It started in nature. A coronavirus that originated in bats has wound up in humans, causing the Covid-19 pandemic.
And it can go back to nature.
The SARS-CoV-2 virus can jump again, from humans, back into animals, back into wildlife, where it can wait, mutate, and change. Perhaps, years from now, it can infect people again.
“If we’re careful — and we’re lucky — there won’t be a wildlife population that becomes infected and becomes an established reservoir that can also infect people,” Sarah Olson, associate director of the health program at the Wildlife Conservation Society, says. “If it does, then we’ve got a long-term issue here, where this virus has the potential to be with us for millennia. And millennia is a long time. The risk may be small, but the consequences are huge.”
Our luck may soon be tested. On December 13, the US Department of Agriculture reported that a wild mink in Utah tested positive for the coronavirus.
“To our knowledge, this is the first free-ranging, native wild animal confirmed with SARS-CoV-2,” the National Veterinary Services Laboratories reported. A genetic analysis of the virus suggested the wild mink picked it up from a nearby mink farm, perhaps via wastewater runoff from the farm.
No other species surrounding the farm were found to be infected, though, and there’s no evidence that Covid-19 is spreading between wild mink. One possibility is the wild mink could have just picked it up from the farm, and has not spread it since.
Another possibility: We haven’t yet detected a bigger outbreak. “This is potentially going to be a more widespread problem in wild mink,” says Stephanie Seifert, a researcher at Washington State University’s school for global animal health. It’s “very unlikely they swabbed the only wild mink with SARS-CoV-2.”
Mink are just one species. There’s no comprehensive analysis of all the animals in the world, whether or not they can get Covid-19 and spread it among themselves, and potentially to other wildlife. The virus can be establishing copies of itself in nature right now, and we would have no real-time way of knowing.
The light at the end of the tunnel for the pandemic is growing brighter, however. Safe, effective vaccines are now starting to be distributed in the US. But the eventual end of the pandemic will likely not mean the end of SARS-CoV-2. It may still sporadically, or more regularly — no one really knows — infect animals and wildlife across the world.
In the right animal host, the virus could lurk for years before an opportune moment to jump back into humans. Over that time, the virus could change a bit, mutating into a form that could evade the current vaccines.
Several species have been infected so far: cats, dogs, lions, tigers, pumas, mink, and, most recently, snow leopards. More species have been shown, in lab studies, to be vulnerable to infection.
But scientists are still investigating: How many more animals could potentially catch SARS-CoV-2 and what will it mean for the course of the pandemic, and, for the health of wildlife?
To avoid the worst, scientists and veterinarians need to know which animals SARS-CoV-2 could potentially infect, and figure out the chances of the virus jumping from humans to the animals, and back to humans again.
Many different types of animals can catch Covid-19
Scientists already know of many types of animals that can catch SARS-CoV-2. They know it because the virus originated in the animal world — likely in bats. And they also know it because they’ve seen several species of animals get infected.
Early on in the pandemic, tigers at the Bronx zoo got sick (three of them had a cough) with the virus. Veterinarians have since found signs of Covid-19 infections in some of the animals that humans spend the most time with.
Jonathan Runstadler, a veterinarian at Tufts University, is running a surveillance study of animals that come in for treatment at the school’s veterinary clinic. So far, they’re finding “a few percent of those domestic pet dogs and cats are developing antibodies to this SARS-CoV-2 virus,” Runstadler says, meaning their bodies have encountered the infection and mounted an immune response.
“It is unknown where the infection or virus they responded to came from,” he says, but the “highest likelihood” scenario was that it came from human household members. Overall, he says it’s not a lot of animals getting infected, but it’s clear that dogs and cats can, in some instances, be infected with the virus.
Cats seem to be more susceptible than dogs, overall (though the cats themselves don’t seem to get very sick). Dogs are a highly diverse species. “So it’s possible that there may be specific breeds or types of dogs that are more susceptible, we don’t really know,” Siefert says.
Other animals have been shown to be much more susceptible not just to infection, but to severe disease and even death. In Denmark, authorities ordered the culling of millions of captive mink after outbreaks occurred on hundreds of farms.
The concern wasn’t just that the virus was spreading among the mink, making them sick, making their breathing difficult, and killing many. It was that the virus had jumped from the mink, and then back into people, with some genetic changes to the virus’s spike protein, which the virus uses to enter cells.
“If the virus does begin circulating in a new species, the results will really be unpredictable,” Angela Rasmussen, a virologist with Georgetown’s Center for Global Health Science and Security, says. The virus is constantly mutating, changing in subtle ways. When it enters a new species, that species’ immune system make it so a significantly altered strain of the virus emerges. “The real question is whether it will change in a way that is more or less detrimental to the human population,” she says.
Once a disease establishes itself in wildlife, “it’s just exponentially harder to control.”
