Most people think of influenza as a seasonal respiratory illness causing fever and body aches during the winter. What makes influenza unique is that it doesn’t disappear when human flu season ends. Influenza A viruses circulate year-round in animals, particularly wild birds, quietly evolving until the right conditions allow them to jump species.
That ability to move between hosts and adapt makes influenza a constant threat that may jump from animals to humans, capable of sparking the next pandemic as seen with the most recent 2009 H1N1 swine flu pandemic.
While farmers know this virus well in poultry and swine, it wasn’t a concern in dairy cattle until March 2024, when H5N1 was confirmed in U.S. herds following unexplained illness and sudden drops in milk production. Nearly two years later, the virus continues to circulate in dairy cattle, raising urgent questions for animal and public health.
Why is the virus affecting milk production?
What makes this outbreak unsettling is not just that H5N1 can infect cattle, but where it replicates. Research shows that the virus primarily targets the udder, meaning the virus is present in milk. Common clinical signs include reduced rumination and feed intake, steep declines in milk production, mastitis and fever.
Because milk is both the economic foundation of the dairy industry and a widely consumed product, this raises understandable concerns about the safety of the food supply. Researchers at U.S. Department of Agriculture-Agricultural Research Service and our work at Ohio State University detected H5N1 viral RNA in retail milk. Importantly, pasteurization effectively inactivates the virus, so commercially pasteurized milk remains safe for consumers. However, unpasteurized raw milk may still be contaminated with infectious H5N1, representing a substantial risk to humans and other animals.
How does the virus infect dairy cattle?
Dairy cattle are highly susceptible to H5N1, requiring a shockingly low amount of virus to establish a robust infection in the udder. In experimental studies that our team conducted at OSU, we show that a single drop of milk from an infected cow could contain enough virus to infect thousands of others. These infected cows quickly develop mastitis, shed massive amounts of virus in their milk and experience sharp drops in milk production, all of which mirror clinical observations in lactating animals on-farm.
How does the virus spread from cow to cow?
Contaminated milking equipment is the leading hypothesis for transmission, as it takes so little virus to trigger an infection in the udder. While this cannot be ruled out, the story appears to be more complicated. On affected farms, even dry cows that never enter the milking parlor have developed antibodies, suggesting they encountered the virus elsewhere.
At the same time, our team at OSU attempted to recreate transmission using contaminated milking equipment under controlled experimental conditions, but were unsuccessful. These findings suggest that the parlor is only one piece of the puzzle, and that other unidentified routes are likely contributing to the spread of H5N1 within dairy herds.
How do we stop the virus from spreading?
No H5N1 vaccine is approved for use in dairy cattle, so farms must rely on biosecurity, early detection and natural immunity. Our experimental studies at OSU show that cows infected during an outbreak develop a strong, long-lasting immune response. When exposed to the same strain more than a year later, cows were protected from disease and did not shed virus in milk. The catch is that immunity is strain-specific; protection against one strain of H5N1 only partly protects against others.
The bigger concern looking ahead is that HPAI continues to circulate in wild birds, a large and constantly changing virus pool. In a short span of time, at least three different introductions of H5N1 from wild birds have already found their way into dairy herds.
Natural immunity may slow spread and has likely contributed to fewer newly positive herds in recent months, according to the USDA’s National Milk Testing Strategy, but it offers only limited protection against the next strain that might arrive.
Is H5N1 in dairy cattle a threat to humans?
There have been 41 confirmed human cases linked to infected dairy cattle, and the true number is likely higher given the limited monitoring of farm workers.
We still don’t know whether cows could become a long-term host like birds or pigs, or whether the virus could evolve in cows in ways that make it more likely to infect people. The close connection between humans and dairy cattle makes H5N1 a serious concern, highlighting the need for surveillance, biosecurity, and proactive measures like vaccination.
(Natalie Tarbuck, PhD student, is a graduate research associate in the College of Veterinary Medicine at Ohio State University. She can be reached at Tarbuck.1@buckeyemail.osu.edu.)
