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How Did Deer Get COVID-19—And What Could It Mean For Humans?

Throughout the pandemic, there've been whispers of the coronavirus infecting and spreading amongst animals: People have passed COVID-19 to their pet cats, and zoo animals have contracted the virus from their infected handlers, according to the Centers for Disease Control and Prevention (CDC). Now, it appears that virus has expanded its reach to the deer population.

And it's not just a few deer that have been infected: New preliminary research shows "strong evidence" of COVID-19 infection among free-living white-tailed deer in Staten Island, New York—nearly 15% of the deer sampled had detectable SARS-CoV-2-specific antibodies, suggesting they'd been previously exposed to the virus. Some of those deer even had the Omicron variant of the virus. Other research—from Texas A&M and Penn State Universities—also found high positivity rates of COVID-19 in white-tailed deer in both Texas and Iowa, respectively.

This research suggests that deer may be an animal reservoir for the SARS-CoV-2 virus, according to Penn State researchers—that they're an outlet for the virus to live, grow, and multiply. This could not only pose a threat for other species, but it could have very real consequences for people as well—now and in the future.

How did deer get COVID-19?

Early on in the pandemic, scientists sought to discover which mammals—other than humans—might be able to contract COVID-19, based on the presence of similar ACE2 receptors that acts as an entry point for the coronavirus. Research published in the Proceedings of the National Academy of Sciences identified 28 species with ACE2 proteins that have a "high propensity" for binding with the virus that causes COVID-19—one of those species was deer (aka cervids).

On top of their ability to contract the virus, deer are also social animals—they congregate and rub noses; and they can also seemingly sneeze and cough. Paired together, deer are able to pass COVID-19 to each other. What's not so obvious is how the virus reached a free-living species like deer in the first place, but scientists have a few theories.

Oladele Ogunseitan, PhD, MPH, a professor of population health and disease prevention at the University of California, Irvine, and UC Presidential Chair, suspects the most likely route of transmission was through wastewater. A lot of the coronavirus is shed out in feces—it goes into the sewer and eventually that sewage can make its way into natural waterways where deer drink, he says.

The second theory is that deer contracted the virus by directly interacting with humans. "Deer are pretty common in urban spaces," Ogunseitan, who focuses on zoonotic disease control, tells Health. They rummage through our trash, eat our garbage, and wander into our lawns. A lot of people want to feed deer — they leave food outside for them or pet them. "Those are all opportunities for human-to-animal transmission and vice-versa," Ogunseitan said.

Finally, we know the virus can cross over into a wide range of species, so there may very well be intermediate hosts like mice or rats. "There could be other animals involved that are more habituated to humans and then are interacting with deer," Christine Johnson, VMD, MVPM, PhD, a professor of epidemiology and ecosystem health at University of California Davis, tells Health.

What does this mean for humans?

Though there are no documented reports of deer passing COVID to a human—mainly because that would be very hard to determine since COVID is so widespread amongst people—it's not out of the question. Research points out that the presence of COVID-19 in deer alone suggests human-to-deer transmission.

"The fact that we found several different SARS-CoV-2 lineages circulating within geographically confined herds…suggests the occurrence of multiple independent spillover events from humans to deer, followed by local deer-to-deer transmission," Vivek Kapur, BVSc, PhD, a professor of microbiology and infectious diseases at Penn State said in a press release. "This also raises the possibility of the spillback from deer back to humans, especially in exurban areas with high deer densities."

The concern isn't only that deer could trigger future outbreaks, even if and when we finally get COVID under control—but that new variants could also emerge in the deer population and spill back over into the human population, says Ogunseitan. The simple fact that deer carry COVID-19 means there are more opportunities for the virus evolve and adapt further. We know this because the same holds true for humans: As long as people are contracting and transmitting SARS-CoV-2, there will continue to be ample opportunities for it to evolve and mutate into new variants.

There's also the potential that SARS-CoV-2 could potentially recombine with another type of coronavirus—one that isn't zoonotic and therefore doesn't affect humans–and turn into one that is able to jump back into humans, says Johnson. There are hundreds of coronaviruses in general, the National Institute of Allergy and Infectious Diseases (NIAID) says, most of which circulate among animals like pigs, bats, cats, and rats. But the seven coronaviruses that currently affect humans began as a spillover event, in which the virus jumped over from animals to humans.

Animal reservoirs, too, ultimately make it much harder to completely eliminate a disease, says Ogunseitan. It's hard enough trying to control a virus in humans, but controlling disease in animals—especially free-roaming animals—is even trickier. "There are major future implications, over the long-term, for disease control in animals and in people," says Johnson.

Can humans do anything to help stop the spread in deer?

Johnson says it's crucial to minimize human contact with wild deer. Keep a distance from wild animals and don't feed them or leave food outside for them.

Part of this effort, she says, is protecting the natural, wild habitat that deer live in. Though it might be tempting to set up a feeding station for deer in your yard or nearby woods, doing so can create a space where infectious diseases can zip through local deer populations. "Any unnatural, high-density congregations is risky," Johnson said.

It's not a surprise to epidemiologists that COVID is circulating in the deer population. What is unusual, however, is how extreme the burden has become in humans. The bigger dilemma is how to control COVID in our population first—to minimize the spread in deer, we must first minimize the spread among ourselves. "The more widespread it is in the human population, the more widespread it becomes in susceptible animal populations," says Johnson. "And then disease control in that setting is even harder."

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.

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