Various studies have found that deer in several states have tested positive for COVID-19. It's unknown where the virus infected the deer, but there's no proof that animals can infect humans.

Health officials, however, are already concerned.

In Wisconsin, health officials have added a few more points to their fall hunting recommendations, including wearing a mask when field dressing deer.

On Monday, the state Department of Health Services updated its deer hunting guidelines.

"Hunters are always encouraged to use good hygiene practices when processing animals to reduce their risk of exposure to many possible disease agents," the agency says on its website. "Incorporating a few additional measures can also help to reduce their risk of possible exposure to the SARS-CoV-2 virus."

Between January 2020 and March 2021, the Animal and Plant Health Inspection Service of the U.S. Department of Agriculture collected 481 samples from deer in Illinois, Michigan, New York, and Pennsylvania. Antibodies to SARS-CoV-2 were identified in 33% of the samples.

Similarly, a study conducted by Pennsylvania State University discovered that over 80% of white-tailed deer samples collected in various locations of Iowa between December 2020 and January 2021 tested positive for SARS-CoV-2.

Suresh Kuchipudi, clinical professor of veterinary and biomedical sciences and associate director of Penn State's Animal Diagnostic Laboratory, said this was the first direct proof of COVID-19 in any free-living species.

COVID-19 spread throughout the human population in 2020 and 2021, prompting some researchers to look for the disease in other species. SARS-CoV-2 was found in mink, otters, tigers, lions, and nonhuman primates, mostly in captivity, as well as white-tailed deer in both captivity and the wild.

The findings have consequences for COVID-19's ecology and long-term persistence, according to Kuchipudi, the university's chair in emerging infectious diseases.

"These include spillover to other free-living or captive animals and potential spillback to human hosts," Kuchipudi said.

When handling COVID-19-infected deer, people should follow the same precautions recommended for preventing human-to-human transmission, such as hand washing, gloves, and masks. According to the world's leading health agencies, getting a COVID-19 vaccine can considerably lower the chance of contracting the virus.

For Massachusetts hunters, the Massachusetts Division of Fisheries and Wildlife has similar advice. Hunters should never handle or eat wild animals that appear sick or are found dead, and should always use gloves and a face shield when handling game. They should also avoid contact with the brain or spinal tissue.