By Kelly Reynolds, PhD

Naegleria fowleri is perhaps one of the most daunting waterborne pathogens known in the United States. Although infections are still rare—with a 1 in 100 million chance of contracting the disease—the fatality rate is nearly 100 percent, with few treatment options available. The parasitic protozoa is actually a free-living amoeba, commonly present in soils and surface waters worldwide with cases reported in diverse locations such as Europe, Australia, New Zealand, Thailand, Africa, India, Korea, Japan, Peru, Venezuela, Panama and the United States. Warm waters above 80o F is where the pathogen multiplies, in conditions specific to the increase of bacterial food sources.

The amoeba infects by entering the nasal passages of swimmers, traveling up the cells of the nervous system to the brain, where it effectively destroys brain tissue, causing severe illness and usually death within about a week.

Naegleria fowleri was first recognized as a human pathogen in 1965, and since then more than 179 cases have been recorded, with 81 reported in this country alone. Infections usually occur in healthy, young people. Because death is so rapid and almost certain, confirmation of the disease is usually established post-mortum, after an examination of brain tissue or cerebrospinal fluid.

The pathogen has been isolated from warm, fresh or brackish water including swimming pools, ponds, lakes, streams, hot springs, thermally polluted waters, dust, soil and sewage. N. fowleri can enter nasal passages when water is forced into the nose via diving or jumping but may also become motile into nasal cavities if the swimmer is submerged underwater. Cases have also occurred in persons with no recent contact with water, suggesting an airborne route where dust particles may have been a carrier of the stable cyst form of the organism.

Evidence of Naegleria presence has mounted over the years. The organism has been isolated from hot water systems of hospitals, where 22 percent of the samples collected from six hospitals were positive. In addition, thermal discharges from power plant facilities greatly contribute to the growth of Naegleria commonly present in the environment. During periods of thermal additions, concentrations of N. fowleri increase by as much as two orders of magnitude.1 Concentrations may not return to previous levels for 30-to-60 days following the cessation of warm water discharges.

A year-round survey of aquatic environments in Tulsa, Okla., showed that N. fowleri could be isolated from 18 percent of the 2,016 processed water and swab samples collected. A study of 30 hot spring spas in Kanagawa, Japan, revealed that Naegleria could be isolated from 46.7 percent of the samples taken.2 Although the species were not the pathogenic N. fowleri, the presence of other free-living protozoa suggest the ability for the persistence of harmful species as well. Finally, in Egypt, Naegleria species were detected in 12 out of 16 swimming pool samples (75 percent), and six out of 10 surface water and canal samples (60 percent). Animals may also become infected with the organism, but little is known about the role of animal transmission to humans either directly or indirectly.

Since 1971, the Centers for Disease Control (CDC) and U.S. Environmental Protection Agency (USEPA) have maintained a collaborative surveillance system for collecting and periodically reporting data relating to occurrences and causes of waterborne disease outbreak.3 During the period from 1995-96, 37 outbreaks from 17 states were attributed to recreational water exposure, affecting approximately 9,129 persons. Of these, 22 were outbreaks of gastroenteritis (59 percent); nine were of dermatitis (24.3 percent); and six were of Primary Amoebic Meningitis (PAM), caused by N. fowleri (16.2 percent), all six of which were fatal.

Morbidity and mortality
Infections with N. fowleri usually result in PAM, a deadly disease of the brain with symptoms usually occurring between 3-to-7 days after infection and including:

  • Swelling and fluid buildup in the brain,
  • Nose bleeds,
  • Swollen lymph nodes,
  • Rapid, shallow breathing,
  • Light sensitivity, drooping eyelids,
  • Severe headache and nausea,
  • Loss of appetite, smell and taste, vomiting,
  • Sore throat, stiff neck and back, cramping, weakness,
  • Confusion, seizures, coma, and
  • Death.

If detected early enough, treatment with antibiotics and anti-fungal medications may prevent the spread of the disease; however, many of the symptoms listed are the result of damage that has already occurred in the brain. At least four persons have been successfully treated following infection.

According to the CDC, there is an average of 1-to-3 infections in the United States each year. In 1995, the six fatal cases of PAM attributed to N. fowleri were all acquired in the summer months (July and August) involving children ranging in ages from 4-to-11 years. Infection was from a shallow lake, pond, canal or river. Five of the six cases occurred in Texas and one in Florida.

