A couple of Canadian researchers may have figured out why cats get COVID and dogs don’t: a mutation in the gene that provides a vector for the novel coronavirus. While a few noted dogs have tested positive for SARS-CoV-2, the virus that causes COVID-19, no dogs have been diagnosed with COVID-19 to date.

Researchers at Dalhousie University in Halifax, Nova Scotia, published a study in which the authors write: “A single genetic change in the host receptor for the virus inherited in cats, but not dogs, correlates with feline susceptibility.”

In other words, during SARS-CoV-2 infection, the virus targets the angiotensin-converting enzyme 2 protein (ACE2). Dogs have a mutation in their ACE2 that gives them a natural resistance to the novel coronavirus. Cats don’t.

That seems to apply to all cats, large and small: the researchers determined that, along with cats, a number of different feline species—cheetahs, leopards, tigers, and lions—are likely to be susceptible to the virus. So are ferrets.

Dogs, bears, pigs, chickens, and ducks are not.

NEWStat reached out to coauthors Graham Dellaire, PhD, a professor of pathology at Dalhousie University, and Sabateeshan Mathavarajah, a doctoral student in the Department of Biochemistry and Molecular Biology at Dalhousie, to find out more.

NEWStat: In a nutshell, why are cats susceptible to SARS-CoV-2 and not dogs?

Graham Dellaire: Dogs have a mutation in the ACE2 receptor at a single amino acid, one of the building blocks of the ACE2 protein. This single mutation weakens the binding of ACE2 by the spike protein of [SARS-CoV-2], the virus that causes COVID-19.

NEWStat: One of the most frustrating things about COVID and companion animals is that even though less than half a dozen dogs in the world have been officially diagnosed with SARS-CoV-2, pet owners are still worried that their dog could get it. If dogs can contract SARS-CoV-2, why haven’t more dogs tested positive?

GD: Dogs can develop antibodies against SARS-CoV-2 when they’re exposed, but the virus replicates poorly, and they do not transmit the virus to each other or to people. This is not the case in cats, who are largely asymptomatic but easily transmit the virus to other cats.

Sabateeshan Mathavarajah: However, the caveat with dogs is that certain breeds may have novel mutations in their ACE2 receptor that increase susceptibility. A future project [that studies] the ACE2 sequences of dogs who have tested positive will help us understand if receptor changes are responsible for why certain breeds are susceptible. German shepherds seem to be the [breed] with the most positive cases. So it’s possible that they carry unique ACE2 mutations that make them susceptible.

NEWStat: Many dog owners continue to worry that their dogs could catch and spread COVID-19. What can veterinarians tell those clients to reassure them that their dogs are safe?

GD: So far in every case of a dog testing positive for SARS-CoV-2, no infective virus has been isolated from [the] dogs. Thus, the risk of your dog giving you COVID-19 is low. [People walking dogs] can relax when their dogs interact with other dogs in public parks. Nonetheless, [an infected person] petting your dog can transmit the virus to you via particles trapped in the dog’s fur—so wash your hands after such encounters.

NEWStat: How big of a risk do cats pose for the spread of COVID to humans?

GD: There have been no documented cases of cats [transmitting] SARS-CoV-2 to humans, only from human to cat and between cats. This suggests the risk is low for most people. But for the elderly or other high-risk populations, such as those with diabetes, it may be prudent to keep their cat inside and wash their hands after interacting with their pet, just in case. Ultimately, [infected] humans pose a bigger risk to endangered cats, like lions and tigers in zoos, than cats do to us.

NEWStat: What are the chances that some of those positive tests in dogs are false positives?

SM: It’s likely that [some] are false positives. A pug reported as positive for the virus ended up having low amounts of the virus in his saliva and the immune response to the virus did not occur. [Given that there were three infected humans in the household], it’s likely that the test was contaminated.”

NEWStat: Health officials announced last week that Buddy, the German shepherd from New York State who tested positive for SARS-CoV-2 in June, died three weeks ago. He had underlying health issues. How could the coronavirus have exacerbated them?

SM: The coronavirus causes a heightened immune response and respiratory issues in most people. However, there’s accumulating evidence that [other systems are] also affected. Therefore, a detailed analysis of the pathology in the German shepherd is [warranted].

Photo credit: © Getty/101cats