Extreme heat may be a crucial factor behind the biggest bird flu outbreak in humans in the US as officials continue to track the virus’ spread.
A heatwave in Colorado likely caused personal protective equipment not to work correctly for workers culling poultry infected with H5N1, a highly pathogenic bird flu. Four people have tested positive for H5N1 and a fifth is also expected to have their case confirmed as bird flu, officials said this week.
It’s the first time a cluster of human cases of bird flu has been reported in the US.
On dairy farms, it has also been difficult for workers to wear PPE, in part because of the heat spreading across much of the US. Colorado confirmed a case of bird flu in a dairy worker earlier this month.
Climate change, which has been linked to intense heat across the country, can amplify outbreaks of deadly diseases like these, experts say. Climate change has also been linked to the emergence and wider spread of pathogens like these, including new migratory patterns for H5N1-infected wild birds.
“When you change a global system like climate so profoundly, small shifts can have seismic impacts, sometimes in unexpected ways,” said Alexandra Phelan, an associate professor at Johns Hopkins Bloomberg School of Public Health.
In Colorado, the workers were culling a flock of egg-laying chickens that had tested positive for H5N1.
They went bird by bird – catching them, putting them in a cart and killing a few dozen birds at a time with carbon dioxide gas.
“It’s a very, very manual – difficult, laborious process,” Julie Gauthier, from the Animal and Plant Health Inspection Service at the US Department of Agriculture (USDA), said at a press briefing on Tuesday.
And it can be dangerous to work in such close and prolonged quarters with animals infected with bird flu, which has a mortality rate of about 50% among people.
It was 104F (40C) outside, but in the chicken houses, it was even hotter.
Between the sweat and the enormous industrial fans to stave off even higher temperatures, workers couldn’t keep their goggles and masks sealed on their faces, Nirav Shah, principal deputy director of the US Centers for Disease Control and Prevention (CDC), told reporters. The fans also spread feathers and debris, which can carry the virus aloft, Shah said.
Of the 160 people working on the farm, 60 went on to develop symptoms, and five tested positive or are presumed to be positive. The remaining 55 workers tested negative for H5N1 at the Colorado state health lab, and some of them were diagnosed with other respiratory illnesses. Only symptomatic workers were tested.
“As the epidemiological investigation proceeds, it is important to note that additional cases may be detected,” Shah said.
Workers will continue culling the chickens, which make up a flock of 1.8m birds, over the next 10 to 14 days, Eric Deeble, acting senior advisor for H5N1 response at USDA, said at the briefing.
“USDA considers it very important for depopulation to continue to stop virus amplification and shedding, prevent further spread and to reduce the viral load in the environment,” Deeble said. “If depopulation were to be paused, workers would still be required to care for and feed animals, prolonging their exposure risk.”
Those involved with culling on the farm – including poultry producers, employees, contractors and USDA employees – are required to wear Tyvek suits, N95 respirators, goggles, boots and gloves while working with infected chickens.
Upon detecting possible cases in people, Colorado immediately alerted the CDC, which dispatched a team of 10 experts – including an industrial hygienist to work on procedures to reduce exposure risks for the workers as they continue to handle infected chickens, Shah said.
The CDC and the state of Colorado “have been joined at the hip”, Shah said – in contrast to other outbreaks at farms around the country, where the CDC typically has had limited access to affected animals and people.
The poultry operation is in the same county as dairy herds that have tested positive for bird flu.
Genomic sequencing shows that these cases are closely related to the outbreak in dairy cows; it seems to have jumped from cows to chickens to people, though it’s not clear how at this point. A USDA report from Michigan last month found the virus may be transported by people between farms.
Climate change can negatively affect human health, making people more vulnerable to extreme weather and spillover events like these, and it remakes animal interactions and migration patterns that can lead to more spillover.
In fact, it may be responsible for the H5N1 outbreak in the Americas.
In late 2021, wild birds carrying the virus managed to fly from northern Europe to Iceland and then to Newfoundland.
“One reason it happened is because of unusually strong winds,” said Jean-Pierre Vaillancourt, a professor of veterinary medicine at University of Montreal. It’s “very unusual” for the birds to be able to travel that far, “but when this happened, we had really unusual climate conditions”, he said.
“It’s not just the extreme heat. It’s the fact that you have unusual climate events,” said Vaillancourt. “It may disrupt usual migratory patterns” and other human and animal behaviors, opening new avenues of risk.
These are the first cases in US poultry workers since 2022, when an inmate tested positive after culling infected chickens in Colorado.
The five workers in 2024 reported conjunctivitis (or “pink eye”) and teary eyes, as well as more typical flu symptoms, like fever, chills, coughing and a sore throat, said Shah. None of them required hospitalization, and they are all recovering, he said.
The CDC has not changed its risk assessment for the general public or its policies for addressing the farm outbreaks, including recommendations on vaccines.
“CDC is not recommending H5 specific vaccine for livestock workers, given the mild symptoms that have been noted thus far, in the absence of any changes to the virus’s genetic makeup,” Shah said.
Vaccines for cows are also being studied.
“We do believe that we can eliminate this within the dairy herds,” Deeble said.
“Enhanced biosecurity would get us to a point where we can arrest the spread, and hopefully in the future, we may get a vaccine which can further facilitate the elimination of this disease from the national herds,” he said.
Officials need to keep climate considerations top of mind in responding to – and potentially eliminating – viruses like these, Phelan said. “Across all areas, governments need to actively and urgently incorporate climate considerations into all health and safety measures more than simply at the surface level.”
