Non-Naegleria amebic meningoencephalitis.

In 1958, the potential for free-living amebas to produce disease in Man was fiLrst reported in the journal Science by Culbertson, Smith, and Minner (1), after a species of Acanthamoeba, thought initially to be a vaccine strain of poliomyelitis virus, was found "contaminating" a batch of polio vaccine. This ameba produced a cytopathic effect within 48 hours when inoculated onto primary monkey kidney cells and proved to be rapidly fatal when inoculated into mice or monkeys, producing a paralytic disease characterized by hemorrhagic necrosis of the cerebral cortex. In 1965, the discovery of natural infections in humans due to free-living amebas occurred when Fowler and Carter (2) in South Australia reported several autopsied cases of remarkably similar but previously undescribed meningoencephalitis. Noted within cerebral tissues were large numbers of peculiar, magenta-colored cells, each containing a single large karyosome and what appeared to be a contractile vacuole. Although these cells had been called "gitter cells" or macrophages by others, they were instead amebas. Because of Culbertson's work, Fowler and Carter believed the organism responsible for the disease they described was an Acanthamoeba; however, subsequently it was shown that the organism was instead Naegleria, a related, free-living amebo-flagellate, but distinctly different from Acanthamoeba. Soon afterwards, the term Primary Amebic Meningoencephalitis or PAM was coined to distinguish a disease process characterized by direct invasion of the central nervous system (CNS) by aerobic, free-living amebas from that produced by Entamoeba histolytica, an anaerobic, coprozoic parasite, which secondarily infects the CNS from a distant site, usually located in the lung and/or liver. PAM produced by Naegleria is a relatively well known entity, both to physicians and public health officials, because the source is fresh water, and infection is usually acquired from swimming in public lakes or pools; or because the organism is known to cause epidemics; or because the infections due to it have been so dramatically devastating and rapidly fatal; or most likely because of all of the aforementioned. Be that as it may, the object of this presentation is to discuss and compare the clinical, pathological and epidemiological features of a less-well understood entity,