Pigs and cows typically only get tested when they get sick; it’s usually up to their veterinarian whether they order tests, and which ones they do. “In contrast to the poultry and the egg industry, where no influenza virus is tolerated, influenza A viruses are very much tolerated in all the U.S. pig facilities,” Gregory Gray, a professor of infectious diseases at the University of Texas Medical Branch and a public health physician who studies zoonotic respiratory viruses, told me. “There’s no systematic sampling of every pig that goes to the market for influenza viruses, and no sampling on the farms for influenza viruses unless there’s illness.”
That’s dangerous, because pigs are known as “mixing bowls” for influenza viruses; they have receptors similar to humans’ and can mix and match different variants that could be well-suited for spread among people. The more densely packed the pig farms are, “the greater the variety of influenza A virus variants,” Gray said. Cows may also have a similar blend of receptors, according to new research that has not yet been peer-reviewed or published.
While avian influenza hits poultry hard, it’s possible that pigs and cows, which have less deadly courses of illness, are spreading the virus with few or no symptoms. “Unfortunately, what always happens with surveillance efforts is that they’re expensive—resources are limited,” Leibler said. Wastewater monitoring, including of the manure and litter stream from commercial food facilities, could help. But that would require some participation from the farms, which have been reluctant to test at all.
