H5N1 bird flu virus has been found in a pig on a farm in Oregon, the first time the virus has been seen in a pig in the United States, the U.S. Department of Agriculture reported Wednesday. A second pig may also have been infected, Oregon authorities later revealed.
The pig confirmed to have been infected was one of five on a farm in Crook County in south-central Oregon that was experiencing an H5N1 outbreak in poultry. The pig did not display signs of illness, but was euthanized and necropsied — the animal equivalent of an autopsy. Tissue testing showed the pig “had virus throughout their body,” said Ryan Scholz, Oregon’s state veterinarian.
Necropsies done on two other pigs showed no evidence of infection, but a swab taken from another pig, one of two “teacup mini” pigs that were co-housed with infected chickens inside a chicken coop, did test positive. Those pigs were also euthanized and results of the testing of their tissues are still pending.
Pigs are sometimes called a “mixing vessel” for flu viruses, because they can be infected with both bird flu viruses and human flu viruses. If the animals are co-infected at the same time with two or more viruses, the viruses can swap genes, potentially creating a hybrid virus that is better able to spread to and among people than bird flu viruses typically are. This phenomenon, called reassortment, is what gave rise to the 2009 H1N1 pandemic.
Because pigs can play this role, flu experts have been worried that the H5N1 virus currently spreading in cows in the United States could make its way to pigs — though any version of the H5N1 virus in pigs would be an unwelcome development.
The USDA said in its statement that it has generated a genetic sequence of virus from poultry on the farm, but it did not indicate what the sequencing revealed. Scholz said Oregon’s Department of Agriculture had been informed that this version of H5N1 was from wild birds. “It is not associated with the dairy strain that we’ve been seeing in other states,” he said.
Several influenza experts STAT spoke to Wednesday were waiting for more detail before drawing conclusions about the significance of this finding.
“If it doesn’t spread from pigs to pigs and it just happened on that one farm, it’s not a big deal. If it starts to spread from pigs to pigs, then it’s much more of a problem,” said Florian Krammer, a flu virologist at Mount Sinai’s Icahn School of Medicine in New York. “If it ends up in large pig populations in the U.S. similar to cows, I think this would be a disaster.”
The USDA said the outbreak was on a backyard farm in Oregon that had poultry and several types of livestock — pigs, sheep, and goats. The farm is not a commercial operation. The entire farm is under a quarantine order and the other animals remain under observation.
Given the risk pigs pose for reassortment of flu viruses, it would be common practice for agricultural authorities investigating an outbreak of bird flu in poultry flocks to test pigs if there were any nearby. The USDA statement said the poultry and the livestock on the affected farm “shared water sources, housing, and equipment.”
Pigs are highly susceptible to flu viruses — human and avian. In fact, over decades, human seasonal flu viruses have found their way into pig populations, where they circulate and reassort. In pigs, the viruses evolve at a slower rate than they do in people, increasing the genetic distance between the viruses that circulate among people and their ancestors circulating among pigs.
From time to time, those viruses spill back into humans, typically infecting farmworkers or people attending agricultural fairs or livestock auctions. Over 500 such infections have been recorded in the United States over the past 15 years, according to the Centers for Disease Control and Prevention. Fortunately, to date these infections have been one-offs — a person becomes infected with a swine influenza virus, but the virus doesn’t pass to anyone else.
But the risk remains.
“Pigs are absolutely full of influenza viruses that in the past were human seasonal influenza viruses or human pandemic influenza viruses. So they have a really, really rich genetic material in them,” said Thomas Peacock, a flu virologist at Britain’s Pirbright Institute, which focuses on controlling viral illnesses in animals.
The fact that many of the viruses in pigs previously circulated in humans indicates they have the genetic know-how to do what H5N1 has not been able to do on its own, but could potentially acquire the capacity to do, if it underwent the right gene swap.
“This is the whole idea that pigs are a mixing vessel in that they are a place where avian and human influenza viruses could potentially mix,” Peacock said.
Studies done over the years have suggested that H5N1 viruses can infect pigs, but do not spread well among them. That said, research published earlier this year by USDA scientists showed that the more recent versions of H5N1 circulating in the United States were better able to infect pigs than earlier versions, said Richard Webby, a flu virologist and director of the World Health Organization’s Collaborating Center for Studies on the Ecology of Influenza in Animals, located at St. Jude Children’s Research Hospital in Memphis, Tenn.
Webby said that were H5N1 to start circulating in pigs, that would be bad news. “To me, that [would] ratchet up the risk substantially from where we are,” he said.
This story has been updated.
