Primary Amoebic Meningoencephalitis

Prepared by Steven Wiersma, MD, MPH
(update from original work by Bill Bigler, PhD)
Bureau of Epidemiology

July 26, 2002

Primary Amoebic Meningoencephalitis (PAM) is a rare disease caused by infection with the amoeba Naegleria fowleri. This amoeba is commonly found in the environment. It is most commonly found in soil or warm, stagnant bodies of fresh water, such as lakes, rivers, and hot springs, unchlorinated pools, and in discharge or holding basins worldwide.1,2,3

Naegleria fowleri trophozoites, cultured from cerebrospinal fluid. These cells have characteristically large nuclei, with a large, dark staining karyosome. The amoebae are very active and extend and retract pseudopods. Trichrome stain. From a patient who died from primary amoebic meningoencephalitis in Virginia. Image: courtesy of Centers for Disease Control

Infection occurs rarely in humans when the amoeba is believed to enter the body through the nose and travel to the brain via the olfactory nerve where it can cause swelling of the brain or linings of the brain. Onset begins abruptly with headache, sometimes with slight upper respiratory inflammation, rapidly progressing to include fever, vomiting, symptoms of meningitis and other central nervous system involvement, followed within a few days by deep coma and death.4 Infections usually result in death within 7-10 days of onset.

There are less than 200 cases of disease reported in the world literature and fewer than 20 have been documented in Florida2,5. Cases are usually reported in children and young adults who have recent exposure to freshwater lakes or streams. The first cases documented in Florida come from Orange County in 1962.6 A review of PAM cases in 1990 included 14 Florida cases as of January 1990. Thirteen of 14 cases were in males and the age of cases ranged between 2 and 23 years of age (mean 12 years) with onset between July and October. Bureau records indicate 2 cases during the 1990s. A 14-year-old boy died from PAM in 1997 after swimming at a canal in Orange County. A second fatality from PAM was confirmed in a 19-year-old Palm Beach County boy in 1999. Other sporadic cases may have occurred during the intervening decade, but these cases are not reportable and therefore complete data are not available.

Naegleria fowleri trophozoite in spinal fluid. Trichrome stain. Note the typically large karyosome and the monopodial locomotion. Image contributed by Texas SHD.

A survey conducted in the early 1970s found that over 46% (12/26) of all lakes surveyed in Florida have the pathogenic amoeba and it is believed that more extensive sampling would result in recovery of the amoeba from most Florida lakes.7 Studies have conclusively shown pathogenic Naegleria to be widely distributed in lake bottom sediment or at the sediment/lake water interface.8

Naegleria fowleri is known to thrive in freshwater where the temperature exceeds 86° F. While studies indicate that chlorination of pools, ponds etc. can destroy cysts and trophozoites of pathogenic Naegleria, there are no practical means of controlling the amoeba in lakes and streams.9

The following guidelines for prevention appear on the website for the Centers for Disease Control and Prevention (CDC), however their scientific basis is not established and their ability to change the low incidence of this disease is unclear.
· Do not swim or jump into warm, stagnant, fresh water, such as ponds or warm water discharge pools, or unchlorinated swimming pools.
· Do not swim in polluted water.
· Do not swim in areas posted as "No Swimming."
· Hold your nose, or use nose plugs when jumping or diving into water.

References
1. John DT. Primary amoebic meningoencephalitis and the biology of Naegleria fowleri. Annu Rev Microbiol 1982;36:101-23.
2. Visvesvara GS, Stehr-Green JK. Epidemiology of free-living amoeba infections. J Protozool 1990 Jul-Aug;37(4):25S-33S.
3. Duma, R. J. et al. Primary amoebic meningoencephalitis. N. Eng. J. Med. 1969;281:1315-23.
4. Lubor, C. amoebic meningoencephalitis. In Med. Microbiology of Infectious Diseases, Brause, A.I., Davis, C.E. and Fierer, J (eds) W. B. Saunders Co. 1981;pp. 1281-1284.
5. Viriyavejakul P, Rochanawutanon M, Sirinavin S. Naegleria meningomyeloencephalitis. Southeast Asian J Trop Med Public Health 1997 Mar;28(1):237-40.
6. Butt, C.G.: Primary amoebic meningoencephalitis. New Eng. J. Med. 1966;274:1473-6.
7. Wellings, F. M. et al. Pathogenic Naegleria: Distribution in nature EPA Research and Development Bulletin No. 600/1-79-018.
8. Wellings, et al. Isolation and identification of pathogenic Naegleria from Florida lakes. Applied and Environ. Micro. 1977;34:661-667.
9. De Jonckheere, J. et al. Differences in destruction of cysts of pathogenic and nonpathogenic Naegleria and Acanthamoeba by chlorine. Appl. Environ. Microbiol. 1976;31:294-297.

Other useful phone numbers:

Florida Poison Information Center
1-800-282-3171

Florida Emergency Information Line
1-800-342-3557

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