Naegleria fowleri is a ubiquitous free-living ameba that is the etiologic agent in primary amebic meningoencephalitis (PAM). Although N fowleri rarely causes disease, it is important because diagnosis can be difficult and PAM is rapidly fatal in more than 95% of cases. In the summer of 2007, 6 fatal cases of N fowleri infection occurred in the United States, all young males.
The earliest known case of N fowleri –associated disease dates back to 1937 and occurred in a patient from Virginia; however, this case was not reported until 1968, when Dos Santos identified the patient during a retrospective review of autopsies. In 1965, Fowler and Carter published the first report of N fowleri –associated CNS disease.
In this initial report of 4 patients in Australia, the authors suggested the etiologic amebae probably belonged to the genus Acanthamoeba; however, subsequent investigation shows these cases were most likely due to N fowleri. Acanthamoeba infections tend to progress slower than N fowleri infections, with insidious symptoms that present weeks or months after exposure; these symptoms are more similar to the presentation of a bacterial brain abscess or tumor than to N fowleri infection.
In 1966, Butt reported the first case of N fowleri meningoencephalitis in the United States and coined the term PAM.
The term was chosen to distinguish the disease caused by N fowleri from the secondary meningoencephalitis due to the extension of Entamoeba histolytica from another site.
N fowleri is a member of the subphylum Sarcodina, superclass Rhizopodea. Rhizopodea includes the free-living amebae Entamoeba histolytica and species of Acanthamoeba, Hartmannella, Balamuthia, Naegleria, and others. Although N fowleri is one of several species in the genus Naegleria, to date, it remains the only Naegleria species known to produce human disease.
Most N fowleri infections have occurred in children and young adults who have had recent exposure to swimming or diving in warm freshwater.
The thermophilic nature of N fowleri allows it to survive in waterways contaminated by thermal discharges from power plants, heated swimming pools, and even hot springs with temperatures up to 45°C (113°F). Most cases of PAM occur during the summer months, when freshwater sources are warm. When water temperatures decrease, N fowleri encyst and enter a dormant stage, which allows them to survive until the next summer.