Bird flu was detected in a pig, and it could be a tipping point threatening human transmission

• A pig has tested positive for H5N1 bird flu in a backyard farm in Oregon.
• Pigs can host both bird and human flu viruses, which can make them a dangerous "mixing bowl."
• Flu season can also heighten the risk of mutations and eventual human transmission of H5N1.

A bird flu virus has jumped one species closer to a human outbreak.

The H5N1 bird flu was detected in a pig in Oregon, the first instance of a swine infection in the US, officials announced on Wednesday.

Pigs get both bird flu viruses and human flu viruses, making them a genetic mixing bowl where H5N1 could gain genetic mutations that help it spread between humans.

That's not inevitable, but "I would say the concern elevated slightly with this pig infection," Stacey Schultz-Cherry, a virus expert at St. Jude Children's Research Hospital and the deputy director of a World Health Organization center on animal and bird influenza, told Business Insider.

Experts' fear is that the situation could escalate.

"If we see pig-to-pig transmission in a sustained way this will be very concerning," Wendy Barclay, a virologist at Imperial College London, told BI in an email.

The 'animal pandemic' that could jump to humans

This strain of bird flu has killed tens of millions of birds worldwide since 2020 and more than 40,000 sea lions and seals in South America. World Health Organization chief scientist Jeremy Farrar has called this an "animal pandemic."

Earlier this year, it made an unexpected jump into US cattle herds.

Since then, H5N1 has even infected a few humans who work closely with livestock in the US. But there has been no sign of sustained human-to-human transmission. Pigs could change that.

Meet the first infected pig

The US Department of Agriculture reported that the infected pig was living in a "backyard farming operation" which is now quarantined.

Two other pigs on the farm have tested negative for H5N1, while results are still pending for two others. Poultry there have also tested positive.

"There is no concern about the safety of the nation's pork supply as a result of this finding," the USDA reported, saying that the pigs at this farm were not intended for commercial food supplies.

Because of the backyard setting, this case doesn't pose the same major outbreak risk as an infection in a commercial pig farm.

"If it starts to spread from pigs to pigs, then it's much more of a problem," Florian Krammer, a flu virologist at Mount Sinai's Icahn School of Medicine in New York, told STAT News. "If it ends up in large pig populations in the US similar to cows, I think this would be a disaster."

Why pig infection could be a tipping point

Pigs play host to both bird flu viruses and human flu viruses. That makes them a dangerous playground for the two to swap genes.

Inside a pig, the H5N1 bird flu virus could pick up genetic mutations that help it adapt to better infect human bodies.

The more pigs are infected, the greater this risk is. However, Schultz-Cherry said, "we would not need to see pig-to-pig transmission to worry about this mixing vessel or this mixing bowl effect."

There's no other animal "at the top of the list" with pigs, Richard Webby, a virologist at St. Jude and director of the WHO animal and bird flu center, told BI in June.

That could just be due to gaps in our understanding of animal influenza. After all, he added, "a few months ago, I would have told you that cows don't get flu."

Bad timing: Flu season is in full swing

The timing of this development is "not ideal," Schultz-Cherry said, because it's the middle of flu season, and pigs can get the strains of flu that circulate among humans.

If this season's flu strains were to overlap with a pig H5N1 infection, that would increase the risk of gene-swapping that helps the bird flu adapt to humans.

"Really it would just take one pig," Schultz-Cherry said.

Getting your seasonal flu vaccine can help, though, especially if you're in contact with cows, pigs, or poultry. Schultz-Cherry said that's because vaccination reduces the amount of flu circulating through the population.

Genomic detective work

Schultz-Cherry was surprised by the pig news but, she said, "I guess I shouldn't be."

The virus is circulating in cattle, chickens, wild birds, and cats all over the country. Mice have tested positive.

Just sharing a drinking water source with these animals could put a pig at risk. The USDA reported that livestock and poultry on the Oregon farm shared water sources, shelter, and other equipment.

Now that officials have samples of the virus that infected the pig, they can sequence its genome. That could hint at which animal it first came from.

Scientists will likely look for mutations in the virus that could help it infect humans more easily, such as a tweak that makes it attach to more mammal-type receptors in a host animal's cells.

"It was one pig," Schultz-Cherry said. "But it is something that will lead to increased monitoring of the situation."