How Would You Diagnose A Naegleria Fowleri Infection?

Naegleria fowleri, also known as “brain eating amoeba”, is a single celled ameba that causes a brain infection known as primary amebic meningoencephalitis (PAM). The ameba is mostly found in warm freshwater (lakes, rivers, hot springs) and soil. Although, Naegleria fowleri infection is rare, the onset of infection is usually acutely fatal. Data shows that from 1962-2018, 145 cases have been reported from US, although only 4 of these infected people survived, making it an aggressively fatal disease with mortality rate of over 97%. It is mostly observed in children and young adults during the summer time. ⁽¹⁾

The infection does not spread via contaminated drinking water; it only causes infection when the contaminated water enters the nose and the ameba travels to the brain to cause brain tissue damage and brain inflammation. Initial symptoms resemble those of bacterial meningitis and start about 5 days after infection. These include nausea, vomiting, fever and headache. With time, neck stiffness, confusion, seizures, loss of balance, altered mental status, hallucinations and coma develop. The symptoms rapidly worsen and cause death within 5 days (ranging from 1-12 days).⁽¹⁾

Primary amebic meningoencephalitis, being a rare, acute and fatal infection, is difficult to diagnose initially, therefore, 75% diagnosis are made after the death of the patient. Naegleria fowleri antigen, nucleic acid and organisms can be diagnosed with laboratory tests including cerebrospinal fluid samples, biopsy or tissue specimens. ⁽¹⁾

CSF is the best specimen for the diagnosis of Naegleria fowleri ameba, which will be identified by trophozoites in the CSF. CSF is obtained through lumbar puncture. There are various methods to examine and visualize N. fowleri ameba, which include direct wet mount microscopy, examination of stained CSF smear, culture, and polymerase chain reaction. However, wet mount microscopy is the diagnostic tool of choice for primary amebic meningoencephalitis. Other CSF findings include bloody CSF with neutrophilia predominating, PMN pleocytosis, elevated opening pressure, RBCs and protein levels along with normal to low CSF glucose levels. Although, head CT scan can show gray matter enhancement and loss of subarachnoid space, the diagnosis is not specific. ⁽²⁾

Direct wet mount examination. The centrifugation of CSF is done at 150 xg for 5 minutes. A drop of extracted sediment suspension is used for the examination on compound light microscopy with 10x or 40x objective on a slide. Using phase contrast microscopy shows trophozoites with lobopodia extension and retraction. Microscopic examination will show motile ameba moving rapidly. Cysts and flagellated stages will be absent for N. fowleri in CSF samples. N. fowleri can also be found with Wright-Giemsa or modified trichrome staining, which will show trophozoites with large karyosome and contractile vacuole. Direct fluorescent antibody staining of CSF smears can be done to detect N. fowleri in the CSF. ⁽²⁾

Culture of ameba. The culture of Naegleria species on non-nutrient agar or agar media is done for increased detection of the species under direct visualization or PCR. Since Naegleria ameba is thermophilic, the culture plate is incubated at 42oC to facilitate the growth of ameba and prevent growth of other ameba. After inspection of the plate, the ameba is identified by tracks left by it after eating bacteria. Further specific testing of these amebas can be done to confirm the presence of Naegleria fowleri species. ⁽²⁾

PCR and other molecular tools such as flow cytometry to track infections to a source for recognizing potential environmental risks on different water bodies. (2)
Histology: N. fowleri causes lesions at the base of brain, cerebellum and brain stem. Lesions are associated with acute necrotizing meningoencephalitis with purulent exudates. Olfactory mucosa and bulbs are the commonly affected sites. Immunofluorescence and immunoperoxidase methods are used to determine N. fowleri ameba trophozoites in the histologic sections of the samples from brain biopsy. These CNS lesions are devoid of any cysts, though.⁽²⁾

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How Would You Diagnose A Naegleria Fowleri Infection?