Meet the Brain-Eating Amoeba

On May 29, 2025, the U.S. Centers for Disease Control and Prevention (CDC)  confirmed  that a 71-year-old woman in Texas died after contracting the “brain-eating amoeba,” Naegleria fowleri, by rinsing her sinuses with tap water from an RV at a campground near Austin. She fell ill four days later and, despite aggressive treatment, suffered seizures and died eight days after symptoms began. While the amoeba was not found in water samples collected later, investigators reported poor water quality and inadequate disinfection at the site. Last summer, three cases of Naegleria fowleri infection were reported in Israel, including a 10-year-old boy. All were linked to exposure to warm freshwater in the Sea of Galilee region, including the Gai Beach Water Park, which was temporarily closed as a precaution.

The disease caused by the brain-eating amoeba is known as Primary Amoebic Meningoencephalitis (PAM). It is extremely rare and highly lethal.  In the United States, an average of just three cases are reported each year, and nearly all result in death. Between 1962 and 2021, only four out of 154 known patients survived, reflecting a fatality rate exceeding 97 percent.

Almost all patients with PAM—most of whom are male—experience symptoms that resemble bacterial meningitis. The early symptoms include a very severe frontal headache (often between the eyes), fever, nausea, and vomiting. Soon afterward, neck stiffness (a classic sign of meningitis) appears. In the specific case of PAM, patients also suffer from a loss of the sense of smell. Neurological symptoms quickly worsen and may include confusion, light sensitivity, hallucinations, and seizures—progressing to coma and, in most cases, death within five days of symptom onset.

Because the disease is so rare and its symptoms mimic those of far more common bacterial meningitis, it is easy to misdiagnose unless healthcare providers are aware of its existence. In one Israeli case, a fast diagnosis was made by Dr. Tamar Gutsman, head of the Infectious Disease Unit at the Rabin Medical Center, who linked the patient’s recent exposure to freshwater in the Sea of Galilee to possible Naegleria fowleri infection.

Each year, an average of three people in the United States are infected by the amoeba. Naegleria fowleri in its various life stages | CDC, via Wikimedia Commons

To Live Free or to Kill

Why Should Exposure to a Water Source Raise Suspicion of Brain-Eating Amoeba Infection in Cases of Encephalitis? 

The brain-eating amoeba, despite its dramatic—and justified—nickname, does not typically live in the human body. Instead, it exists freely in warm freshwater and soil and is found worldwide. Scientifically known as Naegleria fowleri, it is a single-celled, nucleus-containing organism from the genus Naegleria, within the phylum Heterolobosea and the group Discoba.

In warm freshwater, Naegleria fowleri exists primarily in its amoeba form—an amorphous, single-celled organism that moves using pseudopods ("false feet") and feeds on bacteria, yeast, and algae along the bottom. This is known as the trophozoite stage, during which the amoeba can eat, reproduce by cell division, and—accidentally—in rare cases, infect humans. When environmental conditions become unfavorable, such as when food becomes scarce, the amoeba temporarily transforms into a flagellate stage—a pear-shaped, motile cell with two small whip-like flagella that help it swim in search of a better environment. In this form, it does not feed or reproduce and reverts to the trophozoite stage once suitable conditions return.

If escape isn’t possible—such as when a water source dries up or becomes too cold—Naegleria fowleri enters its third life stage: the cyst. In this form, the amoeba becomes a dormant, spherical structure that does not move, feed, or reproduce. Cysts are highly resilient and can survive for months in frozen lakes or years in soil, and may even be dispersed through dust. When conditions improve, the amoeba reactivates and returns to its active form.

Naegleria fowleri is found on every continent except Antarctica and naturally inhabits warm freshwater environments—including ponds, streams, lakes, groundwater, hot springs, warm-water discharges from power plants, and improperly disinfected swimming pools or drinking water systems. It cannot survive in saltwater, such as seawater. The amoeba thrives in temperatures between 35°C and 46°C (95–115°F), making infections more likely during hot weather and summer months.

Naegleria fowleri does not live in the human body but thrives in warm freshwater and soil, and is found around the world.  Life cycle of the amoeba | Illustration: CDC. 

If a person swims or dives in water where Naegleria fowleri is present—especially in a way that splashes water and stirs up sediment from the bottom—water may enter the nasal cavity, carrying amoebas with it. In rare cases, instead of exiting to look for their preferred prey, for reasons not yet fully understood, the amoebas infect the individual, traveling up the nasal passages and making their way toward the brain. 

