Naegleria fowleri
Also known as: brain-eating amoeba
Naegleria fowleri is a free-living amoeba found in warm freshwater that causes primary amebic meningoencephalitis (PAM), a rare but rapidly fatal brain infection acquired when contaminated water enters the nose.
Naegleria fowleri is a thermophilic, free-living amoeba that thrives in warm freshwater — lakes, ponds, hot springs, and inadequately chlorinated pools — especially during summer. Popularly called the "brain-eating amoeba," it causes primary amebic meningoencephalitis (PAM), one of the most lethal infections in clinical medicine.
The route of entry is the key exam fact: infection occurs when water is forced up the nose, typically during swimming or diving. The amoeba attaches to the olfactory mucosa, migrates along the olfactory nerves through the cribriform plate, and invades the frontal lobes of the brain. Swallowing contaminated water does not cause infection, and the disease does not spread person to person.
PAM begins within days of exposure with headache, fever, nausea, and vomiting, then progresses rapidly to stiff neck, altered mental status, seizures, and coma; most patients die within about a week of symptom onset. Cerebrospinal fluid resembles bacterial meningitis — high neutrophils, high protein, low glucose — but shows no organisms on Gram stain; instead, motile trophozoites can be seen on a wet mount of the CSF. Treatment attempts center on amphotericin B, often combined with miltefosine and other agents, but survival remains exceptional.
USMLE Step 1 tests Naegleria fowleri in parasitology as a classic vignette: a healthy young person who swam in a warm freshwater lake days earlier presents with fulminant meningitis, and the CSF wet mount reveals amoebae. Recognize the freshwater-nose-cribriform plate pathway and the trophozoites in CSF.
Key takeaways
- Naegleria fowleri is a free-living amoeba of warm freshwater that causes primary amebic meningoencephalitis (PAM).
- Infection requires water entering the nose; the amoeba reaches the brain via the cribriform plate.
- PAM progresses from headache and fever to death typically within about a week.
- Diagnosis hinges on motile trophozoites in a CSF wet mount despite a negative Gram stain.
- Step 1 vignettes pair recent warm freshwater swimming with rapidly fatal meningoencephalitis.