Update (January 26, 2022): This study has now been published in PNAS.

A study posted by bioRxiv on November 1 finds SARS-CoV-2 in wild and captive white-tailed deer populations in Iowa, with indications that the virus is traveling through both human-to-deer spillover and deer-to-deer transmission. In the study, the authors warn that their observations reveal an “urgent need” for a better understanding about the “ecology and evolution of SARS-CoV-2,” especially regarding the potential for populations of free-roaming animals like deer to become virus reservoirs and eventual sources of spillover back into humans.

The study, which has not yet been peer reviewed, “is the first to provide evidence of widespread dissemination of SARS-CoV-2 into any free-living species, in this case white-tailed deer,” according to the paper, and the authors write that the transmission of the coronavirus from humans to animals poses a challenge for controlling SARS-CoV-2 and can have consequences for the pandemic’s trajectory.

The research team worked with the Iowa Department of Natural Resources, which performs disease surveillance in white-tailed deer. In tests of lymph node samples collected between April 2020 and January 2021, they found that the first SARS-CoV-2–positive samples were collected from far-flung parts of Iowa in September 2020, reports The New York Times. The scientists found that 33 percent of 283 samples collected from both free-living and captive deer in Iowa from April 2020 through December 2020 tested positive for SARS-CoV-2. Notably, 80 of 97, or 82.5 percent, of samples collected between November 23, 2020 and January 10, 2021 were positive, indicating a rapid spread of coronavirus within deer populations as the pandemic progressed, the paper concludes.

The researchers characterized the SARS-CoV-2 genomes from the positive samples and found lineages that reflected those in infected humans in Iowa. The scientists also observed that geographic clusters of the positive cases in deer and humans paralleled each other, suggesting that the deer contracted the virus from humans and then transmitted it through their own populations. Federal scientists from the National Veterinary Services Laboratories verified the study’s findings on November 2, according to the Times.

Suresh Kuchipudi, a Pennsylvania State University veterinary microbiologist and coauthor of the study, tells the Times that the coronavirus infection “was effectively showing up in all parts of the state,” and that the research team was “dumbfounded.” Vivek Kapur, another coauthor also at Penn State, tells the newspaper that “There is no reason to believe that the same thing isn’t happening in other states where deer are present.”

With a population of about 25 million, white-tailed deer is the most abundant wild deer species in the United States, with deer hunting the most popular type of hunting in the United States, according to the paper. According to the Centers for Disease Control and Prevention, there has so far been no evidence of virus transmission from deer or other wildlife back to humans in the US, but the agency nonetheless recommends protective measures such as wearing protective rubber gloves when handling game, avoiding contact between domesticated and wild animals, disinfecting hunting equipment thoroughly, and cooking all game meat to an internal temperature of 165° F or higher.

Tony Goldberg, a veterinarian at the University of Wisconsin-Madison, calls the potential for deer-to-human virus transmission a “game changer” in comments to the Times. “To have a wildlife species become a reservoir after transmission from humans is very rare and unlucky, as if we needed more bad luck.”

The paper’s authors emphasize that continued surveillance of the virus in deer and other susceptible mammals is necessary. They write in their paper that their discovery “has important implications for the ecology and long-term persistence” of SARS-CoV-2.