Naegleria fowleri is only infectious when water containing it enters the nose; it cannot infect a person through drinking contaminated water nor can it spread from person to person. However, infection can occur during sinus rinsing with untreated water—such as with a neti pot—or through religious purification rituals where water enters the nose.

Once inside the nasal cavity, Naegleria fowleri attaches itself to the protective mucosa, penetrates the nasal lining, and migrates along the olfactory nerve into the brain via the cribriform plate a thin bone that separates the nasal cavity from the brain and contains small openings for the olfactory nerve fibers.

The first brain structure it invades is the olfactory bulb, which is directly connected to the olfactory nerve. This region, located at the front of the brain just behind the eyes, is responsible for processing smell. This explains the early symptoms, which typically include a sharp frontal headache and loss of smell. From there, the amoeba spreads quickly through the brain. The body’s immune system mounts an aggressive response, sending large numbers of  white blood cells to the infected area in an attempt to contain the infection, resulting in inflammation and fluid buildup.

The problem is that this battle takes place inside the rigid skull, which cannot expand. As fluid accumulates, brain swelling (edema) and increased intracranial pressure compress the brain, ultimately leading to coma and death.

From the time of infection, symptoms of PAM typically appear within 1 to 12 days, with an average onset around day five. The disease progresses rapidly, and in most cases, the entire course—from infection to death—lasts about ten days.

The brain is warm and moist—just like the environment Naegleria fowleri thrives in. Illustration of the amoeba infecting the brain | Shutterstock, Kateryna Kon

Effective Treatment?

Naegleria fowleri is especially deadly because it causes direct and rapid damage to brain tissue. The disease progresses quickly, and in many cases, it is either not diagnosed until after death or is misdiagnosed—often as bacterial meningitis or viral encephalitis—resulting in delayed or inappropriate treatment. Accurate diagnosis requires a lumbar puncture to collect cerebrospinal fluid (CSF). If the patient had recent exposure to freshwater in the two weeks prior to the onset of symptom —such as during rafting, swimming, or mud play—testing the CSF for amoebas is essential.

Even when the disease is identified early and treatment begins promptly—before significant brain damage occurs, as in the case of a 26-year-old patient in Israel—Naegleria fowleri infection remains extremely aggressive and almost always fatal. There are currently no fully proven or clinically validated treatments effective against the amoeba. However, a treatment protocol exists that includes drugs which have shown to be effective against the amoeba in laboratory studies and animal models, and which appear to have contributed to survival in the few documented cases where patients recovered.

Treatment involves a combination of antibiotics, antifungal medications, and anti-leishmanial agents, administered orally, intravenously, and directly into the cerebrospinal fluid (CSF). Among these is miltefosine, a drug typically used to treat leishmaniasis and breast cancer, which has been used in most of the documented survival cases—including a relatively recently diagnosed patient in Pakistan.

The challenge is that many of these drugs do not easily cross the blood-brain barrier—a cellular layer lining the blood vessels that supply the brain and controlling what enters. While this barrier protects the brain from harmful substances and pathogens, it also blocks many potentially life-saving medications.  To bypass this limitation, drugs must be given at much higher-than-normal doses, which increases the risk of serious side effects, including kidney damage.

In addition to anti-amoebic drugs, patients receive supportive care to reduce inflammation, intracranial pressure, and cerebral edema. This often includes the steroid medication dexamethasone and, in some cases, cerebrospinal fluid (CSF) drainage. Targeted temperature management (TTM), which involves deliberately lowering the body temperature to 32–34°C, has also been used in some instances. Cooling helps relieve brain swelling and may potentially damage the heat-loving amoeba.

Research into improved treatments for Primary Amoebic Meningoencephalitis (PAM) remains limited due to the disease’s rarity. However, since the first reported case—and increasingly in recent years—more cases have been identified, including in previously unaffected regions. This emerging trend is spurring renewed efforts to develop more effective therapeutic strategies. Researchers are investigating drugs with improved brain penetration and reduced tissue toxicity, as well as exploring the potential of mRNA-based vaccines targeting the amoeba.

