Deadly 'brain-eating amoeba' has spread northward in US, CDC warns rising temperatures could be a reason

While the incidence of reported cases of the rare but fatal infection has remained stable, it has expanded northward


                            Deadly 'brain-eating amoeba' has spread northward in US, CDC warns rising temperatures could be a reason
(Getty Images)

A dangerous infection of the brain, which is caused by an amoeba and is usually fatal, has been expanding in the US and has made its way northwards from the southern US, according to US Centers for Disease Control and Prevention (CDC). The culprit behind the expansion may be climate change and rising temperatures, warn scientists.

Known as Naegleria fowleri, it is a free-living amoeba that causes primary amebic meningoencephalitis (PAM), a rare but usually deadly disease. The ameba enters the brain via the nasal passages, causing an acute brain infection that usually results in death within three to seven days of symptom onset. The fatality rate is over 97% and only 4 people out of 148 known infected individuals in the US from 1962 to 2019 have survived. The signs and symptoms can include severe frontal headache, fever, nausea and vomiting. As the disease progresses, people could experience stiff neck, seizures, hallucinations and coma. Because of the rarity of the infection and difficulty in initial detection, many diagnoses are made after the death of the patient.

Historically, US PAM cases have occurred after recreational exposure to warm, untreated freshwater (such as lakes and rivers) in US southern states during the summer. But the new report indicates that the geographic range of exposure locations have expanded northward.

"Our results show a suggested northward expansion of PAM and its potential association with higher temperatures warrants further investigation. Characterizing recreational water exposures could improve risk prediction and prevention strategies, helping to prevent cases, aid natural resource custodians, and reduce the burden on state and local health departments," suggests CDC.

The analysis

For the study, the team analyzed PAM cases reported in the US during 1978-2018 with known or suspected recreational water exposure in a lake, pond, reservoir, river, stream, or outdoor aquatic venue. They included cases with a single known exposure site or multiple sites within an 80 km radius. In the temperature analysis, patients with a “known or imputed date of exposure” were included. Temperature records were obtained from the National Oceanic and Atmospheric Administration’s National Climatic Data Center, from the weather station closest to each exposure location with a maximum distance of 50 km.

Infection typically occurs when people go swimming or diving in warm freshwater places such as lakes and rivers (Getty Images)

Among 120 PAM cases reported to CDC’s free-living amoeba database during 1978-2018, a total of 85 patients had an eligible known or suspected recreational water exposure: 69 patients at a lake, pond, or reservoir; 14 patients at a river or stream; and 2 patients at an outdoor aquatic venue. They excluded 35 patients who were exposed at canals, puddles, or ditches; to geothermally heated water or tap water; at unknown locations; or multiple locations more than 80 km apart. 

According to the analysis, the incidence of PAM associated with recreational water exposures ranged from 0-6 cases per year during 1978-2018 in the US. Researchers found that among case exposures, 74 occurred in the South and 5 in the West. Six case exposures occurred in the Midwest, 5 of which occurred after 2010: Minnesota (2010), Kansas (2011), Minnesota (2012), Indiana (2012), and Kansas (2014).

PAM cases occurred in 33 of the 41 study years. Among 85 cases, 81 (95%) had an exposure date. Air temperatures varied widely in the 2 weeks before exposure, found authors. On average, daily temperatures were higher in the two weeks before exposure than the 20-year average for that date and location: high temperatures were 0.77°C greater, and low temperatures were 0.76°C greater than the 20-year average.

"The rise in cases in the Midwest region after 2010 and increases in maximum and median latitudes of PAM case exposures suggest a northward expansion of N. fowleri exposures associated with lakes, ponds, reservoirs, rivers, streams, and outdoor aquatic venues in the US. We observed an increase in air temperatures in the 2 weeks before exposures compared with 20-year historic averages," say authors. They explain, "It is possible that rising temperatures and consequent increases in recreational water use, such as swimming and water sports, could contribute to the changing epidemiology of PAM."

According to experts, the ameba tolerates a range of conditions by changing its morphology in response to the environment. "It feeds and reproduces in the trophozoite form; assumes a more mobile, flagellated form in low nutrient environments; and forms a resistant cyst in adverse conditions, such as cold temperatures. The thermophilic nature of N. fowleri suggests it might be sensitive to global changes in surface temperature,” they emphasize.

Meanwhile, the experts also found that while the reported incidence of PAM has increased worldwide, the incidence of reported cases of PAM in the US remained stable during 1978-2018. The worldwide trends might reflect changes in international diagnostic capacity, concludes the CDC.

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