We are entering an uncertain phase of the Covid-19 pandemic, where our actions could determine whether the disease shifts to being endemic — a normal part of life, like the seasonal flu or chickenpox.
It isn’t just humans that can get Covid-19. Animals can become infected with SARS-CoV-2 too, adding another unpredictable element that could potentially shape the future pandemic. This is the virus that causes Covid-19.
Critically, the transmission of Covid-19 among free-ranging wildlife — non-domesticated animals living separate from humans — is rarely observed in nature. In 2020, a single wild mink found in Utah marked the first detection of SARS-CoV-2 in a free-ranging, native animal in the United States.
But a study on white-tailed deer posted the pre-print server bioRxiv on November 1 suggests a dramatic shift.
The study indicates that, between April 2020 and January 2021, up to 80 percent of white-tailed deer in Iowa became infected with SARS-CoV-2.
“We have just started testing wild animals for the presence of the virus, and this study shows they can become infected, produce antibodies, and infect other deer,” Krysten L. Schuler, a wildlife disease ecologist and co-director of Cornell Wildlife Health Lab, tells Inverse. Schuler is not affiliated with the study.
What does it mean for a species to become a Covid-19 “reservoir”?
The deer became infected due to “human-to-deer spillover events and deer-to-deer transmission” according to the researchers.
In other words: it wasn’t just humans infecting deer. Deer were transmitting Covid-19 to each other.
Experts are now expressing serious concerns about deer becoming a reservoir for Covid-19.
“A reservoir is any person, animal, or environment where the virus... circulates and multiplies that is capable of passing the virus to another person or animal,” Schuler says.
Based on the study, we know deer can get infected from humans and transmit to each other, but will Covid-19 mutate within deer and spill back over from wildlife to humans or other animal species?
“We don't know if that's the case for deer,” Tony Goldberg, a veterinarian at the University of Wisconsin-Madison who studies the evolution of infectious diseases, tells Inverse.
According to Schuler, “We don’t have answers about transmission to other species or mutations and spillback yet.”
But the knowledge that deer can serve as reservoirs for Covid-19 — even just to members of their own species — is alarming, experts say.
“In general, if animals become a reservoir for a human pathogen, it means it becomes vastly more complex to control that pathogen,” Goldberg explains.
While other diseases like yellow fever and West Nile have seen human-animal spillover, but it’s still rare to see this happen outside of mosquito-borne diseases, Goldberg says.
“I can't honestly think of a perfectly parallel scenario where that happened before.”
Why are some animals susceptible to Covid-19?
Deer, according to Goldberg, are highly susceptible to Covid-19. Animals like great cats are also somewhat vulnerable to Covid-19, and in November, the Monterey Bay Aquarium vaccinated eight sea otters against the coronavirus. Primates are also understandably susceptible, and some gorillas began receiving Covid-19 vaccines earlier this year.
As Goldberg explains:
“I study reverse zoonotic transmission of respiratory viruses from people to great apes in Africa, and there's a great fear that SARS-CoV-2 could infect those species and cause mortality in chimps and gorillas, which are endangered and threatened.”
Meanwhile, rats aren’t that susceptible to Covid-19, according to a 2021 study.
So what makes some animals susceptible to Covid-19 and others not so much?
It has do to with the ACE2 receptor protein, which is on the surface of cells. The spike protein of SARS-CoV-2 binds to this protein receptor in humans and animals, allowing the virus to infect the host and replicate. Some animals’ receptors bind better to SARS-CoV-2.
“There were some research papers that made predictions — based on the viral receptor of different species — which species might be susceptible and which might not,” Goldberg says.
Do animals transmit Covid-19?
While many animals can contract Covid-19, most aren’t good at transmitting Covid-19, Goldberg explains.
Some animals, like the white-tailed deer, can transmit to other members of their own species.
Other animals, like minks, can transmit to humans, but the minks who have transmitted the disease have only been observed in farms — not out in the wild, according to Goldberg. In farms, where large clusters of minks live in close quarters, transmission is likelier to occur than in the wild.
But it’s possible that disease could transmit from deer to minks in the wild, creating a potentially concerning spillover between two species susceptible to Covid-19.
“We know, for example, that mink are susceptible, and there's plenty of mink in the woods here in Wisconsin where there's also deer,” Goldberg says.
What do variants mean for animals getting Covid-19?
As SARS-CoV-2 spreads among humans, it mutates, producing different Covid-19 variants, including the more transmissible Alpha and Delta variants.
According to a recent study published in the journal Veterinary Record, some animals can become infected with these different variants.
“Our study reported, for the first time, the ability for pets to be infected by the Alpha variant,” Luca Ferasin, lead author on the study and a specialist in veterinary cardiology, tells Inverse.
Using PCR and antibody testing, the researchers detected the Alpha variant of Covid-19 in both cats and dogs a few weeks after they developed a kind of cardiac disease known as myocarditis.
“Interestingly, most pet owners had Covid-19 a few weeks before their own pets got sick, suggesting that they developed symptoms of myocarditis after becoming infected,” Ferasin says.
“Our study reported, for the first time, the ability for pets to be infected by the alpha variant.”
We’ve known for a while that humans can transmit Covid-19 to their pets — although it appears to be somewhat rare — and no indication that pets are getting infected at greater rates now because of variants. Instead, it’s exactly what they are becoming infected with which has changed.
“Similarly, we do not have enough scientific evidence to confirm that the Alpha and Delta variants are more transmissible to pets,” Ferasin says.
He adds: “Unfortunately, we do not have any available information to confirm or deny that the virus can mutate in dogs and cats, especially for the small number of observed cases.”
When it comes to the white-tailed deer, it’s possible that the Delta variant could cause an upswing in cases, but there’s no evidence yet.
“The reason why some variants are more transmissible is that they replicate in the host more efficiently and they shed more,” Goldberg says. “So anything that has those properties... has a greater ability to infect other hosts, whether of the same species or different species.”
According to Schuler, many states were prohibited from testing wild species during the pandemic due to a shortage of testing reagents, which needed to be saved for humans.
“I don’t think we know enough about the variants in wildlife yet to know if one is more infectious than other to wildlife,” Schuler says.
“We may never be able to acquire that information.”
What is the future of Covid-19 human-animal spillover?
As far as humans are concerned, this new study doesn’t necessarily change how we should interact with wildlife.
The CDC maintains that “there is no evidence wildlife might be a source of infection for people in the United States” but the agency does offer suggestions on how hunters can keep themselves and wildlife safe during the pandemic.
Goldberg says it may be wise to keep our pets indoors and restrict unsupervised outdoor time in an effort to limit potential interactions with wildlife.
Ultimately, it’s too early to conclude anything about the long-term implications of this study, including the potential for mutations in deers.
“It’s just too early, but we can say that introducing a novel disease agent to a wild population is never good because we have very few tools to manage disease in wildlife,” Schuler says.
There is still fear of what will happen if the deer become a “true reservoir” and transmit the virus back to people, making it “very hard to eliminate and control,” Goldberg says.
It’s possible that the virus will burn out among deer, Goldberg says, causing a low-level infection that’s not very “efficient at infecting the cells of this new species.”
But it’s also possible the virus will replicate and evolve easily in deer hosts, leading to mutations and a potential transmission back to humans.
“We could be unlucky and it could come out of deer in some form that's more harmful to us,” Goldberg says. “I think we can say we're in uncharted territory.”