In the weeks since the U.S. Department of Agriculture announced a bird flu outbreak in dairy cows that had gone undiagnosed for months, bystanders have wondered why it took so long to identify. Experts say there are key scientific and political reasons why the dairy industry was caught off guard by the H5N1 avian influenza virus—and that understanding those factors will be vital to controlling the disease on dairy farms and preventing an outbreak in humans.
“The dairy industry has never had to deal with something like this before,” says Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory and a former dairy veterinarian. “This is probably going to be the most important outbreak in my professional career.”
After weeks of uncertainty—fueled by the unnerving revelation that the outbreak likely began last December and took months to recognize—the federal government is taking some action. The USDA has mandated testing for lactating cows that are being transported across state lines in the hopes of squashing the spread of the virus, which has so far been confirmed in 36 herds across nine states. One human case has been reported this year, but the infection was mild, and more than a month has passed with no new cases confirmed. Epidemiologists have called for more human testing to better monitor the situation, however. Meanwhile Food and Drug Administration testing has shown that viral particles that were found in pasteurized milk were not infectious, suggesting little threat to consumers.
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Authorities’ scramble to understand and control the situation is particularly concerning because scientists have long worried about the potential for an H5N1 strain to jump into humans and cause a pandemic. And bird flu has been devastating poultry farms around the world for years now. Veterinarians and epidemiologists have been on high alert since 2022, when a new strain began tearing through wild birds and even mammals, killing or forcing U.S. farmers to cull some 90 million domestic birds. How could it take months to notice the same virus was spreading in dairy cows?
One reason the outbreak went undetected for so long is that people thought it was unlikely that the virus would jump into cows. Avian influenza is, after all, most common in birds, whereas flus in general have been rare in cows. “The chances of it going from migratory birds to cows were so low,” Poulsen says. “And then it happened.”
Dairy farmers typically worry about contagious bacterial mastitis, an infection of a cow’s udder, or bovine viral diarrhea—illnesses the industry is familiar with and knows how to treat. Avian influenza “wasn’t on our bingo cards for the year,” says Emily Yeiser Stepp, an animal scientist and executive director of the National Dairy Farmers Assuring Responsible Management (FARM) Program. The program, which is housed within a lobbying group called the National Milk Producers Federation, acts as an on-farm social responsibility program for the dairy industry, a mission that includes promoting animal health.
The next challenge was connecting the dots to determine what was infecting the cows—which was difficult because the symptoms of H5N1 in these animals weren’t known in advance and aren’t particularly distinctive. Sick cows weren’t eating well and were producing less milk—and the milk that they did produce was sometimes thick and yellow-hued. But none of these symptoms were particularly unusual, says Meghan Davis, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health and a former dairy veterinarian.
“The number-one presenting complaint to call a vet out to your farm is: [your cow is] off feed, and you can’t figure out why,” Davis says. “So it’s not surprising to me that it took a minute to figure this out.”
Moreover, only about 10 percent of an affected dairy herd seems to get infected with H5N1. And cows aren’t dying; they bounce back to health within a couple of weeks. “Cows are like walking battle tanks,” Poulsen says. But this is also a very different manifestation of avian influenza than that seen in domestic chickens: H5N1 can blaze through and devastate a flock in days, he adds. Wild mammals that have been infected with avian influenza display serious respiratory and even neurological symptoms such as seizures. In contrast, infected cows are tricky to spot. “You need to look for it; it’s not something very apparent,” says Zelmar Rodriguez, a dairy veterinarian at Michigan State University, who has visited farms with H5N1-infected cows.
Because farmers didn’t know they had avian influenza on their hands, the low impact of the disease made it easy for them to shrug off the infection. Once veterinarians pinned down H5N1 as the culprit, the situation became more serious because of the implications for human health. The complications of the dairy outbreak make containing it a challenge, however.
Scientists still aren’t certain how avian influenza jumps between cows. Influenzas are typically considered respiratory diseases, yet cows seem to produce the most virus in their udders, so milking machinery could be carrying virus-laden droplets of milk from one cow to the next. Lactating cows can experience discomfort when they are not milked regularly, so all that’s left for farmers to do is to try to increase safety measures in the milking process itself.
Adding to the problem, the dairy industry moves tens of thousands of female cattle between states every week, Poulsen says, because the animals grow well as calves in hot, dry environments but produce more milk as mature cows in cooler regions. This movement of sick animals likely facilitated early spread between farms.
The USDA has already moved to address this spread: as of April 29, it is requiring that lactating cows test negative for H5N1 before they can cross state lines. Controlling the spread of avian influenza without stopping the complex system of cow movement is a crucial balance, Poulsen says. “We can’t just turn off milk production, and we can’t turn off the supply chain of those cows,” he says.
Testing and other monitoring strategies need to be strengthened even for cows staying in one place, however. “I think that we are a bit behind on the surveillance,” Davis says. In particular, she says she’d like to see cows undergo serological testing, which looks for antibodies in blood that are tailored to a specific pathogen and can signify past exposure to that pathogen. She also notes that existing milk-monitoring systems set up for food safety may offer opportunities for better tracking of H5N1, given the virus’s prevalence in milk. In addition, the same wastewater monitoring infrastructure that scientists developed to keep an eye on COVID is also offering insight on where avian influenza is striking dairy cows, as well as other animal infections.
Tackling avian influenza in dairy animals poses additional logistical complications. Compared with the poultry industry, the dairy industry is more fragmented, Davis says: the latter is made up of many independent farms rather than large, centralized companies that hire farmers on contract.
Another distinction between the industries is that the government has an indemnification system that reimburses poultry farmers for birds that are culled to keep the disease from spreading. By contrast, experts say that killing cows wouldn’t help stem the dairy outbreak because of the animals’ lower infection rate and milder symptoms. Yeiser Stepp isn’t sure yet what financial strategy makes the most sense for the farmers who work with her. She’s also a herd owner who has cows based in states that are not reporting H5N1 cases, and she says her biggest concern is about the fees for calling a vet out to the farm more regularly.
Rodriguez says that at present he thinks the priority needs to be strengthening biosecurity measures that can protect both cows and the humans who care for them. Existing recommendations hold that workers should be wearing gloves during the milking process, but doubling down on this guideline and adding additional personal protective equipment such as disposable coveralls, face shields, masks and goggles are important steps for keeping the virus out of humans. Currently, dairy workers are among the people who are most at risk of avian influenza, and many are disempowered migrant workers who face structural barriers to accessing health care. To determine what comes next, however, scientists need to pin down the outbreak’s many lingering unknowns, such as how the virus is spreading and how long cows remain infectious. “I think that biosecurity is the only thing that we can do right now, at least until we have more answers in terms of the dynamic of the virus,” Rodriguez says.
Despite the stumbles early in the response, Poulsen says, there’s still time to put an end to the dairy outbreak before it becomes a bigger issue for the industry as a whole or for the general population. “We have the best veterinarians and public health scientists in the world,” he says. “We can do it. We just need the funding and the political will.”