ROCHESTER, Minn. A new study of the Iowa white tail deer population reveals that humans gave the animals a dozen variations of the coronavirus in 2020 and 2021, causing asymptomatic infections that the animals then spread rapidly amongst themselves.

While previous research had reported 40% of deer studied had been exposed to the coronavirus, the new findings are the first to document human-to-animal infection in the wild.

Deer-to-human transmission has not been established.

"I think what's the highlight of this research is that this is a free-ranging animal we don't have reason to suspect has any intimate human contact," said Rachel Ruden, a coauthor of the paper and member of the Iowa State College of Veterinary Medicine. "But they're still getting this infection."

Though the coronavirus is predominantly envisioned as airborne, Ruden says deer may have picked up the virus through chance interactions with human fluids, natural behaviors for the animals that then allow rapid transmission within the herd.

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"Right now that's really the million dollar question how are they getting exposed," she said.

"Obviously in a city we expect they are cohabitating with people," Ruden added. "There might not be a reason to expect a really close association with people, but they are sharing a similar environment. On public lands we know 2020 was a year of outdoor recreation."

Ruden says the authors are less mystified by the deer-to-deer transmission.

"Deer aren't following public health guidelines," she said. "If they encounter urine or feces from another deer they might investigate it ... their normal behavior is to interact with these fluids."

COVID-19 spread rapidly among white tail deer last fall

Rachel Ruden.
via Iowa Department of Natural Resources
Rachel Ruden. via Iowa Department of Natural Resources

Working from leftover tissue within standard CWD surveillance collections, the researchers examined 283 lymph glands taken over a 12-month period ending in April of this year.

"We were able to use this nice sample set and look for a virus that would have been sampled by the lymph node that's draining the back of the nasal cavity," Ruden said.

The animals were harvested during hunting season, as well as found as carcasses on roadsides, which is an ideal randomization process, according to Ruden.

They included 151 free-living and 132 captive deer, animals from public lands, game preserves and in urban environments. The tissues were checked for infection, with positive cases then evaluated for genetic identification.

The researchers reported 94, or one-third of the samples, had infections. The first positive samples showed up in late September as 2 of 39 samples tested positive, in animals located 300 miles apart.

That number grew to 4 of 70 samples collected in October, 22 of 77 samples collected in November, and 61 of 75 collected in December. From late November 2020 until early January 2021, 80 of 97 samples tested positive, for an 82% infection rate.

The animals carried 12 lineages of the virus, the authors said, generally matching with strains dominant in the nearby human population, and two versions making up 75% of all lineages.


"Right now that's really the million dollar question — how are they getting exposed."

— Rachel Ruden, Iowa State College of Veterinary Medicine


"The geographic distribution and nesting of clusters of deer and human lineages strongly suggest multiple zooanthroponotic spillover events," the authors wrote, "and deer-to-deer transmission."

The spread of COVID-19 within deer has relevance given the inability to vaccinate the wildlife population and the consequences for further SARS-CoV-2 mutation should deer become a so-called "reservoir" for the virus.

The researchers believe their results underscored the need for a global interspecies approach to disease control known as One Health.

"It's important for us to document these kinds of things and show another way the pandemic has affected our shared experience," Ruden said.

The Centers for Disease Control and Prevention has offered advice that hunters use gloves and masks to handle deer carcasses. Cooking venison to 165 degrees is believed to kill any virus. Being vaccinated against COVID-19 also protects hunters against contracting the illness, according to guidelines from the Association of Fish and Wildlife Agencies.

The authors, a team from Penn State, Iowa State, Cambridge University, The University of Chicago, Houston Methodist and the Iowa DNR, released their findings on Tuesday, Nov. 2, in preprint form, which means it has not been peer reviewed.