Bird flu in U.S. cows caught scientists by surprise. Canadian research has seen it coming since 1953

This story is part of CBC Health's Second Opinion, a weekly analysis of health and medical science news emailed to subscribers on Saturday mornings. If you haven't subscribed yet, you can do that by clicking here.

When U.S. dairy cows began falling ill with a dangerous form of bird flu, many scientists were struck by an unusual pattern: The virus kept showing up in cows' udders, of all places.

Influenza is usually known as a respiratory virus, entering the body through the throat and nose before heading to the lungs. But Canadian virologist Alyson Kelvin wasn't among those shocked by the udder discovery.

"We've known that the cow mammary gland is susceptible to influenza virus infections since at least the '50s," said Kelvin, a longtime influenza researcher who works at the University of Saskatchewan's Vaccine and Infectious Disease Organization. "So this isn't that surprising."

Indeed, decades of scientific study provided early clues that something like this was possible. 

Even though 2024 marked the first time H5N1, a form of influenza A, was reported in dairy cows, researchers knew influenza viruses can target the cells that make up mammary glands.

The little-understood transmission pathway may have also played a role in two human infections linked to the current U.S. outbreak — both of which only involved eye infections, possibly from the virus entering the eye membranes through contaminated milk.

"I think if we paid more attention to [these possibilities]," Kelvin said, "we might have not been so surprised."

So far, cases among U.S. dairy cows have officially spread to more than 50 herds across nine states. 

But multiple scientists who spoke to CBC News in recent weeks say sluggish data-sharing and limited testing — and the detection of harmless viral fragments in the country's processed milk supply — suggest the virus is already more widespread. Genetic sequencing of the virus, showing its evolution, also suggests H5N1 was likely circulating in cows months before the first cases were discovered in March.

That explosive spread appeared to come out of thin air.

Yet decades earlier, researchers from Canada's department of agriculture were conducting "preliminary" experiments on lactating dairy cows to see what would happen when the animals were infected with several types of viruses.

Their 1953 paper showed that a type of human influenza A could infect cows' mammary glands, leading to live virus in milk secretions.

Kelvin's own research, published in 2015, probed the issue further.