Two years after highly pathogenic avian influenza (HPAI) was first identified in U.S. dairy cattle, the industry’s understanding of the disease has changed dramatically.
When the virus emerged in March 2024, veterinarians were making decisions while researchers raced to answer basic questions. Why were dairy cows getting sick? How was the virus spreading between herds? Would infected cows recover? What treatments worked? Could biosecurity contain the outbreak?
Many of those questions no longer lack answers.
Research conducted over the past two years, combined with observations from veterinarians managing affected herds, has transformed HPAI from an unfamiliar emergency into a disease that is increasingly understood. Much of this work and experience was discussed at AABP’s recent Emerging Diseases Conference. Significant questions remain, but the profession now has a much clearer picture of how the virus behaves, how it spreads and how best to manage infected herds.
Here are five of the biggest lessons learned since HPAI entered U.S. dairy cattle.
1. HPAI Behaves Differently in Dairy Cattle than Anyone Expected
Perhaps the biggest surprise during the past two years has been discovering HPAI in dairy cattle is fundamentally different from the disease veterinarians expected based on decades of experience in poultry and wild birds.
Early in the outbreak, researchers quickly realized the virus wasn’t behaving primarily as a respiratory disease in adult dairy cows. Instead, it showed a remarkable affinity for the mammary gland.
“One of the key features of the virus is actually its ability to replicate in the mammary gland,” says Diego Diel, professor at Cornell’s College of Veterinary Medicine. “Because of that, we detect pretty significant viral loads in milk from affected cows. We were pretty surprised with the amount of infectious virus being shed in milk.”
We now understand that the mammary gland contains receptors that allow the virus to efficiently infect mammary tissue, helping explain why milk became one of the defining features of disease in dairy cattle, as well as the shedding of the virus through milk.
This discovery fundamentally changed how HPAI is recognized in the field. Rather than focusing primarily on respiratory disease, HPAI is now associated with sudden milk loss, fever, lethargy, reduced appetite and abnormal milk in lactating cows.
Just as importantly, understanding where the virus replicates has helped guide surveillance efforts, diagnostic testing and future research.
2. The Virus is More Widespread than Clinical Cases Suggest
One of the most important discoveries over the past two years is that clinically sick cows represent only part of the picture. Early outbreaks focused attention on visibly affected animals, but additional research showed many infected cattle never developed obvious clinical disease despite becoming infected.
“Only about 20% of the animals developed clinical disease,” says Jason Lombard, associate professor at Colorado State University. “The majority of the animals exposed and infected remained subclinical during an outbreak.”
That finding changes how we think about disease spread within dairy herds. Rather than assuming outbreaks are driven only by obviously sick animals, we now recognize apparently healthy cattle may also play an important role in transmission.
Diel says field investigations leave little doubt that the virus moves efficiently through dairy populations.
“H5N1 efficiently transmits from cow to cow,” he says. “There’s plenty of field evidence showing the virus transmits within farms and between farms as well. Probably one of the main culprits for dissemination is the large proportion of subclinically affected animals.”
Those observations also explain why HPAI proved so challenging to contain during the early months of the outbreak. By the time the first clinical cows are recognized, additional infected animals are often already present within the herd.
This discovery reinforces the importance of looking beyond visibly sick cows when assessing herd risk.
It also highlights the value of surveillance programs capable of identifying infection before clinical disease becomes widespread.
3. Early Detection is One of the Industry’s Best Tools
If the first year of the outbreak focused on recognizing HPAI, the second focused on finding it sooner.
Surveillance rapidly evolved from an emergency response tool into one of the industry’s most valuable management strategies.
California’s statewide bulk tank milk surveillance program became one example of how systematic testing could improve both disease detection and communication between producers, veterinarians and animal health officials.
“Here in California, they started doing statewide bulk tank surveillance,” recalls Dr. Maxwell Beal, the veterinarian at Mill Creek Veterinary Services who reported the first case of HPAI in California. “Producers had no option not to report it. That actually helped us a lot as practitioners because there were psychological blockages about wanting to report.”
Mandatory surveillance removed much of the hesitation surrounding voluntary reporting while giving veterinarians earlier insight into where the virus was circulating.