Many countries have low autopsy rates and thus some researchers believe that the incidence of N. fowleri and other pathogenic amoeba may be underdiagnosed worldwide. In one U.S. instance, an 11-year-old boy died of what was diagnosed as bacterial meningitis. Following donations of several of his organs to recipient patients, it was later discovered the actual cause of death was PAM caused by N. fowleri.

N. fowleri is ubiquitous worldwide and commonly present in surface waters and soils. Free living amoebae are the main predators of bacterial populations in the environment and play a major role in the ecological balance of many environmental systems. Humans are frequently exposed to the amoebic organism but few infections occur. In most cases it seems a specific set of criteria must be present before infections ensue. Primarily, the organism must not only be present but must grow to high numbers—as evidenced by outbreaks in warm, polluted or stagnant water. In addition, the organism must be introduced to the nasal passages.

Recreational swimmers should be aware of disease symptoms following swimming in warm waters. Swimming in stagnant water is particularly risky, since concentrations of the pathogen increase in this environment by feeding off resident bacteria. Recommendations have been issued from tourism agencies instructing swimmers to avoid submerging themselves completely underwater at hot spring recreational sites, as N. fowleri should be assumed present.

The use of nose plugs or holding the nose when jumping into potentially contaminated water sources is also advised. Warnings from areas where infections have been documented in the past should be considered. Swimming pools should have a chlorine residual of at least 1-to-2 parts per million (ppm). After swimming, remove water from nose by blowing through the nasal passages, to reduce the chance of Naegleria infections.

As an industry, there isn’t much that can be done to completely eliminate exposures to N. fowleri. Education of potential exposures and the recommendation of precautionary measures are primary contributions the water treatment industry can make to the public regarding this highly fatal pathogen. In addition, it’s imperative to educate others on the importance of exercising control, where possible, by maintaining proper disinfectant residuals in public and private pools and spas.


  1. Tyndall, R.L., K.S. Ironside, P.L. Metler, E.L. Tan, T.C. Hazen, and C.B. Fliermans, “Effect of thermal additions on the density and distribution of thermophilic amoebae and pathogenic Naegleria fowleri in a newly created cooling lake,” Applied Environmental Microbiology, 1989. 55(3): p. 722-32.
  2. Kuroki, T., K. Yagita, E. Yabuuchi, K. Agata, T. Ishima, Y. Katsube, and T. Endo, “Isolation of Legionella and free-living amoebae at hot spring spas in Kanagawa, Japan,” Kansenshogaku Zasshi, 1998. 72(10): p. 1050-5.
  3. Levy, D.A., M.S. Bens, G.F. Craun, R.L. Calderon, and B.L. Herwaldt, “Surveillance for waterborne-disease outbreaks–United States, 1995-1996,” Mortality Weekly Report, CDC Surveillance Summary, 1998. 47(5): p. 1-34.

About the author
Dr. Kelly A. Reynolds is a research scientist and microbiologist at the University of Arizona with a focus on the development of rapid methods for detecting human pathogenic viruses in drinking water. She also has been a member of the WC&P Technical Review Committee since 1997.

For more information
More information on Naegleria fowleri can be found online at:

PHOTO CREDITS: Photos courtesy of Tim Osbon, Medical University of South Carolina
Any questions? Guidelines for the control of Amoebic Meningoencephalitis

The typical viral or bacterial meningitis or aseptic meningitis involves only the meningeal cells (thin layers of tissue covering the brain and spinal cord) and the ependymal cells (cells that make up the lining membrane of the ventricles of the brain and the central canal of the spinal cord) and recovery is almost always complete.

Amoebic meningoencephalitis involves more extensive damage to the brain, involving the brain parenchyma (living cells of the soft tissue, i.e., the brain itself) and typically causes death or irreversible brain damage. Other notes include:

Clinical Features
A serious disease of brain, entering via the nasal mucosa and olfactory nerve and causing a syndrome with symptoms that include sore throat, severe headache, neck stiffness and death within 10 days, but usually on the fifth or sixth day.

The amoebae are free-living in water, soil and vegetation.

Incubation Period
Usually three to seven days

Period of Communicability
There is no person-to-person transmission. No isolation, concurrent disinfection or quarantine is necessary.

Amphotericin B, miconazole and rifampicin. Recovery from infection is rare.

Control of Contacts
Investigation of contacts and source of infection should take place.

Preventive Measures

  • Educate public on the risk of swimming in lakes and ponds where water is stagnant or where the infection has been acquired
  • Maintain a residual chlorine of 1-to-2 ppm in swimming pools
  • Chlorinate public water supplies that are naturally warm.
    Source: Department of Human Services, Public Health Division, Government of Victoria, Australia.

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