A new strain of a highly pathogenic bird flu known as H7N9 has surfaced at a poultry farm in Mississippi where chickens are raised for breeding.

The finding of the new strain came as researchers separately reported a potentially positive development: Exposure to human seasonal flu may confer some immunity to H5N1 bird flu.

The new strain found in Noxubee County, Miss., was confirmed March 12 and all of the roughly 46,000 birds either died or were euthanized after the infection spread, according to the U.S. Department of Agriculture’s Animal Plant Health Inspection Service and Mississippi’s Board of Animal Health. None of the birds entered the food supply.

Authorities didn’t say how the birds were infected, although federal wildlife agents had been identifying low-pathogenic versions of the H7N9 virus for several years in wild birds. It is possible that the version found in the chickens is circulating in wild birds, but most researchers think it probably acquired its deadly attributes once it got into the Noxubee chicken operation.

If that’s the case, “my money is on a one-and-done, perhaps with some local spread,” said Richard Webby, an infectious disease expert at St. Jude Children’s Research Hospital in Memphis, Tenn.

Webby said most bird flu outbreaks follow that pattern: A low-pathogenic version is introduced to commercial poultry, and it becomes highly pathogenic once inside.

The introduction of H5N1—the bird flu virus that’s been infecting dairy cows, commercial poultry, pet cats, wild animals and wild birds since March 2024—into poultry and livestock populations was a notable exception to this trend: It was already circulating among wild birds and animals as a highly pathogenic virus.

John Korslund, a veterinarian and former USDA researcher, agreed with Webby and noted that the operation housed breeder broilers: chickens that are grown and maintained for breeding purposes, not for their meat.

This is significant because breeders live for months, if not years.

If a low-pathogenic virus “happens to get into a broiler meat flock, the birds don’t get sick and they go on to slaughter,” he said. But when a breeder flock picks up that virus, “the virus can replicate for weeks … this may well be what happened in Mississippi.”

However, according to USDA rules, routine and periodic testing of breeder birds for low-pathogenic avian influenza is required. In 2017, an outbreak of H7N9 occurred along the Mississippi flyway, probably starting in late February, but reported only in March. A summary report of the outbreak suggested the virus was introduced via wild birds.

As suspected in this case, it is believed it started as “low path” and only became “high path” once it got into commercial operation.

Nevertheless, experts said, if they are wrong and a highly pathogenic virus is circulating in wild birds, it’ll start popping up in other states and sites too.

“Time will tell how nasty it gets this time,” Korslund said.

The key to preventing these kinds of outbreaks—or at least getting ahead of them—is wildlife surveillance, the experts said.

Agencies such as the USDA, the U.S. Fish and Wildlife Service, the U.S. Geological Survey’s Wildlife Health Center, and the National Oceanic and Atmospheric Administration have divisions that are tasked with sampling wild birds and other animals for infectious diseases. The information they gather is then used by agriculture and public health officials to determine where and when to bolster biosecurity, or to keep a lookout.

Without that information, said Angela Rasmussen, a virologist at the University of Saskatchewan’s Vaccine and Infectious Disease Organization in Canada: “We’re flying blind.”

In the positive news that came out this week, a team of international researchers found that ferrets exposed to a common seasonal human flu—H1N1—before being exposed to H5N1, acquire some immunity from the seasonal flu.

Ferrets that weren’t exposed to the seasonal flu before being infected with H5N1 had high levels of the virus in their respiratory tissues, as well as detectable virus in their hearts, spleen, liver and intestines.

In contrast, those that had been exposed to the seasonal flu beforehand had virus only in the respiratory tract—and at pretty low levels.

“The biggest take-home message of our data is that prior human seasonal virus infection can provide some level of protection against the lethality of bird flu,” said Seema Lakdawala, a microbiologist at Emory University in Atlanta and one of the study’s researchers.

Webby, the St. Jude researcher, said the work supports other research that has looked at the potential protectiveness of prior exposure to flu viruses.

“It is for sure playing some role in modulating H5N1 disease in humans,” he said, but was unlikely the only factor. “After all, many people have severe seasonal H1N1 infections each year despite lots of immunity to the virus from previous H1N1 exposures.”

But the finding may help explain why the virus has recently been associated with generally mild disease in people who have been infected. Seventy people in the U.S. have been infected since March 2024, and one person has died. (Four people, including the Louisiana patient who died, have been hospitalized).

Before last year, the virus was thought to have killed roughly 50% of those infected.

Rasmussen said the worry now is that if H5N1 mutates to become transmissible between people, it’ll be young children as well as the old and compromised who are likely to be most affected. Children younger than 5 are less likely to have been exposed to seasonal human influenza viruses than school-aged children and adults—potentially making them more susceptible to the harms of a virus such as H5N1.

In addition, she said, the bird flu viruses circulating in birds and livestock “as far as we know, can’t transmit easily between people. But, if there’s reassortment, then who knows? We don’t know what kind of residual population-level immunity we would have” from a virus such as that.

How seasonal flu vaccines could affect this protection isn’t clear.

“Seasonal vaccines will not provide the same diversity of immune response as natural infection and are unlikely to provide the same level of protection,” said Lakdawala, who is testing this issue in the lab.

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