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  • P.K. Peterson

Some Like it Hot: Naegleria fowleri (“Brain-eating Amoeba”)

“State officials were stunned when the amoeba claimed its first Minnesota victim that year [2015]. The parasite, which causes primary amebic meningoencephalitis, had never been found this far north.”

- Lorna Benson, American health journalist


“Broader education about the disease and efforts to prevent infection in the first place can help stop this killer in its tracks.”

- Claire Panosian Dunavan, M.D., professor, David Geffen School of Medicine, UCLA




Some microbes like it hot; Naegleria fowleri (aka the “brain-eating amoeba) is one of them. This free-living amoeba causes primary amoebic meningoencephalitis (PAM), a highly destructive brain infection. In this Germ Gems post, I explain the nature of the infection, its epidemiology, and why the number of fatal PAM cases is increasing in American youngsters.

Naegleria fowleri pathogenesis. Named after Malcolm Fowler. the Australian pathologist who described the initial cases of PAM, Naegleria fowleri is a eukaryotic, free-living amoeba. As a heat-loving (thermophilic) organism, it thrives in heat and likes warm water. It grows best at high temperatures up to 115°F (46°C) and can survive for short periods at even higher temperatures.


Naegleri fowleri is a free-living amoeba, meaning it only rarely infects humans, in contrast to another amoeba, Entamoeba histolytica, which is the cause of more than 50 million cases of amoebiasis in humans globally per year. Also, unlike Entamoeba histolytica, which is acquired by the fecal-oral route and is mainly a gastrointestinal pathogen, Naegleria fowleri infects people when water containing the parasite enters the body through the nose. This typically happens when people are swimming or diving, or when they put their heads under fresh water in lakes and rivers. (Less commonly, it occurs when people use contaminated tap water to cleanse their noses during religious practices. And in very rare instances, people become infected from recreational water that didn’t contain enough chlorine.)


Once in the nose, Naegleria fowleri amoebas can make their way into the central nervous system causing the usually fatal brain infection, PAM. In its early stages, the signs and symptoms of PAM are similar to bacterial meningitis. The first symptoms of PAM (headache, fever, nausea or vomiting) start within five days of infection. Later symptoms include a stiff neck, confusion, hallucinations, seizures, and coma.


After symptoms start, PAM progresses rapidly and usually causes death within five days. The death rate is over 97%. According to the Center for Disease Control and Prevention (CDC), only four people survived out of 154 known cases of PAM in the U.S. from 1962 to 2021.

Epidemiology of N. fowleri infections. Naegleria fowleri infections in the U.S. are considered rare. As already mentioned, the CDC has recorded 154 cases of PAM from 1962 through the present. A large majority of cases occurred in people infected by exposure to recreational water. Given Naegleria fowleri’s predilection for warm water, most cases have been linked to swimming in southern states, such as Florida and Texas. For unknown reasons, most cases occur in young males, especially those 14 years old and younger.


Why are cases of PAM increasing? The greenhouse gas, carbon dioxide, plays a pivotal role in global warming, including warming of our lakes and rivers. Evidence suggests that the range of Naegleria fowleri is expanding northward, possibly as a result of climate change and warmer temperatures.

Since 2010, an increasing number of cases of PAM have been confirmed at higher latitudes, including in Nebraska, Iowa, Indiana, Maryland, Minnesota and northern California. In July 2015, the Minnesota Department of Health reported the third PAM fatality in the state—a 14-year-old boy who was infected when swimming in Lake Minnewaska. A previous Minnesota case had spurred a nonprofit group, “Swim Above Water,” chaired by the parents of a 11-year-old girl who died of PAM after swimming in a small lake near their home.


In 2022, a total of three cases of fatal PAM were reported in Iowa, Missouri, and Nevada. In a July 19, 2022 MedPage Today article by infectious diseases physician Claire Panosian Dunavan, “Hazardous Waters: Lessons From a Brain-Eating Amoeba,” she poses the question, “So what can be done to lessen PAM’s threat? Among other thoughts, how about widespread education targeting schoolkids, parents, and frontline physicians; posting more warnings at hot springs and other high-risk recreational settings.”


Treatment and prevention of PAM. Currently, PAM is treated with a combination of drugs, often including antifungal agents, amphotericin and fluconazole, antibacterial drugs, azithromycin and rifampin, an antiparasitic agent, miltefosine, and the anti-inflammatory drug, dexamethasone. These drugs have been used to treat patients who survived. But with a fatality rate over 97%, the need for clinical trials of agents that are potentially more effective seems obvious.


The fact that PAM is a rare disease is both good news and bad news. On the side of the bad news, a “rare disease” is generally considered to be a disease that affects fewer than 200,000 Americans at any given time. There are more than 6,800 rare diseases, and altogether, rare diseases affect an estimated 25-30 million Americans. Thus, it seems unlikely that we’ll see randomized clinical trials of anti-amoebic drugs anytime soon.

This sobering appraisal underscores the huge importance of measures to prevent PAM. First and foremost among these is “getting the word out.” Campaigns to alter swimming behaviors that cause water to go up the nose, such as diving or jumping into the water, and “keeping your head above water,” as well as “wearing nose plugs” seem reasonable.

In her article on “Hazardous Waters,” Dr. Dunavan points out that many recent cases of PAM have affected men in the sub-Indian continent who neither splashed nor swam but who performed nasal ablution. She also references a Lancet Infectious Diseases 2020 publication, “Extended summers and prolonged humid conditions due to climate change provide an ideal environment for amoebas to flourish in bodies of water.”


The good news and bad news regarding dealing with the underlying problem of climate change were both on display at the November, 2022 COP (Conference of the Parties) 2027 meeting in Sharm el-Sheikh Egypt. While awareness of the need to address climate change in an equitable manner is clearly mounting, we have a long way to go. Currently, it appears to me that discussions about what needs to be done most urgently often generate more heat than light, which may be a good thing for Naegleria fowleri but certainly isn’t good for Homo sapiens.

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Main Page images courtesy of Shuxian Hu, MD. Dr. Hu is a scientist in the Neuroimmunology Research Laboratory at the University of Minnesota.

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