Microbiology · Free-living amoebae
Naegleria fowleri !
- Parasite
- Exam priority: medium
A warm freshwater amoeba that enters through the nose while swimming and destroys the brain within days. It is very rare but almost always fatal.
Updated 2026-11-01
Also called: brain-eating amoeba (popular name)
At a glance
- Shape
- Amoeboid trophozoite, temporary flagellate form and cyst
- Family
- Vahlkampfiidae
- Type
- Free-living amoeba (amoeboflagellate)
- Stain
- Wet mount of CSF; Giemsa or Wright
- Size
- Trophozoite about 10 to 25 µm
Morphology
- Amoeboid trophozoite with a nucleus with a large karyosome and rounded pseudopods (lobopodia).
- Transient flagellate form with two flagella when conditions change (in water).
- Round smooth-walled cyst; not seen in brain tissue.
Transmission
- Warm fresh water forced into the nose while swimming, diving or rinsing the nose with untreated tap water.
- Not acquired by drinking water.
- Not spread from person to person.
Reservoir: Warm fresh water (lakes, rivers, hot springs), soil and poorly chlorinated water
Isolation precautions
What the care team adds to standard precautions.
- Standard
Life cycle
- It lives free in warm fresh water and soil as a bacteria-eating trophozoite.
- If conditions change it becomes a temporary flagellate; if it dries or cools, it forms cysts.
- Water with trophozoites enters the nose during swimming or nasal rinsing.
- Trophozoites cross the nasal mucosa and follow the olfactory nerves through the cribriform plate.
- In the brain they multiply and cause hemorrhagic necrosis.
Virulence factors
- Thermotolerance: grows in warm water and at body temperature.
- Feeding structures (amoebostomes) and cytolytic enzymes that destroy neurons.
- Intense inflammatory response with brain edema.
Diseases it causes
- Primary amoebic meningoencephalitis: fulminant, with case fatality close to 100%.
Clinical picture
- Onset a few days after swimming: severe headache, fever, nausea and vomiting.
- Altered smell or taste early on.
- Neck stiffness, seizures, confusion and coma; death within days.
- Looks like bacterial meningitis with negative Gram stain and culture.
Diagnosis
- Wet mount of unrefrigerated CSF: motile trophozoites.
- Purulent CSF with many neutrophils and red cells, low glucose and no bacteria on Gram stain.
- Culture on non-nutrient agar with E. coli and PCR at reference labs.
Treatment
Multidrug combinations that include amphotericin B and miltefosine, plus intensive management of brain edema. Few patients survive, almost always with very early diagnosis.
Drug class only; doses and regimens are set by the clinician from the current guideline.
In Pharmacology:
Prevention
- Holding the nose or using a nose clip when jumping or diving into warm fresh water.
- Keeping the head above water in hot springs.
- Using boiled, distilled or sterile water for nasal rinses.
Vaccine: There is no vaccine.
Test yourself
Think of the answer, then tap the question to see it.
How does Naegleria fowleri reach the brain?
Through the nose: it crosses the olfactory mucosa and the cribriform plate along the olfactory nerves.
Which history points to primary amoebic meningoencephalitis?
Swimming, diving or nasal rinsing with warm fresh water a few days before a fulminant meningitis.
How is the amoeba seen in the lab?
On a wet mount of unrefrigerated CSF, as motile trophozoites; culture or PCR confirms it.
Can Naegleria be caught by drinking contaminated water?
No. Infection happens only when water goes up the nose; it is not spread person to person either.
Related
← Back to the group: Free-living amoebae
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: CDC - DPDx - Free Living Amebic Infections (CDC, 2024); Naegleria fowleri Infections | Naegleria fowleri Infection | CDC (CDC, 2025).
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Representative model, for educational purposes only.