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[Microbiology](https://durgical.com/en/microbiology/) · [Free-living amoebae](https://durgical.com/en/microbiology/groups/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)

## Key points

- Fulminant meningoencephalitis after swimming in warm fresh water in a healthy young person.
- Enters through the nose and follows the olfactory nerve through the cribriform plate.
- Purulent CSF with a Gram stain negative for bacteria: look for motile amoebae on wet mount.
- Not acquired by drinking water or from other people.

## 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:

- [Amphotericin B](https://durgical.com/en/pharmacology/amphotericin-b/)
- [Miltefosine](https://durgical.com/en/pharmacology/miltefosine/)

## 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.

## In Colombia

It is exceptional in Colombia, with isolated cases described. It should be suspected in severe meningitis after bathing in rivers, lakes or pools in hot climates.

## Memory aid

«Nose, swim, Naegleria»: three N's of warm water that climbs the smell nerve to the brain.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/naegleria-fowleri/)

## Related

- [Acanthamoeba spp.](https://durgical.com/en/microbiology/acanthamoeba-spp/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

[← Back to the group: Free-living amoebae](https://durgical.com/en/microbiology/groups/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](https://www.cdc.gov/dpdx/freelivingamebic/index.html) (CDC, 2024); [Naegleria fowleri Infections | Naegleria fowleri Infection | CDC](https://www.cdc.gov/naegleria/about/index.html) (CDC, 2025).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

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