Naegleria fowleri: Characteristics, Life Cycle and Lab Diagnosis

Naegleria fowleri, commonly referred to as the “brain-eating amoeba,” is a small, free-living water and soil amebae capable of causing opportunistic infections in humans. It causes a rare but devastating disease called primary amebic meningoencephalitis (PAM).

Naegleria is distributed worldwide in various freshwater habitats (including rivers, lakes, ponds, hot springs, spas, domestic water systems, air-conditioning systems, humidifiers, and cooling towers) and soil.

Naegleria fowleri is not found in saltwater, like in the ocean.

Naegleria fowleri is a thermophilic (heat-loving) organism. It grows best at higher temperatures up to 115°F (46°C) and cand is less likely to be found in the water as temperatures decline below 77°F (25°C). So infection mainly occurs during the summer months of July, August, and September.

Naegleria is not related to the intestinal amebae and does not inhabit the intestines of humans or other mammals.

Naegleria fowleri
A computer-generated representation of Naegleria fowleri in its ameboid trophozoite stage, in its flagellated stage, and in its cyst stage (left to right). Image source: CDC

Stages of Naegleria fowleri


Trophotozie is the only infective stage of Naegleria fowleri. Trophozoites are 10-35 µm long with a granular appearance and a single nucleus. Naegleria fowleri trophozoites are found in cerebrospinal fluid (CSF) and tissue. 


Trophozoites can turn into a temporary, non-feeding, flagellated stage (10-16 µm in length) when stimulated by adverse environmental changes such as a reduced food source. Flagellated forms of Naegleria fowleri are occasionally found in CSF. They revert to the trophozoite stage when favorable conditions return.


If the environment is not conducive to continued feeding and growth (like cold temperatures, food becomes scarce), the ameba or flagellate will form a cyst. The cyst form is spherical and about 7-15 µm in diameter. It has a smooth, single-layered wall with a single nucleus.

Cysts are environmentally resistant, which increases their chances of survival until better environmental conditions occur. Cysts are not seen in brain tissue.

Mode of Transmission

Naegleria fowleri usually infects people when contaminated water enters the body through the nose typically during swimming or diving in warm freshwater places. Naegleria fowleri infection cannot be spread from one person to another.

The occurrence of Naegleria fowleri in freshwater is common but the infection is rare.

Life Cycle

Life cycle of Naegleria fowleri
Life cycle of Naegleria fowleri (Image source: CDC)

Naegleria fowleri has three stages in its life cycle: cyst, trophozoite, and flagellate. The trophozoites replicate by binary division, during which the nuclear membrane remains intact (a process called promitosis).

Trophozoites infect humans or animals by penetrating the nasal tissue and  Migrating to the brain via the olfactory nerves causing primary amebic meningoencephalitis (PAM).

The upper respiratory tract is the true “primary” site of the infection.  Most patients with Naegleria CNS infections have been children or young adults swimming in lakes or rivers. After infecting the nasal mucosa, N. fowleri spreads along the olfactory nerve through the cribriform plate into the brain. After reaching the brain, Naegleria fowleri can cause primary amebic meningoencephalitis (PAM) in healthy individuals.

Swallowing water contaminated with Naegleria does not cause infection.

Sign and Symptoms

N. fowleri causes primary amebic meningoencephalitis (PAM) the symptoms of which are similar to symptoms of bacterial meningitis. Initial symptoms of PAM start about 5 days (range 1 to 9 days) after infection.

  • The initial symptoms may include headache, fever, nausea, or vomiting.
  • Later symptoms can include stiff neck, confusion, lack of attention to people and surroundings, loss of balance, seizures, and hallucinations.

Primary amebic meningoencephalitis is characterized by large areas of hemorrhagic necrosis, especially involving the olfactory bulbs, frontal lobes, base of the brain, proximal spinal cord, and/or temporal lobes of the brain. The parietal lobes and occipital cortex may also be involved. Acute purulent exudate is found in the leptomeninges and within the cortex.

Amebic trophozoites are scattered or may be seen focally in clusters within necrotic cortex. They especially show a predilection for blood vessels; small arteries, arterioles, veins, venules, and capillaries may be surrounded by Naegleria trophozoites containing a large prominent karyosome. This disease progresses rapidly and usually causes death within about 5 days (range 1 to 12 days).

The fatality rate of Nageleria fowleri infection is over 97%.

Laboratory Diagnosis


Cerebrospinal fluid or CNS tissue is the sample of choice for diagnosing meningoencephalitis caused by N. fowleri. Specimen for culture should be collected aseptically and maintained at 20°C to 30°C during transport to the laboratory. If an immediate examination is impossible, the fluid can be kept at room temperature (The amoebae can survive several days at room temperature). The specimen should never be frozen or refrigerated.

Direct Microscopic Examination

Small drops of CSF or tissue suspension should be examined in wet mounts on slides under coverslips by light microscopy under reduced light, or phase-contrast microscopy for motile trophozoites (using a 40x objective). Motile Naegleria may be seen in the preparation (Naegleria is actively motile at 22°C to 25°C). The amebae can also be stained with a variety of stains, such as Giemsa-Wright or a modified trichrome stain.

Morphologic Feature

N. fowleri has two stages-motile trophozoites and non-motile cysts.

Trophozoites of Naegleria are 8-15 μm in diameter and contain large, prominent karyosome. Naegleria has one or more smooth, lobate pseudopods and is actively motile at room temperature. Naegleria species are characterized by a temporary ameboflagellate stage. When Naegleria trophozoites are put in distilled water, they may develop pear-shaped, actively motile forms containing two or more flagella (within 1 to 3 hours).

Cysts of Naegleria fowleri are uninucleate, spherical, 7-15 um in diameter and are surrounded by a relatively thin cyst wall; a feature that makes N. fowleri cyst susceptible to desiccation.  

Identification of Naegleria beyond the genus level is usually achieved by nucleic acid sequencing.

Ameba Culture

Naegleria fowleri can be grown in culture by inoculating it into a growth plate containing lawn culture of Escherichia coli or Klebsiella aerogenes (these bacteria serve as a food source for Naegleria fowleri). The growth plate is then incubated at 42°C to kill other free-living amebae. The appearance of tracks made by an ameba in the lawn culture confirms the presence of thermophilic amebae which could be Naegleria. The presence of Naegleria is then confirmed by PCR or direct visualization.

Antigen detection

Naegleria fowleri antigen present in a sample can be detected using a specific antibody to Naegleria fowleri by indirect immunofluorescence techniques.

Polymerase Chain Reaction

The presence of Naegleria fowleri in CSF or biopsy specimens can be ascertained using molecular techniques. The presence of a specific DNA sequence of this organism can be detected after amplifying it using polymerase chain reaction (PCR).


  1. Clinical Microbiology Procedures Handbook, 4th Edition. ASM Press, Washington, DC. doi: 10.1128/9781683670438.CMPH.ch9.9
  2. Centers for Disease Control and PreventionNational Center for Emerging and Zoonotic Infectious Diseases (NCEZID)Division of Foodborne, Waterborne, and Environmental Diseases (DFWED)
  3. Color Atlas and Textbook of Diagnostic Microbiology, Koneman, 5th edition

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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