From deer and dogs to rats and mink, COVID-19 has spread to the animal world

As SARS-CoV-2 spreads through some animal populations, animals may create a feedback loop as they re-infect humans

By Matthew Rozsa

Staff Writer

Published September 8, 2021 5:40AM (EDT)

A deer stag, dog, rat and mink looking at COVID-19 (Photo illustration by Salon/Getty Images)
A deer stag, dog, rat and mink looking at COVID-19 (Photo illustration by Salon/Getty Images)

For six months out of the year, Dr. Jenessa Gjeltema has a very diverse and unusual roster of patients. The assistant professor of zoological medicine at University of California, Davis provides clinical work for hundreds and hundreds of animals at the Sacramento Zoo, from lions and giraffes to poison dart frogs and two-toed sloths. It doesn't take long to intuit that she cares very deeply for each animal, which is why she was concerned when a meerkat became very sick during the early months of the COVID-19 pandemic.

"The meerkat presented with bloody nasal discharge coming out of its face and was in respiratory distress," Gjeltema recalled. "It was just at the start of the pandemic, when we were getting significant amounts of community spread in our local area, and I was very concerned because we didn't know as much as we do now about how the virus behaves in humans, much less all of the animals that were in our collection."

Fortunately the meerkat was not infected and, after being comprehensively treated by medical experts, made a full recovery. Even so, the anxiety that she felt during this incident still clearly lingers with Gjeltema. 

"It's challenging to work with less than perfect knowledge," she told Salon.

The knowledge we have today about animals and COVID-19 remains quite imperfect, although it has become less so than when Gjeltema had to assist the hapless meerkat. The story of the COVID-19 pandemic centers around a narrative of animal-to-human transition: The prevailing scientific theory is that it originated in a horseshoe bat, reached another animal through one or many "spillover events" (transmissions) and eventually got to a human host. 

Yet zoo animals aren't the only ones who seem to be catching the novel coronavirus. White-tailed deer in both Ohio and Michigan recently tested positive for SARS-CoV-2 antibodies, indicating previous infections. We know that companion animals like cats and dogs can develop COVID-19, while mink farmers are at risk of losing their entire industry because COVID-19 is virulent within that species. 

There are two questions that logically emerge from the broader subject of COVID-19 and animals: The first is how this poses a threat to humans. The second is what it means for the animals themselves.

Currently, the Centers for Disease Control and Prevention (CDC) says that it is unlikely but not impossible for an animal to infect a human with SARS-CoV-2. Even though SARS-CoV-2 likely originated in bats, there is no evidence of any animal species playing a significant role in spreading COVID-19 among people. Nevertheless, the CDC advises people to avoid interacting with unfamiliar wildlife and to enforce vigilant personal hygiene standards after they make contact with strange animals.

Lyndsay Cole, a spokesperson for an Agriculture Department agency known as the Animal and Plant Health Inspection Service (APHIS), elaborated specifically on SARS-CoV-2 and white-tailed deer, which are widely and densely distributed through most of the United States. Scientists know for sure that SARS-CoV-2 antibodies exist in wild white-tailed deer, but they are unclear about how they were exposed to the virus and what impact this exposure will have for the deer, humans and other animals.

"There is no evidence that animals, including deer, are playing a significant role in the spread of SARS-CoV-2 to people," Cole told Salon by email, later adding that "there have been no reports of deer showing clinical signs of infection with the virus." Notably, the tests on the white-tailed deer samples looked for SARS-CoV-2 antibodies rather than the virus itself, which limits how much we know about the nature of what those deer are experiencing.

Lori Ann Burd, the environmental health director at the Center for Biological Diversity, explained that experts are concerned about mink because their behavior makes them susceptible to both developing infections and carrying them to other wildlife.

"Mink are extremely wide-ranging naturally, and they're quite solitary," Burd told Salon, noting that this makes mink extremely stressed when confined to the compact conditions of a mink farm. Indeed, such cramped conditions weaken their immune system and make them susceptible to respiratory diseases like COVID-19. As notoriously intelligent animals, mink can figure out how to escape from captivity and return to the wild, meaning that if they were previously infected by their human handlers, they could spread the disease to other wildlife and create a hotbed for new COVID-19 mutations. There is also the risk that previously infected mink could spread the disease to uninfected people at the farms.

And what about man's best friend, the ever-loyal dog? Sadly there is evidence that our canine companions can die from COVID-19, as Americans learned after Buddy the German shepherd died last year. At the same time, as with other animals, there is no evidence that dogs are major carriers of the disease or particularly likely to be harmed if exposed to it. Health experts agree that it would be cruel and unnecessary for ordinary dog owners to feel unsafe around their companions.

If international statistics are to be believed, cats have more reason to worry about COVID-19 than their supposed rivals. The World Organisation for Animal Health reported that as of last month there were 102 outbreaks of SARS-CoV-2 infections among cats, compared to only 90 among dogs. (They are also more likely to get seriously ill.) Mink had the most outbreaks with 358 while multiple outbreaks were also reported among tigers, lions, pumas and snow leopards.

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While scientists are not entirely clear about why certain animals are more likely to get infected than others, one prevailing theory is that it may have something to do with the ACE2 "receptor" (short for angiotensin-converting enzyme 2), a protein that serves as an entry point for SARS-CoV-2 to penetrate human cells. As a study published last year noted, it is possible to list animals that may or may not be more likely to be infected by the coronavirus based on their structure of these proteins.

The most vulnerable species to COVID-19 were catarrhine primates — a group that include chimpanzees, bonobos, Western lowland gorillas, olive baboons and Sumatran orangutans — but the scientists compiled a database with 410 vertebrates, including 252 mammals, to determine which ones had an ACE2 receptor that was likely to help the SARS-CoV-2 virus. They labeled as "high" risk animals like white-tailed deer, the Chinese hamster, the beluga whale, the giant anteater and the muskrat. At "medium" risk were golden hamsters, wild yak, jaguars, hippopotamuses and American bison. Giant pandas, polar bears, red foxes, dingos and horses were determined to be at "low" risk, while guinea pigs, harbor seals, striped hyenas, Northern elephant seals and Jamaican fruit-eating bats were deemed at "very low" risk.

This information is helpful to Gjeltema, who told Salon that when managing the zoological collection she is particularly worried about primates. At the same time, she is also worried about the exotic felines because of their higher susceptibility, and the otters because they are closely related to mink,  and their bats (for obvious reasons).

When all is said and done, Gjeltema says she has kept her zoo safe by following the basic premises of the CDC's guidelines for humans.

"For example, we have established social distancing as much as possible," Gjeltema explained. "We have all of our keepers wearing face masks. We don't have members of the public interfacing near any of our susceptible animals. Obviously, good hygiene. We have close monitoring."

When it comes to keeping animals safe from COVID-19, and people safe from animals who might have it, Gjeltema at least knows that those ideas will work.

By Matthew Rozsa

Matthew Rozsa is a staff writer at Salon. He received a Master's Degree in History from Rutgers-Newark in 2012 and was awarded a science journalism fellowship from the Metcalf Institute in 2022.

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