(NEXSTAR) – Researchers found coronavirus antibodies in more than a third of the white-tailed deer studied across four U.S. states since January, 2020.

Michigan led the other states – Illinois, New York and Pennsylvania – with a 60-percent positive rate among the samples collected by the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS).

While antibodies where found in the blood samples, it doesn’t necessarily mean that the animals were sick with the disease, only that they had been exposed to the virus via other animals or humans. The research team cited coronavirus outbreaks in farmed mink, contact with humans and contaminated water sources as possible sources of exposure.

The percentage of positive cases from January, 2020 through 2021 is 33%, but that number jumped up to 40% in 2021 samples. The APHIS findings have not yet been formally peer-reviewed.

While none of the deer studied showed signs of critical illness, the findings have researchers worried about the possibility of creating a virus reservoir among animal populations that could continue to infect humans in the future.

“Besides health impacts to wildlife, persistent infections in a novel host could lead to adaptation, strain evolution, and re-emergence of strains with altered transmissibility, pathogenicity, and vaccine escape,” the paper states.

Federal officials, who have said the delta variant is as contagious as chickenpox, and more contagious than the common cold, flu, smallpox and Ebola, said in internal documents obtained by the Washington Post that “the war has changed.”

USDA officials say APHIS is now working closely with the Department of the Interior, the CDC and the Association of Fish and Wildlife Agencies to determine the next steps in the battle against the spread of the delta variant.

There have been 614,249 deaths resulting from 35,226,277 COVID-19 cases in the United States since the start of the pandemic, according to Johns Hopkins University data.