Case 180615 -3

5-year-old, male, Chesapeake Bay retriever

The patient was a hunting dog from Florida, and had a five day history of progressive ataxia and lethargy. Upon presentation to the veterinarian, the patient was minimally conscious with labored breathing and humane euthanasia was elected.

Large portions of the grey and white matter in the cortex and surrounding the ventricles were tan to dark red and liquefied.

One section of cerebral cortex is examined in which the white and grey matter are effaced by a large cavitation. The neuropil is infiltrated by numerous round to ovoid amoebic trophozoites that are 20 to 40 um in diameter. They have abundant eosinophilic, finely granular cytoplasm and a round nucleus with a prominent, central nucleolus. Variable numbers of neutrophils, microglia, and extravasated erythrocytes are scattered throughout the parenchyma. The walls of small diameter vessels are often obscured by eosinophilic, homogenous material (fibrinoid necrosis) and are surrounded by thick cuffs of lymphocytes.

Immunohistochemistry and PCR confirmed the identity of the amoebae as Naegleria fowleri.

Brain (cerebral cortex): Severe, acute, regionally extensive, necrotizing encephalitis with fibrinoid vascular necrosis, perivascular cuffing, and intralesional amoebae

Naegleria fowleri is a free-living amoeboflagellate that exists in cyst, trophozoite, and flagellate forms in soil and warm fresh water worldwide. Infection occurs when trophozoites in the water are flushed into the nasal cavity. The sustentacular cells lining the olfactory neuroepithelium phagocytose the trophozoites, allowing them to penetrate the cribriform plate, enter the subarachnoid space, and ultimately invade the neural parenchyma. The trophozoites produce sucker-like appendages called amebostomes that eat away at the tissue, inducing a cytopathic effect. Other possible virulence factors include the production of cell-lysing proteins (e.g. phospholipase A/B, neuraminidase, elastase, pore-forming proteins) or induction of apoptosis. The amoebas proliferate within the brain, meninges, and cerebral spinal fluid, causing marked necrosis, edema, and fibrinosuppurative exudate especially in subependymal tissue. The trophozoites are round, 8 to 12 um in diameter, and have a round nucleus with a distinctive, centrally located nucleolus. Unlike other free-living pathogenic amoebas, N. fowleri does not form cysts in neural tissue. Other possible differentials for free-living amoebas include Acanthamoeba spp. and Balamuthia mandrillaris.

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