Currently, there’s no clear evidence that the genetic changes that occurred on the mink farm would make the virus more likely to evade a person’s immune system, or diminish the efficacy of a vaccine. But Denmark’s health authorities didn’t want to risk it. So they ordered the culling of all the mink. (Denmark’s health minister who made the decision has since resigned.)
The mink were a bit of a ticking time bomb: The virus spreads easily among mink in farms because they are kept in close quarters (the same ease of transmission happens among humans in close quarters).
Researchers are trying to figure out which animals could spread the virus from humans back to wildlife
It’s relatively easy to keep track of the virus in farmed animals. Their health is regularly monitored. Farmers notice when mink start dying. But what happens if the virus gets into an animal that spreads the virus asymptomatically, or gets into wildlife, which is harder to track?
Once a disease establishes itself in wildlife, Olson says “it’s just exponentially harder to control. I mean, you can barely get people to take vaccines. Imagine wildlife. You just have very limited options.”
The USDA maintains there “is currently no evidence” that the virus has established itself in wild mink populations near the farm where it was found. “It is important that surveillance in wildlife around infected mink farms continue, to identify if the virus enters local wildlife populations,” USDA’s Animal and Plant Health Inspection Service spokesperson said in a statement to Vox.
Researchers can’t study every animal species on Earth and test whether it can carry SARS-CoV-2. They’re focusing their research on animals that could act as conduits between humans and wildlife.
Anna Fagre, a veterinarian and microbiology researcher at Colorado State University, is doing this research on deer mice. In a lab study, Fagre and colleagues revealed that deer mice can contract the virus and spread it among other deer mice.
Deer mice animals are common in rural areas. “We see them, if staying in like cabins in the woods, [deer] mice are going to set up shop there,” Fagre says. Deer mice are known to occasionally spread other viruses, and they exist at the interface between human dwelling and the broader natural world. They could be a conduit ferrying SARS-CoV-2 from humans to other wildlife.
In her lab, “we were able to inoculate and infect these deer mice, and they actually did transmit the virus to other mice they were housed with,” Fagre says. They experienced subtle symptoms like losing a bit of weight and they “get a little bit quiet,” she says (quieter than, um, a mouse). Then, a few days later, they recover. That subtle illness might make it hard to realize if there are, suddenly, a lot of deer mice with the virus. Plus, these are not captive animals. If a viral mutation emerged among them, it would be discovered much later than what occurred in the mink.
“When this preprint [study] came out,” she says, “some people were like, ‘Oh my god, this is so scary: deer mice! We’ll never get rid of the virus if deer mice become infected.’”
For Fagre, her results aren’t a reason to panic. It was just a lab study. The results don’t mean there are deer mice running around rural areas with the virus. They also don’t mean the mice will become a source of future infection for humans.
“There are so many different steps the virus would have to take to spill back from humans into deer mice, and then circulate in deer mice, and then be transmitted back from deer mice into humans,” she says. “I’m not saying it couldn’t happen. It certainly could. Cross-species transmission is what led to the Covid-19 pandemic.” The research helps scientists be vigilant. “It’s important to be aware,” she says.
Rare human-to-animal-to-human jumps could have huge consequences
Being aware of which animals can be infected with the virus helps researchers ask new questions, too. House cats of all sorts seem to be susceptible to the virus. “I live in kind of a rural area of eastern Washington and I’ve actually caught deer mice in my house,” Seifert says. “So I’m like, can my cat, if he kills deer mice, can my cat contract SARS-CoV-2? I don’t know.”
That is unclear. Also unclear: if there are circumstances where a cat could pass the virus to a human. It’s possible, but it has yet to be seen.
“We know that in experimental studies that this can go from cat to cat,” Danielle Adney, a veterinarian-researcher working with the National Institutes of Health, says. “In the real world, it really seems like every animal that’s been reported has a pretty clear link to an infected human. So this is still a pandemic that’s driven almost exclusively by human-to-human contact.”
(Pet owners don’t have to be wary of their cats infecting them. That said: A few of the veterinarians said their colleagues need to be really careful and wear good personal protective equipment and N95 masks when working on cats — particularly if they’re doing dental work.)
But we know that rare events can have devastating consequences. It was rare for SARS-CoV-2 to jump from bats to humans. “I’m very worried about cats,” Rasmussen says. “There’s a lot of feral cats out there in the world. There’s also a lot of people who have outdoor cats that may or may not interact with other feral cats or other outdoor cats. And then if those cats are coming back, and like snuggling with their owners, that’s a potential source for the virus to spill over for future … introductions into the human population.”
She’s not saying this will happen, or that it’s currently happening. She’s saying it’s something to monitor. Because “if it [the virus] got into something like cats, and became widespread among cats, that would be a huge problem in terms of being able to control it long-term.”
It’s still unknown which species ferried the coronavirus from bats to humans in Wuhan, China. It could have been bats, but it could have been another species. Perhaps a similar species is found in other parts of the world and can carry the virus back and forth between humans and animals.
