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U.S. Scientists Surprised by Bird Flu in Cows, But Canadian Research Predicted It Since 1953

When U.S. dairy cows began showing signs of a severe bird flu, many scientists were puzzled by an unusual finding: the virus was consistently found in cows’ udders.

Influenza typically enters the body through the respiratory tract, affecting the throat, nose, and lungs. However, Canadian virologist Alyson Kelvin, from the University of Saskatchewan’s Vaccine and Infectious Disease Organization, wasn’t surprised.

“We’ve known that the cow mammary gland is susceptible to influenza virus infections since at least the ’50s,” said Kelvin, a seasoned influenza researcher. “So this isn’t that surprising.”

Indeed, decades of scientific research hinted at this possibility. While 2024 marked the first report of H5N1, a form of influenza A, in dairy cows, it was known that influenza viruses could target mammary gland cells.

This poorly understood transmission route might have contributed to two human eye infections linked to the U.S. outbreak, potentially from contaminated milk.

“If we paid more attention to these possibilities,” Kelvin noted, “we might not have been so surprised.”

Early research confirmed that flu could infect cow udders. As of now, over 50 herds across nine states have reported cases among U.S. dairy cows. However, scientists believe that limited testing and sluggish data-sharing indicate the virus is more widespread. Genetic sequencing suggests H5N1 circulated in cows months before the first cases were detected in March.

Decades earlier, Canadian agricultural researchers conducted preliminary experiments on lactating dairy cows, showing that a type of human influenza A could infect cows’ mammary glands, leading to live virus in milk secretions. Kelvin’s own 2015 research further explored influenza transmission between mother and infant ferrets, showing that influenza could be present in mammary gland tissue and milk, suggesting a greater role for the mammary gland in infection and immunity.

Federal tests have found no signs of bird flu virus in Canadian retail milk, but Kelvin said her findings were largely ignored. “I had lots of people say, ‘Who cares about the mammary glands?'” she recalled, despite having compelling reasons to pursue further research.

Understanding how flu viruses are transmitted has never been more urgent due to H5N1’s unusual spread, affecting cows, birds, and humans. The two human cases linked to the outbreak involved only eye infections, possibly due to contaminated milk. The U.S. Centers for Disease Control and Prevention highlighted that bird flu could reach eye membranes through splashes of contaminated fluid or hand-to-eye contact.

Past research also showed that virus-filled milk could cause eye infections, a mode of transmission observed in prior bird flu outbreaks.

Before this unusual U.S. cow outbreak, awareness of past research on such transmission routes was limited, said Tom Peacock, a research virologist with the Pirbright Institute in the U.K. He noted that lab-based studies often hint at various infection methods that are not always seen in the wild.

The spread of H5N1 to numerous species worldwide, including in Antarctica, underscores the need for scientists to “expect the unexpected.” A recent Michigan State University study on a dairy farm outbreak revealed severe symptoms among infected cows, including high fevers, dehydration, aborted calves, and reduced milk production.

Immunologist Stephanie Langel pointed out that the shared equipment and messy process of dairy cow milking could facilitate virus transmission. Her research suggests the mammary gland may serve as a vector for H5N1, explaining eye infections in dairy workers and severe health impacts on farm cats drinking raw, virus-filled milk.

One U.S. CDC study found that more than half the cats on a Texas dairy farm died after drinking infected milk. Raw milk could pose a risk to humans as well, although pasteurization makes processed milk safe to drink.

A recent lab study showed that mice given raw milk from infected cows had high virus levels in their respiratory organs, raising concerns about potential risks to humans.

Both Kelvin and Langel called for more research to understand the mechanics of mammary gland infections and their transmission to cows, humans, and other species. Effective use of personal protective equipment among farm workers is also crucial.

Langel emphasized the need to prevent influenza infections in cows’ udders to protect the dairy industry and prevent repeated human exposure. Ongoing research indicates that both avian and human influenza viruses can infect cow mammary gland cells, potentially facilitating human-to-human transmission in the future.

“Understanding women’s health, female health, and lactating animal health is crucial,” Langel said, hoping the outbreak prompts more attention to these areas.

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