Human H5N1 cases in the U.S. are rising. That's bad timing with flu season, bird migrations just months away

The sheer scale of the U.S. bird flu outbreak is hard to fathom. 

More than 100 million farmed birds have been infected with H5N1 since 2022, followed by roughly 170 herds of dairy cows, along with virus detections in more than 200 other mammals — humans included.

Recently, a handful of farm workers in Colorado were infected, marking the country's first human outbreak: Six confirmed cases are linked to culling efforts at one poultry farm, and at least one likely case is tied to culls at another nearby facility. And while the spread may have been chicken-to-human, the virus strain is similar to the form of bird flu tearing through dairy farms across more than a dozen states.

The country's total human infection tally, of less than a dozen confirmed cases since 2022, pales in comparison to the staggering case counts among poultry and livestock. There haven't been any farm worker deaths, and no cases linked to dairy farms have popped up yet in Canada, either.

Yet this new, unusual cluster of human H5N1 cases may be a harbinger of looming challenges to come, all while the broader U.S. outbreak could be surging out of control.

The timing is far from ideal, several scientists told CBC News, with farm worker infections ticking up mere months before the return of the usual flu season, and the fall migration of millions of wild birds — giving this globetrotting virus countless more opportunities to evolve.

"We are looking at, potentially, a huge outbreak that is still expanding, and still growing, and that is not containable," warned virologist Angela Rasmussen, a researcher with the University of Saskatchewan's Vaccine and Infectious Disease Organization. 

"And that increases the risk of more and more human cases, which in turn increases the risk that this virus will become better adapted to humans."

Officials first announced the discovery of several farm worker infections back on July 14, all linked to large-scale culling efforts involving H5N1-infected birds on an egg farm in Colorado.

While there aren't signs of onward human-to-human transmission, sequencing from one of those cases showed the strain is closely related to the virus spreading in dairy cows, which features previously-documented adaptations to mammals, the U.S. Centers for Disease Control and Prevention noted in a recent update.

More reassuring? So far, all the human cases in the U.S. have been mild infections, despite high H5N1 case fatality rates reported globally over the last two decades. Some farm workers in the Colorado cluster had traditional flu symptoms of fever and cough, while others experienced conjunctivitis, suggesting the virus may have snuck in through their exposed eye membranes rather than through the body's respiratory channels.

But given the small number of known human infections in the U.S. to date, and the unusual transmission patterns that don't mimic how this virus would actually spread person-to-person, "we should put no stock at all on what we're seeing in terms of severity," noted McMaster University influenza researcher Matthew Miller, the director of the Michael G. DeGroote Institute for Infectious Disease Research.

If human infections do keep rising into the fall, in Colorado or beyond, experts say the timing would be advantageous to a virus that's already proven quite adept at striking a wide variety of species. And a host of factors, several scientists agreed, may provide opportunities for H5N1 to better adapt to infect and harm more human hosts.

For one thing, the dovetailing of heightened human H5N1 circulation and the return of seasonal flu strains could have dire consequences, said virologist Tom Peacock, a fellow with the Pirbright Institute, a U.K.-based research and surveillance centre for zoonotic viruses.