The bird flu outbreak among dairy cows continues to generate alarm, despite reassuring news that pasteurized milk is unlikely to infect anyone with H5N1. Scientists can’t stop worrying about a nightmare scenario: that the virus will get into pigs and, from there, spark a human pandemic.

Pigs “are the perfect vessels through which an even more virulent strain could emerge,” said Nirav Shah, principal deputy director of the Centers for Disease Control and Prevention at a May 2 briefing by the Council on Foreign Relations.

Pigs are capable of harboring both human flu and bird flu, allowing the viruses to mix and match parts of their genetic material. A 2009 flu pandemic started with a pig-to-human transmission. That strain, called H1N1, wasn’t deadlier than seasonal flu, but that was just a lucky break.

Now is the time to get ahead of 2024’s H5N1 virus with systematic testing of both sick and healthy-looking animals — including pigs. Scientists agree such testing is essential to understanding the situation, and they have the test kits. What they’re missing is a nimble change in policy that would ensure the cooperation of farmers who fear economic ruin if their animals test positive.

Thousands of chickens and turkeys are already monitored — thanks to agreements that reassure farmers they won’t be financially ruined by positive tests. Government compensation for “culling” birds has brought on its own set of controversies, but right now there’s no system to compensate farmers for H5N1 infected cows or pigs, which means they have no incentive to let public health officials do enough testing. The U.S. Department of Agriculture recently required testing of dairy cows only if they were being transferred to other states. It’s up to our political leaders to make further policy changes so that farmers are encouraged to work with scientists — and scientists can do the research they need to do.

This must extend to testing of healthy-looking animals. A recent analysis of the genetic material of the virus suggests it may have been spreading stealthily in cows since last December, long before the first case was detected in late March. Failing to test asymptomatic animals would be a mistake akin to the insufficient testing for Covid-19 in early 2020. That was one of the most egregious public health mistakes of that pandemic — people who didn’t meet very specific criteria (like having recently traveled to China) were unable to get a test, allowing the disease to spread further.

Scientists agree that keeping H5N1 from sparking a human pandemic requires careful monitoring of cows, pigs and farm workers. Shah called the risk of an H5N1 pandemic “not insignificant,” and yet there’s currently no coordinated effort to test asymptomatic farm animals.

And pigs have been silent carriers before. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, said that in 2014 scientists found a flu called H3N2 was being transmitted back and forth between pigs and people, many of them kids, at Ohio state fairs.

Flu likes to bind to a sugar on the surface of cells, and the reason bird influenzas usually doesn’t spread among humans is that our sugars are very different, explained Richard Webby, a specialist in influenza at St. Jude’s Children’s Research Hospital in Memphis.

The cells in a pig’s respiratory tract have both kinds of sugars, so both kinds of virus can get in and swap pieces. The infamous 1918 influenza virus, thought to have originated from a bird flu, was transmitted from humans to pigs in the 1920s, where it continued to evolve. It re-emerged in humans in 1957, 1968 and 2009. In recent years, as bird flu surged through domestic flocks, it’s gained the power to infect dozens of mammal species, including minks, racoons, foxes, seals and porpoises. We really don’t want pigs to be next.

Yet the wider the cow infections spread, the more chances the virus has to jump to pigs. They might get infected through contaminated equipment, or if milk from infected cows gets into their feed. Although pasteurization kills the virus in commercial milk, the raw milk remains highly infectious — it’s the lead suspect in deaths of several farm cats infected with H5N1.

“What’s a bit unclear to me is exactly what’s happening to all this contaminated milk,” said Webby. Could some be getting dumped, raw, where other animals could ingest it?

Osterholm said more surveillance on farms is critical. This virus has already traveled from infected cows to poultry on one farm in Michigan. Scientists need more data to understand how the disease is spreading among cows, which would mean adopting policies modeled on those that allow monitoring poultry.

But right now, farmers who raise pigs and cows see nothing in it for them except lost money and stigma. CDC’s Shah said farmers are often deeply skeptical of the federal government. Farm workers are often fearful of missing work, since they have no paid sick leave, which is hampering the parallel need for farm worker testing.

“Everyone is coming off Covid-19 so fatigued and tired they don’t want to hear about another pandemic,” said Osterholm. But just imagine how we’ll feel if we have to live through another deadly outbreak — and another series of public health mistakes.

F.D. Flam is a Bloomberg Opinion columnist covering science. She is host of the “Follow the Science” podcast.

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