A light microscopy study of the migration of Naegleria fowleri from the nasal submucosa to the central nervous system during the early stage of primary amebic meningoencephalitis in mice

The migratory pathway of Naegleria fowleri from the nasal submucosa to the central nervous system (CNS) during the early stage of primary amebic menin goencephalitis (PAM) was investigated in mice. Twenty-one-day-old CD-1 mice were inoculated by intranasal instillation of 1 x 10(6) amebas. Animals we re divided into 3 groups of 5 and, after being anesthetized, were killed at intervals of 24, 32, and 48 hr postinoculation by transcardial perfusion w ith formaldehyde, acetic acid, and methanol. The heads were decalcified, di vided in the midsagittal plane, and the area of the cribriform plate remove d and embedded in paraffin. Serial sections were cut at 8 mu m and stained with a combination of celestin blue, Harris' hematoxylin, and acid fuchsin for light microscopy. Focal inflammation and amebas were observed in the su bmucosal nerve plexus, olfactory nerves penetrating the cribriform plate, a nd the olfactory bulb of the brain as early as 24 hr postinoculation. The t ime periods selected assured that the disease process would not obliterate soft tissue structures. Earlier studies used moribund mice in which the inf lammation and the number of amebas were overwhelming. The present study pro vides convincing evidence that amebas gain initial access to the CNS throug h olfactory nerves within the cribriform plate during the early stages of P AM.