Humans are the main source of COVID-19, but what happens now that we've spread it to animals?

If there's one thing social media loves, it's a Disney-like moment with a wild animal. There's the much-loved video with the golden retriever who's best friends with a deer, or the one where a woman hand-feeds deer on her porch. But those seemingly innocent interactions between humans and wildlife could be costly during a pandemic.

Humans are currently the main source of transmission for the virus that causes COVID-19, SARS-CoV-2. Globally, the virus has been found in 19 different species, mostly animals that are domestic or living in captivity.

What concerns scientists is the potential for another species to become a reservoir. If SARS-CoV-2 circulates widely enough in another species, there's a risk that animal could become a potential source for new variants and transmission back to humans.

While so far there's no evidence of that happening in the wild, the cycle of transmission seen on mink farms in Denmark, where mink infected by workers spread a new variant back to workers, is exactly the type of situation infectious disease experts want to avoid.

So far, the only known cases of SARS-CoV-2 in the wild were found in mink and deer. Most of the mink that have been infected were on fur farms, but a wild mink did test positive in Utah, in an area where there were outbreaks on local farms.

One big ecosystem

Until recently, most conversations about animals and COVID-19 had to do with its origins, said Scott Weese, an infectious disease veterinarian and a professor at the University of Guelph's Ontario Veterinary College.

He's one of many experts saying that kind of thinking to change to more of a holistic approach, one that considers how all species are interlinked. 

"We had to really kind of drag people kicking and screaming to pay attention to animals," Weese said. 

"We don't have people and everything else. We're all in this big ecosystem." 

People should be aware of the risk of animal transmission, but not worried, he said. At this stage of the pandemic, the average person poses a greater risk to other animals than animals pose to people, and people are far more likely to be exposed to the virus from another person than an animal. 

But the last thing we want to do is gain control of the virus in humans only to discover another species has become a reservoir, he said.

"The risk of variants emerging in the human population will get lower as we vaccinate everyone, if we vaccinate the world. So if we do that, the biggest pool of susceptible individuals and the biggest pool for circulating virus may be wildlife," he said. 

Need for long-term wildlife surveillance 

At Toronto's Sunnybrook Health Science Centre, infectious disease physician and virologist Dr. Samira Mubareka has been working with her team to analyze samples taken from white-tailed deer killed by hunters.

Together with the Canadian Food Inspection Agency's National Centre for Foreign Animal Disease, they analyzed the entire genome sequence of the virus found in Quebec deer and identified the Delta variant.

"Delta started surging over the course of 2021, especially in the fall. So most likely, that's when those deer got infected, when it happened to also be circulating in humans," Mubareka said.

So far the virus has been found in wild white-tailed deer in the northeastern United States, as well as in Quebec, Ontario and Saskatchewan. It's not clear exactly how humans are transmitting the virus to deer, but it could be through contaminated drinking water, direct contact, food or farming. 

Despite humans introducing the virus to deer, there's no evidence of it spilling back into humans. Infected deer haven't displayed symptoms, and the virus doesn't appear to be significantly mutating as it spreads within their population.

But that could change.

It's why Mubareka and her colleagues say it's important to keep up surveillance long-term, to find out what happens next. 

"What we don't know is, what is [the virus] doing now?" Mubareka said.

"For example, if Omicron spilled over into deer, would there be a difference?"

Hunters advised to mask up

Since humans are still the main source of SARS-CoV-2, the best way to prevent animals from getting it is for humans to take precautions.

The Public Health Agency of Canada is advising to avoid close contact with pets and animals as much as possible if you have COVID-19. That includes feeding wildlife.

There are no reports of people catching COVID-19 from preparing or eating meat, but hunters should be cautious while handling carcasses. Public Health also recommends hunters wear a mask when exposed to any respiratory tissues and fluids, and to wear gloves and eye protection while handling and dressing a carcass. 

'The more we can stay away, the better'

Weese, who has been testing the pets of Canadians who have COVID-19, said that so far the virus doesn't seem to have much of an effect on dogs, and domestic cats usually only show minor symptoms.

His advice is to, "treat your animals like you would anyone else. If you're staying away from people, limit your contact with animals."

"If your cat is your support mechanism, know the risk to the cat is fairly low, so you don't lock the cat in the bathroom because you've got COVID. But the more we can stay away, the better," he said.

There have also been reports that other pets, such as ferrets, are susceptible to the virus and show symptoms. 

Rodents are another animal of concern. A pre-print case study based on an outbreak linked to a Hong Kong pet shop suggests there may be evidence of transmission from hamsters to humans. That paper has not yet been peer-reviewed and the latest information available on the Public Health Agency of Canada's website lists hamsters only as a source of transmission to other hamsters.

Part of our lives

Zoos have also taken extra precautions to make sure their animals stay safe. 

While American zoos have experienced outbreaks, no animals at Canadian accredited zoos have tested positive so far. That's partly thank to the safety precautions in place. 

Take the Toronto Zoo, for instance. It had been closed to the public since Jan. 5, but staff were still wearing protective gear when they were around gorillas and other susceptible animals, like big cats. Visitors, when they can return, are asked to wear masks.

Gabriela Mastromonaco, the director of conservation science at the Toronto Zoo, said the pandemic has really highlighted how humans are inextricably linked with the other species around us.

"They're part of our lives every day whether we know it or not. It really is up to us to protect them or not because whatever happens to them is going to come back to us. It is a circle."

She said people tend to forget all the ways our lives interact with other animals through agriculture, food, pets and companion animals, zoo animals and wildlife. 

"COVID heightened our sense of relationships, whether it's with each other or with the animal world."