Of all the news about bird flu, this month has brought some of the most concerning yet. Six people working on a chicken farm in Colorado have tested positive for the virus—the biggest human outbreak detected in the U.S. The country’s tally is now up to 11 since 2022, but that’s almost certainly a significant undercount considering the lack of routine testing.
Since the current strain of bird flu, known as “highly pathogenic avian influenza H5N1,” began spreading around the world in late 2021, it has become something like a “super virus” in its spread among animals, Richard Webby, an influenza expert at St. Jude Children’s Research Hospital in Memphis, told me. Wild birds have been decimated, as have poultry farms: The virus has been detected in more than 100 million birds in 48 states. H5N1 has been around for longer than 25 years, but only recently has it regularly jumped to mammals, infecting cats, sea lions, and bears. In March, it was detected for the first time in American cattle and, since then, has already spread to 163 herds in 13 states.
All of that would be worrying enough without reports of people also falling sick. Everyone who has tested positive in the U.S. has worked closely with farm animals, but each additional case makes the prospect of another human pandemic feel more real. “That’s absolutely the worst-case scenario,” Webby said. It’s a possibility, although not the likeliest one. For now, the virus seems poised to continue its current trajectory: circulating among wild birds, wreaking havoc on poultry farms, and spreading among cattle herds. That outcome wouldn’t be as catastrophic as a pandemic. But it’s still not one to look forward to.
Even with the spate of farmworker infections, the threat of bird flu to humans is, at the moment, considered low. Researchers are keeping an eye out for two red flags. The bigger one would be the virus’s ability to spread between people. All of the people who have tested positive in the U.S. were infected by exposure to sick cows or poultry, and they have not seemed to pass the virus along to anyone else. Symptoms have generally been mild, including respiratory issues, though several people have developed serious cases of conjunctivitis, or pink eye. (No one in the U.S., or globally, has died from this variant of H5N1.) “There is no evidence at this point that this virus is going human to human, and therefore it really does not pose a threat to public health,” Jenna Guthmiller, an immunologist at the University of Colorado Anschutz School of Medicine, told me.
The second warning sign is how the virus itself is changing. So far, H5N1 isn’t very good at getting into human cells and then replicating inside them, abilities that would enable the virus’s spread among people. But that may be changing. In a lab study, virus particles from infected cows showed signs that they were capable of binding to human receptors in the upper respiratory tract.
The current strain of H5N1 has already mutated to infect mammals, and a few genetic changes could be all it takes for the virus to spread more efficiently to humans—or, worse, between them. “We’re at the highest risk of the virus” since the early 2000s, when a different strain of H5N1 led to numerous deadly human infections in East and Southeast Asia, Webby said. Not because the virus itself is necessarily more infectious but because it is spreading among so many different animals, and especially mammals—giving it more opportunities than ever to find a way to replicate in humans. But, again, despite all that transmission—all those chances for the virus to mutate into something that can reliably sicken humans—it hasn’t yet. That could “absolutely” continue to be the norm, David Topham, a flu expert at the University of Rochester Medical Center, told me.
The status quo is still pretty troubling. New cases of bird flu keep popping up in herds across the country, raising fears that it might never be eradicated from cattle. The “most likely” scenario, Webby said, is that this virus will become endemic in birds and dairy cows—a constant presence, regularly causing outbreaks. Right now, infections in poultry tend to align with the migration of wild birds; if cows are constantly infected, chicken outbreaks could become more frequent.
Nothing about endemicity would be good for humans. The consequences would be diminished, but not eliminated. Farmworkers may continue to periodically fall sick, Guthmiller said. The cost of regular animal outbreaks would be exorbitant. The USDA has already allocated more than $2 billion to address surges among poultry and livestock, which includes compensating farmers for animals that have been killed and eggs that have been destroyed to quell the spread.
If the virus continues to regularly sicken cows, it will have even more opportunities to mutate in a way that could allow it to more easily infect humans. In infected cows, virus particles are mostly found in their udders; the virus is thought to spread between the animals through contaminated milking equipment. Research released last week, which has not yet been peer reviewed, indicates that cows can be infected by aerosolized virus; if they can spread the virus through their exhalations and sneezes, they could become infected merely by breathing the same air.
H5N1 is restless—it will continue trying to infect new hosts. Given enough opportunities to mutate, the virus will do so. “It’s like playing the lottery,” Topham said. “We’re giving this virus a lot of tickets.” H5N1 may also be able to combine with flu viruses from different animals. If cows, chickens, and other animals—say, pigs, which aren’t affected by the current outbreak—on the same farm all have different versions of the flu, “that’s your mixing vessel right there,” Topham said. The H1N1 virus that caused the 2009 swine-flu outbreak, for example, was a mix of flu viruses from pigs, humans, and birds.
There is one other possible future—the best-case scenario, which unfortunately is also the least likely. The virus possibly “could disappear,” Webby said. This would partly depend on eradicating it from cows, which he believes is plausible with human intervention and herd immunity. But eliminating the virus in birds—the main animals that get bird flu and spread it—is largely out of human control. H5N1 is particularly lethal in birds, with a mortality rate of up to 100 percent for some species; if it somehow kills enough of them, Guthmiller said, it very well could just fizzle out. “Dumb luck,” as Webby put it, might still prevail.
But a supercharged bird virus with a taste for infecting mammals is not the kind of thing that should be left up to chance. It is fortunate that only 11 farmworkers have been infected—as far as we know. Tools to curtail the spread of bird flu are available, but they’re not being used, or used appropriately. Personal protective equipment is helpful when worn correctly, but doing so isn’t feasible when it involves wearing respirators and Tyvek suits in temperatures that reach 104 degrees Fahrenheit. Unlike many other countries, the U.S. does not vaccinate chickens against H5N1, in part because it’s expensive to do. And cost is also why only 60 farmworkers have been tested for bird flu, giving an imperfect window into the virus’s spread. “It’s going to be a lot more costly to deal with another pandemic than to deal with immunizing our farms,” Topham said.
America’s response has been painfully shortsighted, and the country is paying the price: Had bird flu been kept in check earlier, it might never have made it into cows, and might never have developed the mutations that allow it to flirt so closely with human-to-human transmission. At this point, bird flu’s future has no good options—only one that’s bad, another that’s abysmal, and one that relies on nothing but dumb luck.
