State wildlife officials tell GBH News that COVID-19 antibodies have been found in 15 percent of white tail deer tested in Massachusetts as part of a USDA-led study.
COVID-19 has been detected in deer in other states, but the new study provides the first confirmation that Massachusetts deer have been infected with the virus. Of the 558 Massachusetts deer tested so far, COVID-19 antibodies were discovered in 86 samples. While deer-to-human transmission has only happened in one documented case, experts’ primary concern is that widespread infection in the deer population offers additional sites for the virus to mutate.
It's extremely unlikely that most people could be infected by a deer. Any deer-to-human transmission would likely happen through the air, which would require close contact like when one human passes the virus to another, says Martin Feehan, the state deer and moose biologist for the Massachusetts Division of Fisheries and Wildlife. Feehan said there's no evidence that COVID-19 can be spread by ingestion of meat.
"It's clear that any kind of transmission that would occur from deer to humans, if that occurs, would be incredibly rare," Feehan said. "For the most part, any time that people are interacting with deer, it's an outdoor setting, which should really minimize a lot of that risk."
Most of the deer tested for COVID-19 were hunted during the first week of shotgun season, which begins the weekend after Thanksgiving. And USDA genetic sequencing confirmed that two samples were the delta variant — while samples tested weeks later from New York were omicron, Feehan said.
But experts say infections in deer do raise the concern about the possible development of new variants in the animals that might potentially then be reintroduced into humans. Some studies of COVID-19 in deer have found large numbers of mutations of the virus.
"Anytime there's a lot of virus floating around in other species, it can change in ways that are very hard to predict," said Andrew Lover, an epidemiologist at UMass Amherst. "And even if it's pretty unlikely for it to spill back into humans again — if or when it does, it could be a very, very different virus."
One Canadian study did find a human case of COVID-19 with mutations that suggested the infection had come from a deer, though NPR reported vaccines still appeared to offer strong protection against that particular variant.
"One of the reasons why it's so important to get control over a virus like this is that the more people or animals that it's infecting, the more opportunities and the faster that it can evolve in, and the more concerned we are about variants," said Elinor Karlsson, the director of vertebrate genomics at the Broad Institute. But so far, she said, the known variants like delta and omicron appear to come from human mutations. "There's no evidence yet that we have mutations that are evolving in other species that could infect humans."
Other species that haven't had the same level of scrutiny may be getting infected as well, she added. Scientists from the Cummings School of Veterinary Medicine at Tufts University have been testing animals from domestic, wildlife and zoo environments for the virus, and say they've seen rare cases as a result of human exposures.
"I think if the deer are telling us anything at this point in time, it's that to understand these kinds of viruses, we need to be testing outside of humans and looking in the animal populations as well, much more so than we are now," Karlsson said.
"For me, possibly one of the most interesting parts of the whole story is, how are the deer getting infected?" she added. "I think understanding that is going to help us really understand a new kind of dimension to the risk story here."