Experimental Naegleria fowleri meningoencephalitis in sheep: light and electron microscopic studies.

A sheep infected intranasally with Naegleria fowleri of human origin died 7 days after inoculation. The olfactory lobes were distinctly soft and friable. Histologic findings indicated suppurative leptomeningitis and hemorrhagic necrosis in the olfactory lobes. Protozoa were disseminated in the necrotic areas, particularly in perivascular locations; vasculitis was also observed. Meningitis and perivascular cuffing with lymphocytes were evident in the cerebrum, cerebellum, pons, medulla, and cervical spinal cord. Electron microscopy disclosed trophozoites only in the olfactory lobes. The amebae contained a central nucleus with a distinct, electron-dense nucleolus. The cytoplasm contained myelinated figures, lipid-like vacuoles, open vesicles, electron-dense granules, mitochondria, numerous free ribosomes, scant rough endoplasmic reticulum, and occasionally a phagocytized erythrocyte. Trophozoites were grouped close to arterioles, except when phagocytized by a neutrophil or endothelial cell.