With the U.S. bird flu outbreak uncontained,

 scientists see growing risks

JULY 25, 2024 6:00 AM ET

For nearly four months, the spread of bird flu in the nation’s dairy cattle has stoked fears that, if left unchecked, the virus could eventually unleash a pandemic.

The recent cluster of human cases connected to poultry farms in Colorado only underscores that the threat remains real.

Genetic sequencing of the virus collected from the sickened poultry workers closely resembles what’s circulating in dairy herds, suggesting that cattle somehow introduced the virus into the poultry flock.

At one massive poultry facility, workers culled the birds under particularly dangerous circumstances.

As health officials describe it, they struggled to properly wear protective equipment over their mouth, nose and eyes as they handled thousands of sick birds in a sweltering barn, with industrial fans blowing feathers and other potentially virus-laden material into the air.

Given these conditions, it’s far from surprising that people would catch the virus themselves, says Jennifer Nuzzo, an epidemiologist at Brown University.

“It’s gambling with people’s lives,” she says. “There’s no other way to describe it.”

State and federal health officials are still investigating the scope of the outbreak, although so far all of those who’ve tested positive have only had mild, flu-like symptoms.

Nuzzo says the spillover at the poultry farms drives home the risks of having a viral reservoir in dairy herds that offers the virus ample opportunities to jump between species and potentially adapt to mammals.

“Every time you give an avian virus a chance to infect a human, it’s like buying a ticket for a lottery you don't want to win,” says Troy Sutton, a virologist at Penn State University who studies transmission of bird flu.

Based on the newest research, here’s what scientists are learning — and concerned about — as they study the virus.

Certain mutations could make the virus more dangerous

Luckily, there’s no indication that we have drawn the dreaded “winning ticket.” At least not yet.

The virus working its way through dairy cattle is still fundamentally well-suited to infect birds, not people; however, there are clearly some mutations helping it maintain a foothold in mammals, says Thomas Peacock, a virologist at the Pirbright Institute in the U.K. who studies avian influenza.

“I wouldn't be surprised if it's quite a lot more infectious at the same doses, than a purely avian virus that would have been seen last year in poultry,” says Peacock, adding that this may give it a leg up when it comes to infecting humans, too.

Scientists are still trying to get a better sense of what exactly these changes in the dairy cattle version of H5N1 are doing, but he says there are certainly some mutations that are already enhancing its ability to replicate in mammalian cells.

Of most concern would be the virus evolving to get better at using the type of receptors found in the upper respiratory tract of humans. Such a change could let bird flu spread easily via the airborne route between people similar to the seasonal influenza viruses that typically infect humans.

Peacock and other scientists are closely watching for changes in hemagglutinin proteins on the surface of the virus which would be ground zero for this development.

New study suggest virus may be better at infecting mammals than other types of bird flu:

There are still many outstanding questions about how exactly humans are catching the virus.

One possibility, raised by federal health officials, is that a splash of milk, say, in the eyes or elsewhere could explain some of the infections in dairy workers, particularly those whose only symptoms were conjunctivitis. There’s also speculation that aerosolized milk could be another source of infection.

While the virus still doesn’t do well in the upper respiratory tract of humans, Peacock says, evidently there's a degree of replication because nasal swabs are testing positive for low amounts of viral genetic material, at least in some of the human infections.

Research on avian influenza predating the dairy cattle outbreak has shown that, with only a few mutations artificially inserted, airborne transmission can take place between ferrets, which are used as a model of human infection.

Since the dairy cattle outbreak began, scientists have begun to analyze how this version of the virus spreads, in hopes of understanding the threat it poses to humans.