Why dangerous bird flu is spreading faster and farther than first thought in U.S. cattle

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A dangerous type of bird flu virus discovered in the lung of a U.S. dairy cow that didn't show symptoms. Viral particles identified in processed, pasteurized milk. Genetic sequences showing distinct changes in this H5N1 strain that's been rapidly spreading throughout American cattle.

Those were just a handful of the rapid-fire developments this week as an unprecedented H5N1 outbreak among U.S. dairy cattle continued to evolve.

Scientists now warn this form of avian influenza is likely more widespread in cows, and was transmitting for longer than official reports suggest. And while American officials are ramping up testing — all in an effort to keep sick cows from being moved between states — others say we're already several steps behind the spread of a disease that could pose a major threat to human health.

Michael Worobey, a researcher from B.C. who's now head of the department of ecology and evolutionary biology at the University of Arizona, said these cattle infections may have been "flying under our radar for months," providing ongoing opportunities for this virus to acquire adaptations that could lead to a flu pandemic.

"I think, in many ways, this is the biggest news story in the world right now."

After the U.S. Department of Agriculture posted more than 230 genetic sequences from the country's growing H5N1 outbreak last Sunday, Worobey was among the scientists racing to analyze the complex set of data.

The sequences were from cattle — amid an outbreak impacting more than two-dozen herds across nine U.S. states — but also several bird species, cats, skunks and raccoons.

It's possible that local birds passed the virus to local farms, in different states, and they're all spreading a genetically similar lineage, he said. But Worobey believes what's far more likely is that the cattle outbreak has a single origin point.

In an interview with CBC News, he outlined H5N1's evolutionary tree, with a distinct branch of sequences linked to cattle. All those sequences share the same mutations, Worobey added. 

"This is the hallmark of a single jump, that's at the root of this outbreak," he said.

And that jump likely occurred earlier than the first known cattle infections reported in late March. 

A single introduction could have happened as far back as November or December, with a distinct lineage of H5N1 then spreading undetected for months, Worobey said. He added missing details in the initial U.S. data dump — including exact cattle locations and dates — made it tough to know for sure.

The cattle sequences also contain "at least two distinct mutations" that are known for increasing the risk of a flu virus infecting and transmitting in humans, Worobey added.