Philadelphia, August 14, 2025 – Since its emergence in US dairy cattle, highly pathogenic avian influenza (bird flu, H5N1) has defied control, spreading to other species and disrupting every stage of the dairy sector. A newly published invited review in the Journal of Dairy Science presents the most comprehensive look yet at the virus's impact and calls for a unified response. With current science pointing to complex transmission dynamics and broad consequences, the authors argue that only a One Health approach, taking into account the intersection of animal and human health with that of the environment, can effectively combat H5N1 and prevent future outbreaks.
Jason Lombard, DVM, MS, of Colorado State University, Fort Collins, Colorado, lead author of the review, explained, "The H5N1 outbreak in dairy cattle is unlike anything we have seen before—it has an exceptional ability to spread between farms and among cows within a farm—and is the first time an infectious disease of this magnitude has affected the US dairy sector in nearly 100 years. It has revealed challenges we still haven't solved in our ability to detect, report, and manage new diseases."
The review, building on both the current body of literature and best first-hand accounts and unpublished data of the outbreak, describes how the H5N1 virus, previously known for outbreaks in poultry, spilled over from wild birds into dairy herds and a range of other animal species, and maps the broad impacts of the virus to date.
"Our team wanted to describe the effects of H5N1 [on] agriculture as well as animal and human health in the US and demonstrate that a One Health perspective—or one that takes the health of animals, humans, and the environment as a holistic interconnected network into account—will be essential to navigating this outbreak," Dr. Lombard added.
First, the team outlines the latest science on how bird flu is impacting the US food supply, of which poultry, beef, and dairy play a key role. "Before anything else, it's critical to convey that pasteurization remains a very effective tool in inactivating the virus in milk, which means our commercial supply of milk and dairy products remains safe, as are properly cooked beef, poultry, and eggs, based on several US Department of Agriculture Food Safety and Inspection Service studies," Dr. Lombard emphasized. Still, reduced production and supply disruptions have driven up prices for consumers, and the virus's broader impacts—from farm operations and management to worker health—continue to ripple across the dairy industry.
Stopping the spread of H5N1 starts with an understanding of exactly how the virus spilled over into cattle, and the routes it is taking to spread among dairy cattle and between farms—both of which are still unknown. The evidence suggests that cows' initial exposure may occur through feed or water contaminated by wild birds, although it is possible the virus could also spread through aerosols (tiny airborne particles) or via fomites (contaminated surfaces or objects). After their initial exposure, cows can easily spread the virus to others in their herd.
"The H5N1 virus attaches to sialic acid receptors, which are abundant in the udders and respiratory tracts of cows, which means breathing in the virus and coming in contact with infected milk are both options for spread on the farm," outlined Dr. Lombard.
"We have information from experimental and field-based studies, but right now, we are missing some key pieces of the puzzle regarding cow-to-cow viral transmission, and more information is needed," added Chloe Stenkamp-Strahm, DVM, PhD, of Colorado State University and co-author of the article.
Dr. Lombard and the co-authors were clear that understanding transmission will be critical to stopping the spread of H5N1, and that this is uniquely challenging because of the structure and complexity of the dairy industry in which cattle, humans, other animals, and equipment are frequently in close contact and moved from location to location.
"Overall, it's important to think of our dairy operations as ecosystems," said co-author Brian McCluskey, DVM, PhD, DACVPM, of Colorado State University. "It's not just cows that make up a dairy farm, but a dynamic and interconnected community of living organisms—birds, cats, flies, and people—as well as their physical environments—water, vehicles, bedding, housing, and equipment." All of which interact with each other to potentially spread pathogens like H5N1, which is why tracking, understanding, and mitigating the virus is complicated and will require the collaboration of multiple partners: producers, herd veterinarians, government and state partners, and researchers.
The human element of this outbreak is the final area the team reviewed, touching on the impact on human health to date and the potential for a larger H5N1 human pandemic in the future. "We've seen 70 confirmed H5N1 cases associated with this outbreak, most of whom were people working with cattle or poultry," said co-author Cherissa Abdul-Hamid, DVM, MPH, DACVPM, of the Texas Department of State Health Services in Lubbock, Texas. These cases have generally been mild, mostly involving conjunctivitis (pink eye) or mild flu-like symptoms. The article is clear, however, that H5N1 extends beyond physical illness; the outbreak has exacerbated mental health challenges for dairy workers, who often face pre-existing disparities in healthcare access, as well as for veterinarians and producers, who contend with increased workloads and financial, and emotional strain.
The virus has not only affected the dairy industry, but the "cattle strain" has also greatly affected the poultry industry. "According to the American Association of Avian Pathologists, more than 28 million commercial birds on almost 100 premises have been depopulated, and there are many gaps in our knowledge of how it is being transmitted from cattle to poultry," said co-author Kay Russo, DVM, MAHM, DACPV, of RSM Consulting in Fort Collins, Colorado.
The virus's potential for causing a larger human health event is much less clear and will require significantly more data than we have available. "So far, we don't truly have enough information to predict the likelihood of H5N1 developing into a virus that can spread [from] human to human and trigger a pandemic," said co-author Carol Cardona, DVM, PhD, of the University of Minnesota in St. Paul, Minnesota, "but it is safe to say the potential increases as we continue to fail to control the virus."
How can all of these impacts be addressed in the dairy industry? Controlling the spread of H5N1 will demand a coordinated effort across the dairy sector to strengthen biosecurity practices on farms (including the role of dairy workers), broad viral surveillance and reporting, a review of current control measures, and potentially a vaccination option—all while protecting the livelihoods of dairy producers. Although the article referenced several published One Health recommendations, it emphasized the three from a recent RAND article as particularly impactful:
- Declare H5N1 a public health emergency and mandate ongoing nationwide testing of representative samples of dairy and beef cattle, free of charge to farmers. Implement increased testing of farm and dairy workers, livestock vaccinations, and indemnity payments.
- Expand the US Centers for Disease Control and Prevention's National Wastewater Surveillance program to test for H5N1 and associated variants from different hosts.
- Make H5N1 vaccines available to anyone likely to have been exposed to infected animals, and prepare a plan for nationwide distribution.
Paul Kononoff, PhD, editor in chief of the Journal of Dairy Science, underscored the importance of the article: "While there are still many unknowns and more science still to understand, this outbreak is a clear reminder that the health of our dairy herds, farm workers, and the broader dairy community and the general public are deeply interconnected. This invited review helps to equip producers, veterinarians, and policymakers with an evidence-based perspective on why One Health is essential for navigating the challenges and opportunities this outbreak presents."
