The link between climate change and a spate of rare disease outbreaks in 2023

A 16-month-old boy was playing in a splash pad at a country club in Little Rock, Arkansas, this summer when water with a very rare and deadly brain-eating amoeba went up his nose. He died a few days later in the hospital. The toddler was not the first person in the United States to contract the freshwater amoeba, Naegleria fowleri, this year. In February, a man in Florida is dead after flushing his sinuses with unboiled water — the first Naegleria fowleri-linked death to occur in the winter in the USA

2023 was also an active year for Vibrio vulnificus, a type of flesh-eating bacteria. There was 11 deaths linked to the bacteria in Florida, three deaths in North Carolina, and another three deaths in New York and Connecticut. Then there was the first locally transmitted case of mosquito-borne dengue fever in Southern California in October, followed by another case a few weeks later.

Scientists have warned that climate change will alter the prevalence and distribution of diseases in the US, particularly those caused by temperature-sensitive pathogens. This year’s spate of rare diseases may have come as a surprise to the uninitiated, but researchers studying the way climate change affects disease say 2023 represents the continuation of a trend they expect will become more pronounced over time: The geographic spread of pathogens and the timing of their emergence undergoes a shift.

“These are generally the patterns we would expect,” said Rachel Baker, assistant professor of epidemiology, environment and society at Brown University. “Things are starting to move northward, expanding out of the tropics.” The number of outbreaks Americans see each year, said Colin Carlson, a global change biologist who studies the link between global climate change, biodiversity loss and emerging infectious diseases at Georgetown University, “is going to continue to increase.”

This is because climate change can have a profound effect on the factors that cause disease, such as temperature, extreme weather and even human behavior. A 2021 study found that water temperature was one of the top environmental factors affecting the distribution and abundance of Naegleria fowleri, which thrives in water temperatures above 100 degrees Fahrenheit but can also survive freezing winters by forming cysts in lake or pond sediment. The amoeba infects humans when it enters the nasal passage and from there the brain. “As surface water temperatures increase with climate change, it is likely that this amoeba will pose a greater threat to human health,” the study said.

Vibrio bacteria, which have been called the “microbial barometer of climate change,” are affected in a similar way. The ocean has absorbed the vast majority of human-caused warming over the past century and a half, and sea surface temperatures, particularly along the nation’s coasts, have starts to rise sharply because of that. Studies that mapped Vibrio vulnificus growth show the bacteria extending northwards along the eastern coastline of the US in lockstep with rising temperatures. Hotter summers also lead to more people seeking bodies of water to cool off in, which can influence the number of human exposures to the bacteria, a study said. Humans become infected by eating contaminated shellfish or exposing an open wound—no matter how small—to Vibrio-contaminated water.

Mosquitoes breed in warm, humid conditions and can spread diseases such as dengue when they bite people. Studies show the mosquito species that carries dengue, which is endemic in many parts of the global south, is moving north into new territory as temperatures climb and flooding becomes more frequent and extreme. A study warned as of 2019 that much of the southeastern US is likely to become inhospitable to dengue by 2050.

Other warmth-loving pathogens and carriers of pathogens are also on the move—some of them affecting thousands of people a year. Valley fever, a fungal disease that can progresses into a disfiguring and fatal disease, spread through a West that is drier and warmer than it used to be. The lone star tick, an aggressive hunter that often leaves the people it bites with a lifelong allergy to red meat, is expanding northward as winter temperatures become milder and longer breeding seasons allow for a larger and more dispersed tick population.

The effect that rising temperatures are having on these diseases doesn’t necessarily mean that every death linked to a brain-eating amoeba or Vibrio that occurred this year wouldn’t have happened in the absence of climate change—rare pathogens claimed lives long before they did. was anthropogenic. warming has begun to change the planet’s dynamics. Future analyzes could look individually at the outbreaks that occurred in 2023 to determine whether rising temperatures or another climate change-related factor played a role. What is clear is that climate change is creating more opportunities for rare infectious diseases to emerge. Daniel R. Brooks, a professor of evolutionary biology at the University of Toronto and author of a book on climate change and emerging diseasescall it “pathogen pollution,” or “the accumulation of very small upstarts.”

State and local health departments have few tools at their disposal to predict outbreaks of aberrant diseases, and doctors are often unfamiliar with diseases that are not endemic to their region. But health institutions can take steps to limit the spread of rare climate-driven pathogens. Medical schools can include climate-sensitive diseases in their curricula so that their students know how to recognize these emerging threats, regardless of where in the US they end up. A rapid test for Naegleria fowleri in water samples already exists and can be used by health departments to test swimming pools and other summertime hot spots for the amoeba. States can do real time monitoring beaches for Vibrio bacteria via satellite. Cities can monitor the larvae of the mosquito species that spread dengue and other diseases and spray pesticides to reduce the numbers of adult mosquitoes.

“If we proactively looked for pathogens before they caused disease, we could better anticipate local outbreaks,” Brooks said. In other words, he said, we must “find them before they find us.”