Veterinary pathologist Dr. David Swayne believes rapid surveillance has become one of the profession’s strongest defenses against future outbreaks.
“The quicker the diagnostics through surveillance, the quicker you can do eradication before it becomes established and spreads farm to farm,” he says.
Although surveillance programs continue to evolve, the past two years have demonstrated that early detection provides veterinarians and producers with valuable time to strengthen biosecurity, prepare employees and make informed management decisions before widespread clinical disease develops.
Perhaps more importantly, surveillance has shifted HPAI management from reacting to outbreaks toward anticipating them.
4. Practical Management Beats Aggressive Intervention
The first HPAI outbreaks left veterinarians with few evidence-based treatment recommendations. Producers wanted answers, and quickly. Every new report seemed to bring another suggested therapy or management strategy, but field experience gradually showed more intervention didn’t always produce better outcomes.
Instead, veterinarians found success by focusing on supportive care and using labor strategically.
“Aggressive treatment seemed to have an overkill aspect to it,” Beal says. “If you’re drenching twice a day, you’re going to do a thousand animals today, and you tire your people out so much they start making mistakes. Setting parameters for which cows really needed treatment seemed to be the best middle ground.”
That lesson extended beyond medicine.
Managing an HPAI outbreak required balancing animal care with the practical realities of operating a dairy. Employees were often working long hours caring for sick cattle while trying to maintain daily operations. Protocols that appeared comprehensive on paper sometimes proved unsustainable in the field.
Instead of treating every potentially affected cow, many veterinarians shifted toward targeted supportive care focused on animals most likely to benefit from intervention.
“My advice was always, she’s got a high fever, she looks pretty bad, let’s go ahead and treat her,” Beal says. “But do we need to treat everything in the pen?”
That experience helped reshape outbreak management. Rather than searching for a single treatment that would stop HPAI, veterinarians increasingly emphasized early recognition, supportive care, labor efficiency and realistic expectations for recovery.
It also reinforced a broader principle of emerging disease management: Evidence often develops more slowly than urgency. Successful practitioners learned to balance scientific uncertainty with practical decision-making while adapting recommendations as new information became available.
5. HPAI isn’t Going Away; Researchers are Still Looking for Answers
Despite the progress made over the past two years, important questions remain.
Researchers continue to investigate the relative importance of different transmission routes, how long infected cattle may shed virus under certain circumstances and how the virus will continue to evolve as it circulates among dairy herds and wildlife.
“The big questions that still remain are the modes of transmission in dairy cows,” Diel says. “Could it be milking equipment? Could it be aerosol transmission? I suspect it’s more than one. I don’t think it’s only one method of transmission.”
That complexity reflects one of the biggest lessons from the outbreak.
Rather than identifying a single explanation for every new infection, researchers increasingly recognize that multiple transmission pathways are likely contributing to disease spread under different management systems and production environments.
Lombard says his team is also investigating why some cattle appear to shed virus longer than expected.
“Some of the prolonged detections we’re seeing may be due to a handful of persistently shedding cows,” he says.
Meanwhile, the virus itself continues to evolve.
“These viruses are always changing,” Swayne says. “Even the current virus we’re facing in the U.S. has been changing since its first appearance 30 years ago.”
Those realities mean HPAI research is far from finished. New discoveries will continue to shape recommendations for surveillance, biosecurity and herd management in the years ahead.
Yet the conversation surrounding HPAI has changed considerably since 2024.
Two years ago, we were still trying to understand what the virus was doing in dairy cattle. Today, we are refining how to manage it.
Research has clarified how the virus behaves in dairy cattle, field experience has improved clinical management and surveillance systems now provide earlier warning than was possible during the first months of the outbreak.
None of those advances eliminate the threat posed by HPAI. Instead, they provide veterinarians with something they didn’t have two years ago: experience backed by evidence.
HPAI is no longer viewed solely as an emerging disease emergency. It has become another health challenge to account for when managing modern dairy herds. Continued surveillance, ongoing research and strong biosecurity will remain essential as the virus evolves, but the profession is no longer responding from a position of uncertainty.
