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State College -- Scientists from several institutions including the USDA, Cornell, and Penn State are currently struggling to figure out how exactly deer keep being exposed to SARS-CoV-2, among other related questions.

The issue has been of great concern to both ecologists and epidemiologists, who worry that the virus will mutate and circle back to humans or infect other animals. The matter has even caught the eye of the U.S. government: following initial studies, the American Rescue Plan Act allocated $6 million specifically towards researching the coronavirus in white-tailed deer.

How are deer getting the virus?

The short answer: We don't know.

Have you been so close to a live white-tailed deer that you could breathe on it lately? Neither have we. With the exception of frequently-handled deer on farms, the obvious explanation of SARS-CoV-2 spreading from humans to deer through aerosols seems unlikely. Unlikely isn't the same as impossible, though, so this possibility remains on the table.

A Penn State study of deer in Iowa from April to December of 2020 found an increase in exposed deer during the peak of hunting season, but this doesn't necessarily mean that hunters are breathing COVID at deer.

The study only analyzed 283 deer, which were part of the state's Chronic Wasting Disease surveillance program. With the limited sample size and the awareness that many CWD tests are conducted on donated deer parts from hunters (meaning that more specimens in general will pop up during hunting season), it's not easy to pin this on hunters with bad breath.

The "breathing on deer" guess has garnered widespread ridicule. A PBS Wisconsin interview with Professor Tony Goldberg, a professor of epidemiology for the University of Wisconsin School of Veterinary Medicine, presented a more likely scenario: let's say I go to a barbecue at my sister's house with a can of chewing tobacco, or hypothetical-me has a habit of spitting. If I have SARS-CoV-2 and I spit in the potted plant on her front step, the deer that she keeps catching on her security camera will end up eating my germy saliva right along with the plant. If chewing tobacco is involved, the deer may even specifically be attracted to it.

An equally plausible explanation is deer drinking contaminated water or eating contaminated garbage. One of the ways that the spread of COVID is being monitored is through sewage samples; to put it bluntly, the virus continues to survive in people's poop for a while.

Deer could very well be sipping virus-laden wastewater or eating trashed corn cobs that an infected person nibbled. Whether the virus can spread from person to person through fecal matter is currently unknown - it's hard to find willing test subjects for that kind of thing.

A third theory suggests that people have spread SARS-CoV-2 to a different animal that then gave it to deer. Pets and farm or zoo animals getting the coronavirus is relatively common -- someone's cat getting COVID and sneezing at a deer isn't the most outrageous idea.

What we do know based on the research from Penn State and the USDA is that human-to-deer SARS-CoV-2 viral transmission has taken place several times: every time a new COVID variant spreads through people in the U.S., somehow the deer end up with the very same one.

Are the deer getting sick?

Nope. Deer that have been exposed to SARS-CoV-2, whether they have antibodies or active viruses in their system, have shown no sign of illness so far.

Should we be worried about hunting/venison?

Not really. One should exercise caution with diseases when harvesting, of course, but there is no evidence that eating or preparing deer meet can transmit COVID to humans at this time. Although, animal-to-human transmission is not unheard of: in late 2020, mink on a farm caught the virus from humans and then transmitted it back to people, leading to the culling of tens of millions of the weaselly critters.

Just keep your tools clean, wash your hands, and cook your venison to a safe temperature.

If the deer aren't getting sick or making people sick, why is this a concern?

Imagine that the COVID-19 pandemic is over and I can finally stop writing articles about it. We've managed to eradicate it, become immune to all known varieties, vaccinated it out, or it mutated into such a weak form that it's only a mild inconvenience. The only Delta Variants we have to concern ourselves with are faucet design choices and all is right with the world.

Then, a white-tailed deer on a farm in Minnesota nudges a wild deer through the fence with its nose. A week or so later, the deer then nudges a farmer with its nose, exhaling a form of SARS-CoV-2 that's mutated so much that the farmer's immune system doesn't know how to fight it.

Suddenly we're back to square one as the deer farmer spreads the disease to everyone they know and so-on.

Unlike the mink farm COVID cases, the 2009 swine flu, and the 2014 bird flu, diseases transmissible to humans from animals are much harder to control when spread to wild populations. Animals that are contained within farms can be culled to control diseases -- a disaster for businesses, but not a terrible ecological tragedy.

There could be a terrible ecological tragedy, though.

Humans aren't the only organisms we need to consider. Mink aren't completely confined to farms. Unlike deer, mink do experience severe illness when exposed to the virus. If deer spread the virus to susceptible animals like mink, it could devastate wild animal populations.

The above scenarios are just a few worst-case possibilities. If we're lucky, the virus will just sit around in the deer population as-is, doing basically nothing. At this time, USDA data suggests that the chances of wild animals spreading COVID to people is low.

What's next?

In addition to academic institutions, the Pennsylvania Game Commission is performing testing on deer specimens donated by hunters and road-killed deer in an attempt to assess how widespread SARS-CoV-2 is in Pennsylvania deer. Less extensive testing will be performed on elk.

There's not much that the public can do about this one. Keep using common sense while interacting with wild animals and hope for the best.

The USDA continues to track confirmed SARS-CoV-2 infections in animals within the United States. A regularly-updated page with public records of animal infections is available here.

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