Free-living amebic meningoencephalitides: comparative study

Primary amebic meningoencephalitis (PAM) is an infectious disease essentially confined to the central nervous system (CNS) and caused by free-living ameboflagellates of the genus Naegleria (N) and Acanthamoeba (A). N produces an acute fulminant, usually fatal hemorrhagic-necrotizing meningoencephalitis, associated with an inflammatory reaction composed of neutrophils, eosinophils, macrophages and a few lymphocytes. The incubation period is between 3-7 days. Typically, PAM occurs in healthy, young individuals, who frequently have a history of swimming or washing their face in infested waters. The portal of entry into the CNS is through the olfactory neuroepithelium, at the level of the cribriform plate and invasion of the amyelinic submucosal nervous plexus. Trophozoites are the only amebic forms found in the lesions. By contrast, A produces a sub-acute or chronic granulamotous meningoencephalitis (AM) with multinucleated foreign body giant cells, lymphocytes and monocytes. Cysts and trophozoites may be present in the lesions. AM have been reported in chronically ill debilitated individuals, some of them under immunosuppressive therapy, without history of recent swimming. The portal of entry into the CNS appears to be by hematogenous route. The incubation period is unknown, but perhaps more than 10 days. This comparison indicates that infection due to Naegleria produces a clearly defined clinco-pathological entity, which differs significantly from that due to Acanthamoeba, and both species of amebus should be considered in the differential diagnosis of amebic meningoencephalitis.