The world of livestock watched, sampled, studied and tried to control the highly pathogenic avian influenza (HPAI) for nearly four years as it moved through flocks of laying hens, turkeys, ducks and meat chickens, relieved it was not their industry being dealt a wicked blow.

And then it showed up in dairy cattle – moving rapidly between herds, elusive to early detection and thus far impossible to pinpoint a mode of transmission. The disease has left the dairy industry scrambling for treatment, fast-tracking vaccines, developing containment strategies, implementing biosecurity protocols and reassuring the public that dairy products are not the source of a potential foodborne public health crisis.

HPAI is a poultry disease that can also be called H5N1 or bird flu. For lack of a better term in livestock, the USDA has labeled the cattle disease as HPAI. As of February, 17 states have reported nearly 1,000 cases in dairy herds and 67 human cases. Most new cases in cattle and humans are being reported in California.

“It’s a new presentation of an old disease,” said Nora Wineland, DVM, a state veterinarian for the Michigan Department of Agriculture during a panel discussion at the Great Lakes Regional Dairy Conference.

Led by Zelmar Rodriguez, DVM, Michigan State University, the panel included Wineland; Jason Lombard, DVM, Colorado State University; Barb Petersen, DVM, Sunrise Veterinary Service, Texas; Doug Chapin, Michigan Milk Producers Association board chairperson and Chapin Family Farms; and Nathan Brearley, Brearley Farms of Portland, Michigan. The group shared their experiences, tracking, research and the industry response to H5N1.

Rodriguez began with a disease timeline beginning in March 2024 when HPAI was first detected at a Texas dairy, a spillover from migratory waterfowl. Cases followed quickly in Michigan, Ohio, California and New Mexico.

The first human case was reported the following month in an individual who had mild symptoms. The same month, the USDA ordered testing for HPAI for all interstate dairy cattle movement. States soon began implementing a mixed bag of biosecurity orders that included limited access to farms and sanitation for farm pickup and delivery vehicles.

In August, the first case of HPAI was detected in a hog. Although not the same virus subtype found in cattle, it was a disturbing finding given that swine are a species known to get multiple influenza strains that can recombine into something more serious.

The USDA’s National Milk Testing Strategy Surveillance launched polymerase chain reaction (PCR) testing in December 2024 with 42 states enrolled, pulling a monthly raw sample from each farm. The underlying strategy is to identify if the virus is present on any given farm. A positive result can – depending on the state – generally result in the quarantine of cattle movement to and from the farm.

Since December 2024, the D1.1 HPAI genotype was found in a Nevada milk silo. D1.1 is the genotype found in poultry cases, indicating another spillover from wild migratory birds. Prior to this detection, B3.13 had been the only strain found in cattle. Both are different genotypes of the same virus. It’s like comparing two breeds in a species – both are the same species but have different physical traits.

A Louisiana man with underlying health conditions and exposure to a backyard poultry flock was the first human avian influenza genotype D1.1-related death reported on Jan. 6.

Transmission modes

That the Texas outbreak was followed quickly by the Michigan outbreak pointed to cattle movement early on as the mode of transmission, and the topic of cattle movement still contains the deepest information library on transmission mode.

“We know the movement of cattle brought it to Michigan,” Lombard said. “But once it got here, it moved by multiple methods.” Each new transmission model created more questions than answers. He said some experts are convinced aerosol transmission is the culprit. “That is concerning because there’s not a lot you can do to prevent air movement across your farm.”

“What we heard in California is that it essentially went through like a wildfire. And just like a fire will go and burn and then back-burn, that's what they saw with HPAI,” Lombard said. “Farms that didn’t get it when HPAI first moved through an area would later become infected. It seems like the riskiest thing is proximity.”

Lombard initially believed transmission through milk movement was spreading HPAI; however, after finding only one positive result in 126 samples from 18 milk trucks tested in Michigan, that theory lost traction.

Dairy producers in Colorado reported that employees who worked with infected cows experienced high rates of conjunctivitis, while coworkers who didn’t work with cows were fine. That connection became worrisome for workers who feared spreading it to their families. Whether these workers had mild HPAI cases is unknown. The Centers for Disease Control and Prevention (CDC) maintains that human-to-human transmission doesn’t occur.

