Diagnosis & Detection
- Clinicians: For 24/7 diagnostic assistance, specimen collection guidance, shipping instructions, and treatment recommendations, please contact the CDC Emergency Operations Center at 770-488-7100. More detailed guidance is under Resources for Public Health & Medical Professionals.
Primary amebic meningoencephalitis (PAM) is a serious infection and inflammation of the brain caused by Naegleria fowleri 1, 2. The disease is diagnosed using specific laboratory tests available in only a few laboratories in the United States. Because of the rarity of the infection and difficulty in initial detection, about 75% of diagnoses are made after the death of the patient.
PAM and Naegleria fowleri infection can be diagnosed in the laboratory by detecting 3:
- Naegleria fowleri organisms in cerebrospinal fluid (CSF), biopsy, or tissue specimens, or
- Naegleria fowleri nucleic acid in CSF, biopsy, or tissue specimens, or
- Naegleria fowleri antigen in CSF, biopsy, or tissue specimens.
Naegleria fowleri test methods
Direct visualization 4, 5: The motile amebae can often be seen moving rapidly under a microscope when looking at a fresh sample of CSF. The amebae can also be stained with a variety of stains, such as Giemsa-Wright or a modified trichrome stain, for identification.
Antigen Detection 4, 5: A specific antibody to Naegleria fowleri can be used in conjunction with another antibody that deposits a chemical (immunohistochemistry [IHC]) or glows under specific types of light (indirect immunofluorescence [IIF]) to directly stain the amebic antigens in tissue.
Polymerase Chain Reaction (PCR) 6, 7, 8: Specific molecular tools can amplify DNA from the amebae in CSF or tissue to specifically identify if the amebae are present. Looking at strains or subtypes of Naegleria fowleri can be done, but little is known about the natural populations in the environment, which makes it difficult to interpret what the findings mean.
Ameba culture 9: The amebae can be grown in culture to increase the likelihood of detecting the ameba by direct visualization or PCR. The sample is added to a growth plate covered in bacteria that can serve as a food source for Naegleria fowleri. The initial screening is accomplished by incubating the plate at a higher temperature (108°F/42°C) that kills most free-living amebae, while selecting for heat-loving (thermophilic) amebae, such as Naegleria fowleri or other amebae. This initial screen shows up as tracks made by an ameba as it moves across the plate eating the bacteria. If there are no amebae on the plate grown at the higher temperature, then Naegleria fowleri is not present. If heat-loving (thermophilic) amebae are present on the plate grown at the higher temperature, then these amebae undergo further specific testing to determine whether Naegleria fowleri is present since other free-living amebae can also be heat-loving (thermophilic). (NOTE: Amebae 10-12, including thermophilic amebae 13 other than Naegleria fowleri, can be common in water systems but none of these other amebae cause primary amebic meningoencephalitis (PAM)).
Environmental Detection: Water samples can be collected, concentrated, and put into culture to grow and select for Naegleria fowleri. Samples can be tested using the serologic or molecular methods described above.
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