By Kelly A. Reynolds, MSPH, PhD

Avid readers of On Tap may recall previous articles on Naegleria fowleri, a pathogenic amoeba that occurs naturally in water environments (On Tap, January 2006; June 2000). The water-based pathogen is making media headlines (“Brain Eating Amoeba” Associate Press, September 30, 2007) as the annual number of deaths in the US again spiked in 2007.

Populations commonly exposed
The causative agent is a parasitic protozoa, Naegleria fowleri. Naegleria fowleri is a free-living amoeba, commonly present in soils and surface waters worldwide (cases reported in diverse locations such as Europe, Australia, New Zealand, Thailand, Africa, India, Korea, Japan, Peru, Venezuela, Panama and the United States). In warm waters (usually above 80oF), the pathogen multiplies, due to an increase in bacterial and algal food sources. The organism has been isolated in recreational freshwater supplies, hot springs, warm water discharges from industrial plants, poorly maintained swimming pools (including therapeutic pools), aquariums, soil and even tap water.

Naegleria fowleri 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 fowleri commonly present in the environment. During periods of thermal additions, the concentrations of Naegleria 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.

In a year-round survey of aquatic environments in Tulsa, Okla. Naegleria fowleri was isolated from 18 percent of the 2,016 processed water and swab samples collected. In Egypt, Naegleria species were detected in 12 out of 16 (75 percent) swimming pool samples; six out of 10 (60 percent) surface water and canal samples. Although first recognized as a human pathogen in 1965, increased awareness and improved diagnostics may account for the apparent increase in incidence. Because death is so rapid and almost certain, confirmation of the disease is usually done post-mortum upon examination of brain tissue or cerebrospinal fluid.

Naegleria is not transmitted via person-to-person contact and it is unable to infect via the gastrointestinal route (i.e., via food and water consumption). There is evidence, however, of possible tap water transmission via bathtub exposures. Humans are infected when the amoeba enters the nasal passages, traveling up the cells of the nervous system to the brain, where it destroys brain tissue, causing severe illness and usually death within one to two weeks. Naegleria fowleri is thought to enter nasal passages when water is forced into the nose via diving or jumping. Although infections are still rare (one in 100 million chance of contracting the disease), the fatality rate is nearly 100 percent, with few treatment options available.

Recent outbreaks
According to the US Centers for Disease Control and Prevention (CDC), an average of one to three Naegleria infections occur in the US each year. From 1995 to 2004, there have been 23 documented deaths, but in 2007 alone, six fatal cases were identified. Infections usually occur in healthy, young people who have a history of exposure to swimming or diving in warm fresh water within the last seven to 14 days. All six cases this year were associated with recreational water exposures; i.e., freshwater lakes, rivers and streams and in males between the ages of 10 and 22. All of the cases were in warm climatic regions (Florida, Texas and Arizona). The sixth case occurred in Arizona, and involved a 14-year-old boy who swam in Lake Havasu who survived for nine days with severe and persistent headaches. Most often, the first symptoms develop two to five days after the last exposure to contaminated water. The illness begins suddenly with the abrupt onset of fever, headache, nausea and vomiting.

If detected early enough, treatment with antibiotics and anti-fungal medications may prevent death; however, many of the symptoms are due to damage having already occurred in the brain. Still, at least four persons have been successfully treated following infection.

Tap water exposures
In 2002, two five-year-old children, both living in Maricopa County, Arizona, died of meningitis caused by Naegleria fowleri. The children did not know each other and did not attend the same school but became sick on the same day and died within 72 hours following hospital admission. Both families reported that the children routinely played in the bathtub. Public health officials were surprised to find that tap water was the only common exposure route of the two children. The water supply was an untreated groundwater source from a private water company serving 6,000 residents at the time of the outbreak. This was the first evidence linking Naegleria to a groundwater source. The groundwater supply from one boy’s home and the refrigerator filter from the nearby home of the second boy’s grandparents, where he reportedly spent a lot of time, were both positive for Naegleria fowleri. A follow-up survey of bathroom and kitchen pipes and sink traps found that 17/19 samples were positive for Naegleria fowleri. Filtered bathwater from both of the homes also tested positive.2

Stagnant water in the domestic setting is also a concern. Outbreak conditions suggest that regrowth of the amoeba in stagnant sections of the domestic water supply may account for higher levels and increased incidence of disease. Those who bathe in untreated groundwater supplies or in tap water supplies with no residual disinfectant may be at increased risk of exposure despite avoidance of recreational water contacts. Domestic water supply contamination was reported in South Australia and traced to growth in overland pipes during a period of hot weather. The exposure sources included a backyard wading pool and bathtub.

The recent national media frenzy was spawned not only by the 2007 spike in CDC-reported cases but also following the release of information from the University of Arizona researcher, Dr. Charles Gerba, stating that Naegleria is commonly isolated from tap water utilizing groundwater sources in Tucson, Ariz. Such findings were surprising since Naegleria outbreaks are primarily associated with surface water exposures. The Arizona study found that twelve out of thirty-five wells initially tested positive with five confirmed positives after subsequent sampling for the organism. Tucson water is disinfected with chlorine prior to distribution. Chlorine is known to be effective at killing Naegleria fowleri; however, not all groundwater supplies are currently disinfected.

Reducing your risk
We know that Naegleria fowleri is ubiquitous worldwide and commonly present in surface waters, groundwater and soils of the US. In fact, 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.

Recreational water continues to be the primary transmission route for Naegleria fowleri and the CDC reports that there will always be a low-level risk of Naegleria infection associated with swimming in warm, freshwater lakes (see Figure 1). Persons who engage in water sports are at increased risk given that the organism must invade the body via entry into the nasal passages where it travels to the brain. More frequent head immersion events and the potential for more forceful entry of water and the amoeba into the nasal passages facilitates the necessary exposure route for the germ to cause disease in humans.

Recreational swimmers should be aware of disease symptoms following swimming in warm waters and especially stagnant water. Recommendations have been issued from tourism agencies, instructing swimmers to avoid submerging themselves completely underwater at hot spring recreational sites, as Naegleria fowleri should be assumed present.

The use of nose-plugs or holding one’s 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 one to two ppm. After swimming, remove water from nose by blowing. In the rare instance of characteristic neurological symptomology, physicians should be made aware of recent recreational water exposures.

Regarding tap water exposures, consumer education of domestic exposure routes and the point-of-entry precautionary measures is the primary contribution the water treatment industry can make to the public regarding this highly fatal pathogen. The organism is susceptible to chlorine disinfection, irradiation and it’s large size (eight to 12 µm) also allows for the use of appropriate filtration units. Using only treated bath waters is a valid consideration and water from wade pools should never be reused.

Following domestic water outbreaks of Naegleria fowleri in Australia, recommendations from the Environmental Health Directorate of Australia include letting bath and shower taps run for a few minutes before use and to test pool and spa disinfectant levels every day. In general, they recommend that water you bathe, wade or swim in should be “clean, cool and chlorinated.”


  1. Tyndall, R.L., et al. 1989. “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, 55: 722-32.
  2. Marciano-Cabral et al., 2003. “Identification of Naegleria fowleri in domestic water sources by nested PCR”. Applied and Environmental Microbiology. 69: 5864-69.

About the author
Dr. Kelly A. Reynolds is an associate professor at the University of Arizona College of Public Health. She holds a Master of Science Degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds has been a member of the WC&P Technical Review Committee since 1997. She can be reached via email at



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