Bird flu has picked up speed - and may no longer be containable

Since late 2021, the highly pathogenic H5N1 bird flu has devastated wild bird populations, disrupted agriculture, and reached an unexpected range of mammals.

Entire flocks have been wiped out. Eggs, meat, and even household birds have all taken a hit. Many hoped that the standard approach – culling infected and exposed domestic birds – would halt the virus. It didn’t.

The virus kept spreading across species and across borders. It reached wild birds that used to be mostly on the sidelines of bird flu stories. That shift raised a big question: what, exactly, changed?

Tracking the bird flu outbreak

A team led by Louise H. Moncla from the School of Veterinary Medicine decided to track how the recent wave of H5N1 entered and spread across North America. Their study focuses on the first 18 months of the outbreak on this continent.

“The picture for HPAI influenza has really changed for North America and the U.S. in the last couple of years,” said Moncla.

“This used to be a virus that primarily circulated in Asia, Northern Africa, and domestic birds. But in more recent years, we’ve seen increasing outbreaks across Europe, associated with wild birds, and since 2022, we’ve also had similar outbreaks in our North American birds.”

Viruses spread by wild birds

To understand that shift, the researchers pulled together information from publicly available databases. These included the Canadian Food Inspection Agency and the United States Department of Agriculture Animal and Plant Health Inspection Service.

The team combined these records with genomic sequencing of the viruses and maps of major migratory flyways that birds use to travel across the continent.

“The main conclusion from this study is that that outbreak was really different from all of the past ones we have had in North America because these viruses were spread primarily by wild migrating birds,” said Moncla.

“Our data pinpoint the Anseriformes, which are ducks, geese, and swans. This was happening in Europe – Europe had almost the exact same thing happen that we had in 2022. They just had it 2 years earlier.”

Evolutionary shift in the bird flu virus

According to the study, an evolutionary shift around 2020 helped H5N1 adapt better to wild birds.

Once that happened, migrating flocks could carry the virus much more efficiently, covering huge distances as they moved between wintering and breeding grounds.

Instead of mostly bouncing around inside barns and sheds full of poultry, the virus began to move with birds that freely cross borders.

However, H5N1 viruses in North America are still classified as foreign animal diseases, noted Moncla.

“Our policy is based on the idea that these viruses come from elsewhere and don’t circulate continuously in our birds here,” she said. “Our study shows that this is no longer the case, and so we need to update our policy to align with this reality.”

Backyard flocks as an early alarm

The team also looked at different kinds of domestic bird settings. They found that agricultural outbreaks often traced back to repeated introductions of the virus from wild birds, not simply spread between farms.

Backyard flocks played a special role in this pattern. These are small groups of fewer than 1,000 domestic birds, as defined by the U.S. Department of Agriculture and the World Animal Health Organization.

On average, they were infected approximately nine days earlier than commercial poultry, which suggests they can act as an early warning signal.

“These populations have a lot of different epidemiological features,” said Moncla. “The farms are smaller. They tend to have less biosecurity. These birds have a much higher likelihood of being raised outdoors with potentially more access to wild birds.”

Pressure on commercial farms

Earlier highly pathogenic avian influenza viruses spread very efficiently between domestic chickens and turkeys.

When officials shut down transmission in commercial farms, the outbreaks usually faded.

With this version of H5N1, wild migrating birds can keep bringing the virus back, even after a farm has done the hard work of cleaning up and culling.

That constant pressure from outside makes control much harder for farmers and regulators. Even if one farm tightens up its practices, migrating ducks, geese, or swans passing overhead may still be carrying new viral strains into the region.

Preventing the spread of bird flu

The practical question is what to do next. When asked about solutions, Moncla summed up the answer in a way many people in agriculture will recognize.

“A series of boring things,” said Moncla. “We need to keep investing in biosecurity – biosecurity does work – making sure that people have good biosecurity plans, both to prevent transmission to other farms but also to prevent wild birds from interacting with their domestic birds.”

According to Moncla, there is also a need for a layered approach to encourage adherence to protocols that prevent the introduction of bird flu viruses.

“At some point we probably will need to investigate vaccinating domestic birds as a possibility,” said Moncla. Investing in novel ways to keep domestic and wild bird separated would also help reduce spillovers, she added.

Continuous surveillance in wild birds, especially Anseriformes waterfowl, would help track the virus and reconstruct how each new outbreak starts.

Can we forecast bird flu risk?

The study points toward another idea: using data to see trouble coming before it hits.

“Our lab is really interested in risk modeling,” said Moncla. “If we had a better understanding of how these viruses are circulating in wild birds and the kind of degree to which different migratory birds are driving transmission, could we have something like a forecasting system for risk over time?”

For example, if risk is highest in a particular region in September, people with backyard birds in that area could be told to make sure that they are adhering fully to their biosecurity plan during that month.

While it is unlikely that the disease will ever completely go away or be solved, what we can do is try to manage it from getting into agricultural animals, concluded Moncla.

The full study was published in the journal Nature.

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