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

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

Abstract. Primary amoebic meningoencephalitis (PAM) was induced in mice by intranasal inoculation of Naegleria fowleri (Singh et Das, 1970) to study the role of the blood vessels and lungs in the early and later stages in this disease. Upon culturing blood and lung tissue obtained at 24-, 36-, 48-, 72-, 96-, and 120-hour time periods, it was found that amoebae grew only from blood and lung tissue obtained at the 96 and 120 hour time periods. Paraffin sections of the head revealed small foci of acute inflammation and amoebae within the olfactory bulb of the central nervous system (CNS) at 24 hours. Amoebae were not observed within blood vessels of the CNS until 96 and 120 hours. Also, amoebae were observed within the connective tissue surrounding blood vessels and sutures of the skull, bone marrow, and venous sinusoids between the skull bone tables at 96 and 120 hours. No amoebae or acute inflammatory reactions were observed in the lung sections from any time period and indirect immunofluorescence microscopy was negative for N. fowleri. This study provides evidence that neither blood vessels nor lungs provide routes for N. fowleri to the CNS during the early stages of PAM and that amoebae enter veins of the CNS and bone marrow during later stages of the disease.

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