White-tailed deer are getting coronavirus infections: How big of a problem is that?

Since making the leap to people, the SARS-CoV-2 virus that causes COVID-19 has wrought havoc on human populations. Now the virus appears to be surging among some wild animals as well. A new US government study of white-tailed deer reported that many had been infected, raising the potential that even if the virus is eventually controlled or even eradicated in humans, another common animal could provide it a reservoir and spawn future outbreaks.

The US Department of Agriculture’s Animal and Plant Health Inspection Service tested 481 samples collected between January 2020 and March 2021 from deer in Illinois, New York, Michigan, and Pennsylvania and found that 33 percent tested positive for antibodies to the COVID-19 virus. While many animals appeared to have been infected—more than 60 percent of samples in Michigan tested positive, for example—the agency said there were no reports of deer appearing clinically ill.

Researchers want to know how the deer got infected in the first place. Linda Saif, a virologist at The Ohio State University told Nature that a critical question is “how the virus spread to deer and if it will spread from infected deer to other wildlife or to domestic livestock such as cattle.” The animal and plant health service said the deer could have been exposed to the virus by people, other deer, other animals, or the environment.

Laboratory experiments have shown that deer—along with several other animals, including cats, nonhuman primates and deer mice—are susceptible to SARS-CoV-2 infection. For the most part infected animals don’t get very sick.

Researchers want to know which species are susceptible to the virus in part to better understand the potential for “spillback”—or the reverse of the natural spillover from animals to people that many scientists believe caused the pandemic. With spillback, infected people could spread the disease to other species and create a new reservoir for the virus. Transmission among animals could also lead to new variants. In experiments, researchers have shown that the virus begins to mutate quickly after spreading among just a handful of animals.

While it’s not known whether deer can spread the virus to people—the government researchers believe the risk is low—there have already been documented cases of farmed minks spreading the disease to workers.  Hundreds of people in the Netherlands were infected with mink-related variant viruses last year.

White-tailed deer are common in North America.