Primary amoebic meningoencephalitis laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Hardik Patel, M.D.

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Overview

Primary amoebic meningoencephalitis 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. It can be diagnosed in the laboratory by detecting Naegleria fowleri organisms, nucleic acid, or antigen in cerebrospinal fluid (CSF), biopsy, or tissue specimens.

Laboratory Findings

CSF Analysis

Cerebrospinal fluid (CSF) studies of patients infected with Naegleria fowleri typically demonstrate a pattern similar to bacterial meningitis with an elevated opening pressure, a polymorphonuclear pleocytosis, normal or low glucose, and elevated protein. However the observations of blood in the CSF and/or motile amoeba are clues to a potential diagnosis of PAM.

Direct Visualization

The motile Naegleria fowleri 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.

Trophozoite of Naegleria fowleri in CSF, stained with H&E

Trophozoite of Naegleria fowleri in CSF, stained with trichrome. Image courtesy of the Texas State Health Department.

Indirect Immunofluorescence (IIF) assay for Naegleria fowleri, 1000x oil magnification

Copyleft images obtained courtesy of http://www.cdc.gov/parasites/naegleria/diagnosis-hcp.html

Immunohistochemistry (IHC)

A specific antibody to Naegleria fowleri can be used in conjunction with another antibody that deposits a chemical or glows under specific types of light (indirect fluorescent antibody [IFA]) to directly stain the amebae in tissue.

Polymerase Chain Reaction (PCR)

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.

Amoeba Culture

The sample is added to a growth plate covered in bacteria that can serve as a food source for Naegleria fowleri. Incubating at higher temperatures selects for Naegleria fowleri growth, which can be seen as tracks made by the ameba as it moves across the plate eating the bacteria. Growing Naegleria fowleri in mammalian cell culture and looking for toxic cell effects is also possible.

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.