In the near-term, vaccines will help avoid the virus jumping back from animals to humans. But 10, 20 years from now, how many people will still be vaccinated and immune to Covid-19? No one knows. Thinking about Covid-19 in animals is to think about the bigger picture, to think on a longer timeline. Covid-19 could essentially hide in animals for years, waiting, subtly mutating and changing, before making a jump back into humans.
What’s hard about this topic is all the (literal) moving, crawling, trotting, scampering pieces: There are so many species, interacting with us in so many ways, interacting with other members of their own species in so many ways, interacting with other species in so many ways. In that sense, studying Covid-19 in animals is an opportunity to better understand the complicated ways diseases spread from animals to humans, and back again. That could help keep SARS-CoV-2 at bay, but it could also help prevent future pandemics.
The research on Covid-19 and animals has uncovered some good news, too.
“Luckily, ducks and chickens and pigs have all been shown not to be susceptible, in laboratory studies, and cows have really low susceptibility,” Fagre says. That means that the situation that happened on mink farms is unlikely to occur on farms where these more common animals are raised as livestock.
It’s not just about human health, but animal health as well
Veterinarians can think of a lot of potentially scary scenarios here. Not all of them are scary in terms of human health, but for animal health, too.
Scientists have conducted broad surveys of animal biology, noting which animals have a cellular receptor similar to the ACE-2 receptor in humans. This is a protein found on the surface of many human cells that the virus uses as a front door to start hijacking the cell and replicating within it.
At the top of the list of potentially most at-risk animals are some of the most critically endangered species on the planet, and some of our closest genetic relatives in the natural world.
At the Bwindi Impenetrable Forest National Park in Uganda, veterinarian and conservationist Gladys Kalema-Zikusoka is concerned about a potential outbreak among the park’s 460 mountain gorillas, which represent nearly half of all mountain gorillas left in the wild.
Gorillas share 98.4 percent of their DNA with humans. They have a similar immune system, and have similar cellular proteins through which SARS-CoV-2 enters to infect the body. If one of these precious few gorillas got infected, Kalema-Zikusoka worries they would get sick and die. Worse, the disease could spread rapidly among them.
“They don’t know how to social distance,” Kalema-Zikusoka says. Likewise, there’s no putting a mask on a 300-plus-pound wild gorilla. “They’re always grooming each other, they’re always moving together as a group. So if one of them gets Covid-19, it’s very easy for the rest of them to get it.”
The virus, she says, plainly “is a threat to the gorillas,” as well as to chimpanzees and orangutans, which also share an overwhelming amount of DNA with humans. It’s not easy to treat a wild gorilla if it gets sick. And if one does, she says, the plan is to quarantine potentially exposed gorillas via 24-hour monitoring by park workers in the forest.
“You can’t provide the same level of intensive treatment to a wild gorilla as you would a human being, who you can put in a hospital ward, put on a ventilator for days and days,” she says. Instead, they would try to treat the gorillas in their own habitat, shooting pharmaceutical-loaded darts at the animals, if need be.
“The best we can do,” she adds, “is teach people to social distance from them.” Since the pandemic began, all people visiting the gorillas in Bwindi Impenetrable National Park in Uganda must wear masks, they must get their temperatures checked, and they must stay 10 meters (32 feet) away from the animals.
Just as Covid-19 threatens gorilla conservation in Uganda, in North America, researchers are worried about bats. In recent years, millions of North American bats have died of a fungal disease called white nose syndrome. The pandemic threatens bats, for one, because it has basically shut down research on live bats. There’s fear that humans could give the bats the virus, and start an outbreak among them. “We don’t know if that can happen and which species that can happen in,” Siefert says. But considering how this virus likely originated in bats, scientists don’t want to risk it.
There’s no knowing what SARS-CoV-2 would do to bats in North America, or which species it could infect. Perhaps more would get sick and die. If infected, North American bats could become a reservoir species for SARS-CoV-2, a potential source of the virus for other wildlife, and down the line, for more human infections.
All of the veterinarians I spoke to stressed: Whatever is happening with Covid-19 in animals right now, it’s not as critical, or dire, as the situation in humans. It makes obvious sense that there are more resources currently going into tracking the spread among people, than tracking the spread among animals.
“Thousands of people are dying every day from this virus,” Fagre says. “Everyone’s first priority isn’t screening a bunch of wild rodents to see if they’ve been exposed.”
But down the line, perhaps we should prioritize them. Covid-19 is leaving a lot of shadow imprints on the world. It has upended lives and industries. But it’s also potentially burrowing itself back into nature, where it will wait. This virus came from nature, and it may very well return there. Scientists ought to track it as it does.
“This isn’t going to be the last spillover event,” Olson says, where a virus jumps from animals to humans. “We owe it to future generations to get our act together here.”
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