A definitive diagnosis of the disease is made through lumbar puncture to collect cerebrospinal fluid (CSF).  Illustration of the amoeba alongside immune cells in the CSF | Kateryna Kon, Shutterstock

More Awareness, More Cases

The first documented cases of what is now known as Primary Amoebic Meningoencephalitis (PAM) were reported in 1965 by pathologists Malcolm Fowler and Rodney Carter from Adelaide, Australia. They published a paper describing the first four known cases of the disease. All cases occurred in the northern part of Spencer Gulf in South Australia.  One case, involving a 9-year-old boy, occurred in 1961, and three others in 1965: two 8-year-old girls and a 28-year-old man. All were admitted to hospital with symptoms resembling bacterial meningitis that did not respond to antibiotic treatment, and all died shortly after admission.

During autopsies, Fowler and Carter found amoebas in the patients’ brains and cerebrospinal fluid, but not elsewhere in the body. Drawing on earlier studies in which free-living, non-parasitic amoebas introduced into the noses of mice caused fatal brain infections, they hypothesized that the patients had been infected in a similar way: the amoebas entered through the nose and traveled directly to the brain.

Following the publication of Fowler and Carter’s paper, physicians and researchers began re-examining past encephalitis cases, leading to the retrospective identification of additional PAM cases. In 1966, American pathologist Cecil Butt reported on three boys from central Florida who had died after being infected with the amoeba—two of them as early as 1962. He coined the term Primary Amoebic Meningoencephalitis to emphasize that the brain inflammation resulted from a primary, or direct, amoebic infection of the brain. In 1970, Carter named the species Naegleria fowleri in honor of Fowler, “who was the first to recognize the disease it causes.

The earliest confirmed case of PAM known today was found in a preserved brain specimen at a pathological museum in London, scheduled for disposal. Museum records had labeled the specimen as showing “carcinomatous infiltration of the leptomeninges. April 1909. Boy from Essex.” However, a re-examination of the sample in the late 1960s revealed that the supposed cancerous growths were actually Naegleria fowleri amoeba cells and the tissue damage they had caused.

Since the discovery of PAM, hundreds of cases have been reported worldwide—most occurring during warm-weather months.  People bathing in canals in Lahore, Pakistan | | From Siddiqui and Khan 2014

Since the disease was first identified, hundreds of cases have been reported worldwide—most often during warmer seasons. The majority of documented cases have occurred in the United States, Pakistan, Mexico, India, Australia, and the Czech Republic. Surprisingly, relatively few PAM cases have been reported from tropical regions, despite their warm climates. This likely reflects underreporting, due to limited awareness, a lack of diagnostic resources, and the tendency for this rare disease to be overlooked amid the prevalence of more common diseases in these areas. Even in the United States, most PAM cases likely go unreported. One estimate suggests that for every three confirmed deaths from the disease, another 13 deaths caused by Naegleria go undiagnosed.

As awareness of the disease grows, so does the number of correctly diagnosed infections. In Pakistan, for example, the first documented case of PAM was reported in Karachi in October 2008. By October 2019—just 11 years later—146 cases has been identified, a figure approaching the total number reported in the United States over a span of 59 years. Almost all the patients in Pakistan were men, who became infected after introducing water deep into their nasal passages during the Islamic purification ritual wudu’ (وُضوء)—despite the fact that such deep nasal rinsing is not a required part of the practice.

The vast majority of PAM case reports in Pakistan come from Aga Khan University Hospital in Karachi, a private medical facility. It is highly likely that many more cases in Pakistan go undiagnosed—especially in poorer regions of the country, where there is limited awareness of the disease and inadequate access to the diagnostic equipment needed to detect the amoeba. Thus, even with increasing awareness, the known cases likely represent only the tip of the iceberg.

In Pakistan, nearly all PAM patients are men, often infected by forcefully rinsing water deep into the nose during the Islamic purification ritual wudu’ (وُضوء)—a deep rinsing not strictly required by religious guidelines. A man rinsing his nose | From Siddiqui and Khan 2014

Warming Up, Spreading Out

The global increase in cases of Primary Amoebic Meningoencephalitis (PAM) appears to be driven not only by greater awareness and improved diagnosis, but also by rising global temperatures caused by the climate crisis, which allow Naegleria fowleri to thrive and spread to new regions.

In Karachi, for example, the climate has always been favorable to Naegleria fowleri—hot and humid, particularly in summer. But in recent years, average temperatures have risen, and the city has experienced more frequent and intense heatwaves. These shifts, likely attributed to climate change, may be contributing to the amoeba's increased prevalence in the area.