Birds and waterfowl could carry the virus on their feet and feathers, but blaming wildlife beyond the initial spillovers doesn’t hold up either. “Flu is a respiratory virus that cows shed more in the milk than they do from the respiratory system, but the pace at which it moves through a herd doesn’t support milk as being the only mode of transmission,” Lombard said. “There has to be an aerosol component to disease spread, but when we go on farms and do a lot of air sampling, we rarely find it in air or outside of the parlor.”

Transmission through milking machines is another theory, but the few trials that tried to replicate mechanical transmission failed. “I still think the parlor is a risky place because there are a lot of aerosols generated with milk getting on surfaces and hosed off,” Lombard said. “It is an environment that creates aerosols.”

Human and animal toll

Nathan Brearley’s farm is a closed herd. When they broke with HPAI, they felt alone and unprepared to care for animals in the magnitude it required.

“To have half the herd of 500 cows sick from the disease and have to treat 250 to 300 of them twice a day for rumination, temperature and hydration therapy stretched us beyond thin,” Brearley said. “I was angry. We were cruising along and getting better every year, and this thing attacks us out of nowhere. There is the economic loss from milk production drop, treatments and all the labor involved. And we don’t know how these cows will perform in the next lactation.”

The farm’s Smaxtec biometric tracking system gave an early warning on elevated cow temperature, which may have saved the farm from an even larger hit.

“People aren’t willing to report if they have broke, and it’s a lonely place to be in,” Brearley said. “I eventually decided I didn’t have to do this alone and reached out to extension and others who helped us get through the crisis.” Petersen added that there was a lot of understandable fear and even grief going through stages of trying to understand the situation. “You don’t know what the outcomes are going to be,” said Petersen.

Biosecurity

Proximity to an infected herd seems to be the looming hazard, and an infected dairy close to a commercial poultry operation – or vice versa – is an uncomfortable position.

“We were really behind once we figured out what we were dealing with, and many farms had already gone through it,” Petersen said.

“In the end, people implemented everything – change your clothes; leave clothes and boots at the dairy; disinfect tires; protect your eyes, nose and mouth,” Lombard said. “We did everything we thought could make a difference at the time.”

“I don’t think anybody can give you all of the things that you can do for biosecurity because we still don’t know exactly how it’s being transmitted,” Petersen said.

Industry response

Chapin said one of the biggest challenges for cooperatives was navigating decisions from the USDA and the state with limited knowledge.

“At the end of the day, let’s make sure we’re asking our producers to do things based on science and not emotion,” Chapin said.

When an FDA press release announced HPAI in dairy, the industry communicated to consumers that dairy products were safe. 

“We accomplished that, and it’s clear today that our consumers believe it because we had record intake of dairy products in the U.S. last year,” Chapin said. “We didn’t lose faith with our consumers.”

Immunity and vaccinations

Lombard is tracking some previously infected Colorado herds for antibodies, sampling around 30 cows per farm for 10-12 weeks. About 90% of the cows had some sort of antibody response during the sampling period.

Two entire herds in Colorado with 1,900 and 3,500 head were tested, and around 30% were seropositive or had some antibodies about six months after infection.

“So, this makes me nervous about vaccination because if we are only getting less than six months of immunity duration in cows that were naturally infected, we would expect an even decreased rate of protection with a vaccination,” Lombard said.

Another caveat is that high-producing cows may not have been able to produce high levels of antibodies when they were infected because the energy was directed toward milk production – leading to yet another question on ideal timing to administer a vaccination. Producers are generally accepting of proven vaccines that provide reasonable efficacy. However, there could be pushback on vaccines with international trade implications.

More unknown than known

Lombard sampled a small number of 45-day-old calves on farms during H5N1 clinical duration on a farm and found mixed outcomes. Thirteen of 15 sampled developed antibodies on the farm where calves were retained at the milking farm. Only two out of 15 calves developed antibodies in the herd where they were raised off-site at another facility. Future performance of heifers and cows that have been infected remains unanswered, along with potential damage to mammary tissue in infected milking females.

Beef cattle are not being tested, and clinical signs aren’t showing up in lactating beef females if they are infected. There is no real information on HPAI in the beef herd or if it is connected to transmission in any way.

HPAI has led the dairy and poultry industries through a maze filled with rabbit holes where hypotheses are not definitive, and answers lead to more questions.

Producers can reference resource information through their cooperatives, state departments of agriculture or consult the USDA Animal and Plant Health Inspection Service (APHIS) website.