The wind may be partly to blame for bird flu spread between farms, a new study suggests

Even if it is moving through the air, experts say, that doesn’t necessarily make bird flu a bigger threat to people. (Callista Images / Image Source / Getty Images via CNN Newsource)

In February 2024, government veterinarians in the Czech Republic investigating a bird flu outbreak found themselves in the middle of a puzzling case.

The highly pathogenic H5N1 virus was devastating a flock of chickens on a highly secure farm, home of a breeding program that produced hybrid birds with specially coloured feathers and eggs. A nearby facility owned by the same company also saw its chickens become infected.

How had the virus gotten in? The facilities were state-of-the-art. The water on the chicken farms was filtered, supplied by their own wells. The chicken houses had giant fans that created one-way air flow through the barns. The facility was surrounded by sturdy, well-maintained fencing that kept wild animals out. Employees weren’t even allowed to keep their own chickens at home.

Their investigation led the researchers to conclude that this particular case involved a distinct set of “just right” conditions that allowed the virus to blow onto the chicken farm through the wind.

Wild birds that carry flu viruses in their guts and shed them in their poop are believed to be the main way H5N1 has been introduced onto farms. Still, experts say the windborne spread of bird flu has been suspected before.

“The whole idea of potentially being sort of wind-driven has been out there for a while,” said Dr. Richard Webby, who directs the World Health Organization’s Collaborating Center for Studies on the Ecology of Influenza in Animals.

“It’s incredibly hard to actually measure,” he said, “to get the definitive data to say yes or no.”

Webby wasn’t involved with the new study but said he’s been in touch with a veterinarian in California who believes wind may have played a role in the recent spread of H5N1 between some cattle herds in the Central Valley.

But even if it is moving through the air, he said, that doesn’t necessarily make bird flu a bigger threat to people.

For one thing, the current versions of the H5N1 virus that are infecting animals don’t seem very good at infecting humans. The more the virus spreads, the more opportunities it has to hone its craft and get better at breaking into human cells, but for the moment, there’s no sign that it’s made this leap to being a fully human pathogen.

For another, it takes just a tiny amount of virus to make a bird sick, and it would probably take a lot more to infect a human. Particles, even viruses, disperse in air, meaning any virus in the wind is likely to be present in only very small amounts.

How the virus jumped between farms

In the case of the chicken farm in the Czech Republic, scientists’ only clue about the origins of the outbreak came from a nearby duck-fattening farm that, a week earlier, had also seen bird flu tear through its flocks.

The duck farm, with 50,000 birds, was completely different from the chicken facility. It was near a lake that hosted wild ducks. The farmed ducks were kept in buildings with natural ventilation and a basic level of biosecurity – the precautions taken to safeguard animals on a farm.

The course of the infection on the duck farm looked very different than what played out with the chickens.

Bird flu swept through the duck houses like wildfire. On February 4, the first day of the outbreak, 800 ducks died. Within two days, 5,000 had died. On February 7 and 9, the entire flock was depopulated to control the spread of the infection and prevent greater suffering for the birds.

The chickens at the breeding facility were different in another way, too: They got sick slowly. Over the course of a week, they gradually stopped eating and drinking as much, and the farm owners noticed that a few birds were dying, mostly near the barns’ air intake vents.

Eventually, the infection spread to both barns in the breeding facility and another barn at the second location. Thousands of birds were lost, including critical breeding stock.

The government’s veterinary investigators collected samples of the viruses that had infected the ducks, the chickens at the breeding facility and chickens at the second farm to see what they could learn.

Three H5N1 strains sequenced from the duck farm were genetically identical to strains found in the birds that first got sick on each farm, suggesting that the duck farm had been the source of the chicken outbreak.

But how? The duck farm was nearly 5 miles west of the chicken facilities, and investigators couldn’t find any physical links between the two. None of the people who worked on the duck farm ever went to the chicken farms; even the contractors who dropped off supplies or came to collect waste were different. The investigators said they ruled out any human connections between the farms.

There were no major bodies of water near the chicken farm, which steered them away from the idea that wild birds might have carried in the flu virus.

Weather holds a clue

Then, the researchers checked the weather patterns for the week the chickens got sick. There had been a steady breeze from the west. There was extensive cloud cover, which would have blocked germ-killing UV light from the sun. The temperature was cool but not freezing, between 40 and 50 degrees, and viruses love cold air.

In other words, perfect conditions to transport a virus and allow it to survive a long trip.

Dr. Kamil Sedlak, who directs the State Veterinary Institute in Prague and is senior author on the study, says that after they explored all the possibilities, windborne spread was the best fit in this case.

“I think that under certain specific conditions, the spread of the avian influenza virus by wind can occur,” he said.

The Czech researchers published their study as a preprint, ahead of peer review.

Webby said the virus particles could also have hitched a ride on something larger, like dander from the ducks, and that’s what allowed them to travel so far.

The study authors didn’t try to sample the air near the duck or chicken farms for virus, and they note that previous air sampling studies in and around swine barns and poultry houses have detected high levels of avian influenza viruses. Concentrations of the virus quickly dissipate, though, as air travels away from barns.

That may be why the chickens got sick more slowly: They were getting a lower dose of the virus from very low levels carried in the air. The fact that the first chickens to die were near the air intake vents was another clue.

“I think that they’re making a compelling case based on the knowledge they had of the farms,” said Dr. Montse Torremorell, professor and chair of Veterinary Population Medicine at the University of Minnesota, who was not involved with the new study.

Torremorell conducted air sampling around poultry barns during the last major outbreak of avian influenza in the United States in 2015. She found high levels of infectious virus blowing out of the large exhaust fans in poultry houses.

It’s not just a matter of how much virus is blowing out into the air, she said, but also where it lands. Those plumes of infectious aerosols may end up on clothing, equipment and vehicles that can also carry the virus from one place to the next.

Layered approaches to protection

Torremorell said airborne spread should be something farms consider when they’re thinking about ways to protect their livestock and flocks.

“I think the airborne transmission should be on the table,” she said.

But it’s tricky to bring up because people might hear “airborne spread” and feel powerless to stop the virus.

“Often, people say, ‘Oh, it’s in the wind. I cannot do anything about it.’ Then they stop doing the basic biosecurity that it’s also very important that they have to do,” she said.

But she says they’re forgetting lessons from the Covid-19 pandemic about how stopping the spread of a virus requires layered approaches to protection. Current precautions like restricting entry to farms and wearing personal protective equipment should be kept in place, but filtering the air in barns could also help protect animals and farmworkers from bird flu, too.

Dr. Michael Osterholm, who directs the Center for Infectious Disease Research and Policy at the University of Minnesota, also thinks windborne spread is playing a role in H5N1 transmission now.

“I think there is more wind-driven H5N1 occurring right now in the community because of the number of aquatic of waterfowl that are infected,” he said.

Waterfowl like ducks and geese poop in lakes, he says, and wind blowing over that water may carry the virus to nearby farms.

“We’re in unprecedented territory right now,” Osterholm said. “It’s all over the country.”

Osterholm thinks airborne or windborne spread might account for some infections in which a source of the virus – like contact with a sick animal – couldn’t be identified. The recent case of the three veterinarians who tested positive for antibodies against the H5N1 virus at a conference comes to mind. Two of the three had no known exposure to sick animals.

“I think it’s very low-risk for humans to be infected with the virus like that, but I think it happens,” he said.

Article written by Brenda Goodman, CNN