This past spring, Amanda Goldberg crouched in the leafy undergrowth of a southwestern Virginia forest and attempted to swab a mouse for COVID. No luck; its nose was too tiny for her tools. “You never think about nostrils until you start having to swab an animal,” Goldberg, a conservation biologist at Virginia Tech University, told me. Larger-nosed creatures that she and her team had trapped, such as raccoons and foxes, had no issue with nose swabs—but for mice, throat samples had to do. The swabs fit reasonably well into their mouths, she said, though they endured a fair bit of munching.
Goldberg’s throat-swabbing endeavors were part of a study she and her colleagues devised to answer an unexplored question: How common is COVID in wildlife? Of the 333 forest animals her team swabbed around Blacksburg, Virginia, spanning 18 species, one—an opossum—tested positive. This was to be expected, Goldberg said; catching a wild animal that happened to have an active infection right when it was swabbed was like finding Waldo. But the researchers also collected blood samples, and those were more telling about whether the animals had experienced previous bouts with COVID. Analysis by researchers at Virginia Tech revealed antibodies across 24 animals spanning six species, including the opossum, the Eastern gray squirrel, and two types of mice. “Our minds were blown,” Goldberg said. “It was basically every species we sent” to the lab.
That animals can get COVID is one of the earliest things we learned about the virus. Despite the endless debate over its origins, SARS-CoV-2 most likely jumped from an animal through an intermediate host to humans in Wuhan. Since then, it has since spread back to a range of animals. People have passed it to household pets, such as dogs and cats, and to a Disney movie’s worth of beasts, including lions, hippos, hyenas, tigers, mink, and hamsters. Three years into the pandemic, animals are still falling sick with COVID, just as we are. COVID is likely circulating more widely in animals than we are aware of, Edward Holmes, a biologist at the University of Sydney, told me. “In all my 30-plus years of doing work on this subject, I have never seen a virus that can infect so many animal species,” he said. More than 500 other mammal species are predicted to be highly susceptible to infection.
Given that most people nowadays aren’t fretting too much about human-to-human spread, it makes sense that animal-to-human spread has largely been forgotten. But even when there are so many other pandemic concerns, animal COVID can’t be ignored. The consequences of sustained animal transmission are exactly the same as they are in people: The more COVID spreads, the more opportunities the virus has to evolve into new variants. What’s most alarming is the chance that one of those variants could spill back into humans. As we’ve known since the pandemic started, SARS-CoV-2 is not a human virus, but one that can infect multiple animals, including humans. As long as animals are still getting COVID, we’re not out of the doghouse either.
Perhaps part of the reason COVID in animals has been overlooked—apart from the fact that they’re not people—is that most species don’t seem to get very sick. Animals that have gotten infected generally exhibit mild symptoms—typically some coughing and sluggishness, as in pumas and lions. But our research has gone only fur-deep. “We certainly can’t ask them, ‘Are you feeling headaches, or sluggish?’” said Goldberg, who worries about long-term or invisible symptoms going undiagnosed in species. And so animal COVID has lingered unchecked, increasing the chances that it could mean something bad for us.
The good news is that the overall risk of getting COVID from animals is considered low, according to the CDC. This is partly explained by evolutionary theory, which predicts that most variants that emerge in an animal population will have adapted to become better at infecting the host animal—not us. But some of them, strictly by chance, “could be highly transmissible or virulent in humans,” Holmes said. “It’s an unpredictable process.” His concern is not that animals will start infecting people en masse—your neighbors are far likelier to do that than raccoons—but that in animals, SARS-CoV-2 could form new variants that can spill over into people. Some scientists believe that Omicron emerged this way in mice, though evidence remains scant.
A troubling sign is that there’s already some evidence that COVID has made its way from humans to animals, where it mutated, and then made its way back into humans. Take white-tailed deer, by now a well-known COVID host. Every fall, hunters take to the golden meadows and reddening forests of southwestern Ontario to shoot the deer, giving researchers an opportunity to test some of the hunted animals for COVID. The species has been infected with the same variants circulating widely in humans—a handful of Staten Island deer caught Omicron last winter, for example—which suggests that people are infecting them. How the deer get infected still isn’t clear: Extended face time with humans, nosing around in trash, or slurping up our wastewater are all possibilities.
The researchers in Canada found not only that some of the animals tested positive, but also that the variant they carried had never before been seen in humans, indicating that the virus had been spreading and mutating within the population for a long time, Brad Pickering, a research scientist for the Canadian government who studied the deer, told me. In fact, the new variant is among the most evolutionarily divergent ones identified so far. But despite its differences, it appeared to have infected at least one person who had interacted with deer the week before falling ill. “We can’t make a direct link between them,” Pickering said, but the fact that such a highly diverged deer variant was detected in a human is very suggestive of how that person got sick.
This research adds to the small but growing body of evidence that the COVID we spread to animals could come back to bite us. Fortunately, this particular spillback does not appear to have had serious consequences for humans; rogue deer variants don’t seem to be circulating in southern Canada. But this is not the sole documented instance of animal-to-human spread: People have been infected by mink in the Netherlands, hamsters in Hong Kong, and a cat in Thailand. Other spillbacks have probably occurred and gone unnoticed. So far, no data show that the animal variants that have spread to humans are more dangerous for us. Even if a potential animal variant isn't the next Omicron, it could still be better at dodging our existing treatments and vaccines, Pickering said.
But there is also, frankly, a lack of data. Local wildlife-surveillance efforts led by researchers like Goldberg and Pickering are ongoing, but they do not exist in most countries, Holmes said. An international database of known animal infections, maintained by Complexity Science Hub Vienna, is a promising start. An interactive map shows the locations of previously infected animals, including large hairy armadillos (Argentina), manatees (Brazil), and cats (everywhere). At the very least, with animal COVID, “we need to know what species it’s in, in what abundance, and genetically, what those variants look like,” Holmes said. “It’s absolutely critical to know where [the virus] is going.” Without this, there is no way of knowing how often spillback occurs and whether it puts humans at risk. And we can’t tell whether new COVID variants are also putting animals in danger, Goldberg said; a devastating Omicron-like variant could emerge in their populations too.
The steps we need to take to mitigate the animal-COVID problem—and prevent other zoonotic diseases from jumping into humans—are clear, even if they don’t seem to be happening. Eliminating wet markets where wild animals are sold is an obvious preventive measure, but it has been difficult to implement because the livelihoods and diets of many people, especially in the global South, depend on them. As climate change and land development decimate even more habitats, wildlife will be forced into ever-closer quarters with us, fostering an even more efficient exchange of viruses between species. Unlike mask wearing and other straightforward options for curbing the human spread of COVID, preventing its transmission to, from, and among animals will require major upheavals to the way our societies run, likely far greater than we are willing to commit to.
Humans tend to act like COVID ends up afflicting us after traveling through a long chain of species. But to think so is like living in the Middle Ages, Holmes said, when the Earth was considered the center of the universe. As we learned then, we are not that important: Humans are but a node in an immense network of species that viruses move through in many directions. Just as animal viruses infect us, human viruses can spread to animals (measles, for example, kills a variety of great apes). There are definitely bigger problems than animal COVID—no one needs to hunker down for fear of sneezing deer—but as long as animals keep getting infected, we can’t overlook what that means for us. Paying attention to animal COVID often starts with a single swab—and a snout to stick it in.