How U.S. Farms Could Start a Bird Flu Pandemic

Without a sharp pivot in state and federal policies, the bird flu virus that has bedeviled American farms is likely to find a firm foothold among dairy cattle, scientists are warning.

And that means bird flu may soon pose a permanent threat to other animals and to people.

So far, this virus, H5N1, does not easily infect humans, and the risk to the public remains low. But the longer the virus circulates in cattle, the more chances it gains to acquire the mutations necessary to set off an influenza pandemic.

“I think the window is closing on our ability to contain the outbreak,” said Dr. Krutika Kuppalli, an infectious-disease physician who worked at the World Health Organization until April.

“We’re so quick to blame China for what happened with SARS-CoV-2, but we’re not doing any better right now,” she added. “That’s how pandemics happen.”

Half a year into the outbreak, H5N1 shows no signs of receding in U.S. dairy cattle or in the workers who tend them. In recent weeks, the virus has spread into poultry and workers.

As of Wednesday, infections had been reported in 192 herds of cattle in 13 states, and in 13 people. Nine were workers at poultry farms close to dairy farms in Colorado.

Earlier this month, the state reported that H5N1 had also been diagnosed in six domestic cats, including two indoor cats with no direct exposure to the virus.

Yet fundamental questions about the outbreak remain unanswered.

Researchers do not know how many farms are being investigated for the virus, how many cows are infected in each state, how and how often the virus jumps into people and other animals, what the course of the illness is in people and animals and whether cows can be infected more than once.

“We need to understand the extent of the circulation in dairy cattle in the U.S., which we don’t,” said Dr. Maria Van Kerkhove, the acting director of pandemic preparedness and prevention at the W.H.O.

She lauded the Agriculture Department’s financial incentives to encourage farmers to cooperate with investigations but said “a hell of a lot more needs to be done.”

The government’s response to the outbreak may be complicated by politics during an election year and by the fact that oversight is led by a federal department that is tasked with both regulating and promoting the agricultural industry.

Federal officials have downplayed the risks to animals, saying the virus causes only mild illness in cows. But a study published in late July showed that cows on affected farms died at twice the normal rate and that some were infected without any outward symptoms.

In theory, nothing about this outbreak should make it difficult to contain, Dr. Van Kerkhove and other experts said. Unlike other influenza viruses, this version of H5N1 does not appear to spread efficiently through the respiratory pathway in cattle.

Instead, in most cases, infections seem to be transmitted through contaminated milk or viral particles on milking machines, vehicles or other objects, such as clothing of farmworkers.

“It’s actually good news,” said Dr. Juergen Richt, a veterinarian and virologist at Kansas State University who led the study.

“If we want to control or eradicate this disease, we just have to focus on the mechanical transmission or anthropogenic transmission,” he said.

Federal officials have said findings like these undergird the belief that they can stop the virus.

“I do believe the response is adequate,” Eric Deeble, an Agriculture Department official, told reporters on Aug. 13.

He has also said the outbreak is containable because there is no wildlife reservoir of the virus — no species in which it is naturally at home.

But experts outside the government disagreed, saying the current measures were not enough to snuff out the outbreak. The virus is entrenched in wild birds, including waterfowl, and in a wide range of mammals, including house mice, cats and raccoons.

“Wishful thinking is a wonderful thing, but it doesn’t necessarily bring you the result that you need,” said Michael Osterholm, an infectious-disease expert at the University of Minnesota. “We’re still totally in a state of confusion.”

Ideally, farms would “bulk test” milk pooled from many cows at once and restrict movement of cattle and farmworkers until the virus was eradicated.

But federal rules require testing only when cattle are moved between states. And many states require testing only of cows that are visibly ill.

So far, Colorado is the only affected state that requires bulk testing of milk, a decision that led to the identification of 10 additional infected herds within two weeks of the July 22 order.

The Agriculture Department has also tried to encourage testing through a voluntary program. Of the roughly 24,000 farms that sell milk in the country, only 30 are participating.

The program has resulted in the identification of herds with infected cows and is “an indication that the system is working as designed,” a department spokesman said in an emailed statement.

Given the risk to their businesses, few farm owners have taken up offers of compensation to set up testing or biosecurity. Many are staffed by migrant workers who fear deportation.

“Right now those guys are feeling very vulnerable, and very, very few are willing to cooperate,” said Dr. Gregory Gray, an infectious-disease epidemiologist at the University of Texas Medical Branch. “Those that are cooperating, in some cases, I think, are regretting that they cooperated.”

Dr. Gray and his colleagues visited two Texas farms in April that had reported sick cattle in the previous 30 days. Of the 14 workers who agreed to have blood drawn, two had antibodies to H5N1, indicating exposure to the virus.

Two-thirds of milk samples from the farms showed signs of live virus, suggesting that infections in both animals and people have been more widespread than official tallies indicate.

So far the virus has not cropped up in cattle in other nations, perhaps because they do not move animals between farms at the scale that Americans do.

Genetic data suggest that the U.S. outbreak stemmed from a single spillover of the virus from birds into cattle and then spread to other parts of the country.

“At that time, there was a lot of virus in wild birds, but that seems to have quietened, so there may not be another spillover event,” said Tom Peacock, a virologist at the Pirbright Institute in Britain.

There is a slim chance that the virus will burn through susceptible cattle herds and disappear, at least for a while, scientists say. But that might take months or even years, if it happens at all.

More likely, the virus will become enzootic — endemic or rooted in animals — much as other viruses have in pigs. Swine farms never rid themselves of a new virus, because susceptible piglets are constantly introduced into the population.

The same may happen among dairy cattle in the United States, Dr. Gray said: “What we see in the swine farms is something we hope we never see in the dairy farms, where you get multiple strains of influenza that might mix and generate novel viruses.”

Already the outbreak in cattle is imperiling poultry — and people.

The virus found in Colorado poultry farms appeared to have come from dairy cattle, and it resulted in the culling of 1.8 million birds. Nine workers involved in the slaughter became infected.

“If this continues at this level, the dairy industry is going to sink the poultry industry,” said Dr. Peacock.

“They’ve had every possible warning that this is a virus that could go pandemic,” he added, referring to federal officials.

Swine farms typically have strict rules to contain new pathogens. Workers are not allowed to move between farms on the same day, for example, and must quarantine themselves in between. When they arrive, they are required to shower and wear gear provided by the farm.

Placing similar restrictions on dairy farms is likely to be harder, because cows are kept alive longer and need far more space. But if dairy farms adopt these measures, “most likely this will be the way to control it,” said Dr. Richt, the Kansas State virologist.

Most experts said it would be premature, and most likely unhelpful, to immunize farm workers with the current vaccines. But vaccinating cattle might be a workable option.

It is easier to make animal vaccines more effective against a virus, with ingredients that may not be tolerated in humans. “That does give me a little bit of optimism,” said Troy Sutton, an influenza expert at Pennsylvania State University.

Still, it may not be possible to end the outbreak by focusing on only cattle. Scientists have found the cattle version of the virus in blackbirds in Texas, suggesting that the birds could carry the virus to new farms.

“The idea that we would have a flu pandemic anytime soon, I think the weight of that politically, economically, in terms of all of our mental health, is just too much to bear at the moment,” said Dr. Van Kerkhove of the W.H.O.

“Everyone’s tired from Covid, everyone’s tired from mpox, everyone’s tired from climate change and war and all that,” she added. “But right now, we don’t get to be tired.”