The single-celled organism Naegleria fowleri is commonly known as the “brain-eating amoeba” because it causes Primary Amebic Meningoencephalitis (PAM). PAM is a severe, rapidly progressing infection of the brain and central nervous system. Although cases are extremely rare, the resulting illness is nearly always fatal, with a mortality rate exceeding 97%. Public concern stems from the amoeba’s environmental presence in freshwater sources across the United States.
How the Amoeba Causes Infection
Naegleria fowleri is a thermophilic organism that thrives in warm environments, primarily warm freshwater bodies and soil globally. It is typically found in lakes, rivers, ponds, and hot springs, growing best in water temperatures between 77°F and 115°F. The amoeba can also inhabit poorly maintained swimming pools, water heaters, and untreated municipal water supplies. While the amoeba exists in various forms, the trophozoite stage is responsible for human infection.
Infection occurs exclusively when water containing the amoeba is forced up the nose, usually during activities like swimming or diving. Swallowing contaminated water does not cause infection, as stomach acid destroys the amoeba. Once in the nasal cavity, the amoeba travels along the olfactory nerve, gaining direct access to the brain. There, the organism begins to destroy brain tissue, leading to the rapid onset of PAM.
The amoeba consumes brain cells and nervous tissue in the human host, causing severe inflammation. Symptoms of PAM, which often resemble bacterial meningitis, typically appear within one to twelve days after exposure. Initial signs include fever, severe headache, nausea, and vomiting. As the infection progresses, symptoms worsen to include a stiff neck, confusion, and hallucinations. Death usually occurs within one to 18 days of symptom onset.
States with Reported Cases
Historically, the majority of Naegleria fowleri infections in the United States have been concentrated in Southern states, where water temperatures are consistently warm throughout the summer months. The Centers for Disease Control and Prevention (CDC) tracks these cases, showing a high prevalence in states along the Southern tier. Texas, Florida, and Louisiana have reported the largest numbers of cases since tracking began. These states provide the ideal habitat for the amoeba to proliferate during the peak summer recreation season.
Although the incidence of PAM remains very low, the geographic distribution of reported cases has expanded in recent years. Cases are increasingly documented in states farther north than the historical range, including Midwestern states such as Kansas, Minnesota, and Indiana. A significant number of these northern cases have occurred since 2010. Data shows the maximum latitude of case exposure has shifted northward by approximately 8.2 miles per year over four decades.
This shift is linked to rising surface water temperatures necessary for the organism’s survival and growth. As climate patterns result in warmer water temperatures for longer periods, bodies of water in more temperate zones become hospitable to Naegleria fowleri. Cases have been reported as far north as Minnesota and Nebraska, regions where the amoeba was previously rarely detected. CDC data indicates the amoeba’s potential habitat is expanding, introducing risk to new areas.
Essential Prevention Measures
Prevention strategies focus on minimizing water entering the nasal passage, as this is the only route of infection in warm freshwater environments. When engaging in water activities in warm lakes, rivers, or hot springs, use nose clips or hold the nose shut. This physical barrier prevents contaminated water from reaching the nasal passages. Individuals should also avoid activities that forcibly push water up the nose, such as jumping, diving, or plunging into warm freshwater.
Since Naegleria fowleri often resides in the sediment at the bottom of a water body, avoid digging into or stirring up the mud or sand in shallow, warm areas. Disturbing the sediment can release the amoeba into the water column, increasing the risk of exposure.
Precautionary measures also apply to nasal rinsing practices using devices like neti pots or bulb syringes. For nasal irrigation, individuals must use water that has been sterilized to kill potential organisms. This requires using only distilled or sterile water, or tap water that has been boiled for at least one minute and allowed to cool. Utilizing untreated tap water for nasal rinsing poses a risk, as the amoeba can be present in inadequately treated water systems.