This trend is particularly evident in the United States, where Naegleria has been monitored for many years. For decades, PAM cases were limited to warm southern states such as Florida and Texas. However, since 2010, infections have also been reported in central and northern states—a change attributed to climate change.

One notable case occurred in the summer of 2010, when a 7-year-old girl in Minnesota contracted the amoeba and died after swimming in a lake during a period of unusually high temperatures. The infection occurred roughly 900 kilometers north of the previously known northernmost case. Two years later, a 9-year-old boy died from PAM after swimming in the same lake during another exceptionally hot summer. Since then, more PAM cases associated with high temperatures have been reported in northern U.S. states, including Indiana, Maryland, northern California, Iowa, and Nebraska.

In short, global warming leads to higher temperatures and warmer waters—conditions in which Naegleria fowleri thrives. Rising heat also drives more people to seek relief in the water, increasing the risk of exposure to the amoeba. Additionally, climate change may cause droughts in some regions, prompting people to store water, such as rainwater. These stored supplies can become ideal breeding grounds for the amoeba, further elevating the risk of infection.

However, even complete dryness may not offer protection from the amoeba. Its cysts have been detected in airborne dust, particularly during the dry Harmattan season in West Africa. It’s possible that inhalation could allows these cysts to reactivate in the nasal passages and cause disease. Yet, in these same dry African regions, live amoebas have been found in the nasal passages of children who have not developed illness. Similarly, healthy individuals worldwide have been found to carry antibodies against Naegleria fowleri, suggesting past exposure without illness. In other words, there are documented cases in which people were exposed to the amoeba, it entered their nasal passages, but did not cause disease.

Rising temperatures drive more people to enter the water to cool off, increasing the risk of exposure to the amoeba. Boys swimming in a water source in Pakistan | From Siddiqui and Khan 2014

Rare, and Easy to Prevent

Though deadly, Naegleria fowleri infections are extremely rare and primarily affect healthy individuals engaging in routine recreational activities—one reason the disease often draws media attention. However, as Nebraska State Epidemiologist Matthew Donahue put it, "Millions of recreational water exposures occur each year, while only 0 to 8 Naegleria fowleri infections are identified each year."

As NPR emphasized, despite millions of recreational water exposures yeach year, there have been only 31 reported infections in the United States in the 10 years from 2012 to 2021. In comparison, nearly 4,000 people died from unintentional drowning each year during that same time period.

Still, while the likelihood of infection is exceptionally low and should not cause undue fear, it is important not to ignore the risk, given the infection’s almost universal fatality.

Fortunately, the risk of infection can be reduced through relatively simple measures. Proper maintenance of disinfection systems in swimming pools and water parks, cooling the water, and chlorination (adding chlorine to disinfect water) help prevent the amoeba from thriving. Chlorinating drinking water supplies is also crucial. In South Australia, where PAM was first identified, no infections have occurred since 1981, thanks to a public awareness campaign and routine chlorination of water sources. In contrast, in 2016, a teenage girl died after being contracting Naegleria fowleri at a water park in North Carolina, where the water had not been adequately disinfected.

While natural water sources cannot be chlorinated or cooled, you can still reduce your risk of infection by following a few simple precautions. One option is to avoid entering or swimming in warm freshwater during hot seasons, when water temperatures are highest—though admittedly, that’s not the most appealing advice. 

More easily implementable steps include avoiding jumping into warm freshwater, or at least pinching your nose if you do, to prevent water from entering the nasal passages. When swimming, try to keep your head above water and avoid full submersion—especially in hot springs. Also, try not to disturb the bottom, as this can stir up amoebas from the sediment. Using nose plugs is strongly recommended, especially if you enjoy diving.

Since the amoeba infects only through the nose, it’s important to exercise caution during any activity that could force water into the nasal passages. In practices where water is intentionally introduced into the nose—such as certain religious purification rituals or the use of a neti pot—experts recommend using only distilled water from a sealed container, or water that has been boiled and cooled. Tap water should never be used for these purposes unless it has been properly treated.

By following these guidelines and continuing to monitor water sources for contamination, we can reduce the already low risk of infection and continue to enjoy freshwater activities—even on a warming planet.

 

This article was originally published on the blog "Noam's Ark – Biological Thoughts"