebal motility, hindering detection); PCR for confirmation; 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 relevant water-exposure history.
Treatment: amphotericin B (IV and intrathecal) combined with other agents (miltefosine, azithromycin, rifampicin, fluconazole) has been used in rare reported survivors, but overall prognosis remains very poor.
Prevention: avoiding nasal water exposure in warm freshwater bodies, use of nose clips while swimming in such environments, proper chlorination/maintenance of swimming pools.