The US government said on Friday it had confirmed the world’s first cases of Covid-19 in deer, expanding the list of animals known to have tested positive for the disease.

The US Department of Agriculture reported infections of SARS-CoV-2, the virus that causes Covid-19, in wild white-tailed deer in the state of Ohio, according to a statement. There were no reports of deer showing symptoms of infection, the USDA said.

“We do not know how the deer were exposed to SARS-CoV-2,” USDA spokeswoman Lyndsay Cole wrote in an e-mail to Reuters. “It’s possible they were exposed through people, the environment, other deer, or another animal species.”

The USDA has previously reported Covid-19 in animals including dogs, cats, tigers, lions, snow leopards, otters, gorillas and minks.

Worldwide, most animal infections were reported in species that had close contact with a person with Covid-19, according to the agency.

The USDA reported last month that white-tailed deer populations in Illinois, Michigan, New York, and Pennsylvania were exposed to SARS-CoV-2, based on a study that analyzed serum samples from free-ranging deer for antibodies to the disease.

The Ohio State University College of Veterinary Medicine collected samples from the infected deer in Ohio from January to March as part of ongoing studies, the USDA said. The samples were presumed to be positive for Covid-19 in university tests, and the cases were confirmed at USDA’s National Veterinary Services Laboratories, according to the statement.