Last updated on September 9th, 2020 at 10:53 am
As a result of a story where a Malayan tiger in the Bronx Zoo contracted COVID-19, many people have begun worrying about dogs, cats, and COVID-19. Is there a worry? Can our pets get sick from the virus? Can they infect humans with it?
Of course, like everything in science, the evidence is not completely clear, especially since this virus has only been recognized since early December 2019. We still don’t have a complete picture of the pathophysiology of the disease, so answering questions about dogs, cats, and COVID-19 is going to be somewhat difficult.
However, we do have some early data, so I’m going to review it as best as I can.
All about coronaviruses
Coronaviruses (there are seven that infect humans) are species of virus belonging to the subfamily Coronavirinae in the family Coronaviridae, in the order Nidovirales. They are an RNA virus that contains around 26-32 thousand nucleobases each.
One of the myths about coronaviruses, stated by Donald Trump and many others, is that it’s related to the flu. No, the influenza viruses aren’t even closely related to coronaviruses – they’re actually in two separate phyla, meaning that they are as closely related to one another as a human is to a lobster. In other words, they aren’t closely related.
Coronaviruses probably arose around 55 million years ago, so it co-evolved with many mammals and birds. Evidence of coronaviruses in humans goes back 10,000 years or more.
In fact, there are coronaviruses that can infect cats and dogs, and there is a vaccine available for dogs (Nobivac from Merck).
This 2019 coronavirus outbreak is known as COVID-19 and is caused by the SARS-CoV-2 virus, which is closely related to other SARS-related viruses. The virus is spread easily by small droplets from infected individuals when they breathe or cough. The time from exposure to the SARS-CoV-2 virus to onset of symptoms is generally between 2 and 14 days.
Early symptoms are often referred to as “flu-like” but that’s a general term that is used for many diseases. Again, that does not mean that coronaviruses are related to influenza, just that they can share some symptoms.
As I mentioned above, severe acute respiratory syndrome (SARS) is caused by SARS-associated coronavirus (SARS-CoV), which means that the current coronavirus outbreak is related to SARS. In addition, the disease known as the Middle East respiratory syndrome (MERS) is caused by the Middle East Respiratory Syndrome Coronavirus (MERS-CoV).
Coronaviruses, like the SARS-CoV-2, MERS-CoV and SARS-CoV species, infect the upper respiratory and gastrointestinal tracts of mammals and birds. Non-human animals that can be infected by one or more species of coronaviruses include mice, dogs, cats, bats, beluga whale, pigs, ferrets, rabbits, cattle, turkeys, and many other animals.
Mostly, these viruses don’t move between species very often, but it is not unknown. Thus, we don’t know if SARS-CoV-2, as an example, has a reservoir in another species. That’s why examining dogs, cats, and COVID-19 is important, especially for those of us who have them as our companions during this time of social isolation.
Dogs, cats, and COVID-19 study
In a paper published on 8 April 2020 in Science by J. Shi et al. examined whether various domesticated animals were susceptible to infections by SARS-CoV-2. There is some good news and some very troubling news.
Right at the top, they determined that domestic cats were susceptible to COVID-19.
Researchers intranasally inoculated cats, ages 6 to 9 months and, to monitor respiratory droplet transmission, they placed an uninfected cat in a cage adjacent to each of the three infected cats. The researchers found SARS-CoV-2 RNA in the soft palate and tonsils of both a virus-inoculated cat and an exposed cat with viral RNA-positive feces.
The exposed cat also had viral RNA in its nasal passages and trachea, which the authors stated that it indicated “that respiratory droplet transmission had occurred in this pair of cats.”
The researchers replicated the study in younger cats and found:
…massive lesions in the nasal and tracheal mucosa epitheliums, and lungs.SARS-CoV-2 can replicate efficiently in cats, with younger cats being more permissive and, perhaps more importantly, the virus can transmit between cats via the airborne route.
Even though this study was very small, it does indicate that our pet cats are able to pass the virus between each other. Of course, we don’t know if they can pass the virus back to us, something that will require further study.
The researchers also found that ferrets can be infected with the SARS-CoV-2 virus, but they don’t seem to exhibit severe complications like humans. As I wrote previously, preclinical testing of vaccine candidates requires an animal model that can contract the disease, and this could be good evidence that the ferret would be a good animal model for vaccine testing.
The researchers then examined if the virus had any effect on dogs. They intranasally inoculated five beagles and put them with two uninoculated beagles. Viral RNA was found in two of the inoculated dogs on day 2 and one on day 6.
However, the researchers wrote, “infectious virus was not detected in any swabs collected from these dogs.”
On day 14, the researchers collected blood from all seven dogs. Two of the inoculated dogs seroconverted (that is, the virus caused detectable levels of antibodies against the virus) while the rest of the inoculated and non-inoculated dogs were seronegative. They concluded that dogs had a “low susceptibility” to the SARS-CoV-2 virus.
The researchers also conducted these same studies in pigs, chickens, and ducks. They found no viral RNA in any virus-inoculated and non-inoculated animals.
The researchers attempted to provide a biologically plausible explanation for their results. They said that cats and ferrets only have two amino acid differences from humans in SARS-CoV-2 spike-contacting regions of angiotensin-converting enzyme 2 (ACE2), the receptor SARS-CoV-2 uses to enter cells.
In other words, it appears that the cell receptor that attaches to the virus is very similar between cats, ferrets, and humans.
Furthermore, the researchers mentioned that cats that have been surveilled in Wuhan, China have been found to be seropositive for SARS-CoV-2.
What does this mean?
So what does this research mean for dogs, cats, and COVID-19? Frankly, I don’t know.
I think dogs are not an issue, because the virus doesn’t seem to replicate in dogs. So, you can keep them as a pet without too much worry.
As for cats, the answer is much more complicated. If your cat is socially isolated with you, then I wouldn’t worry very much. Of course, if you contract the virus and you have a cat, you both will need to be quarantined, but right now, I can’t find anyone making any official recommendations.
J. Shi et al. mentioned that some of the cats that contracted the virus became very aggressive. That could be concerning if there are young children around.
If your cat is an outdoor cat (something that troubles me from an environmental perspective), it is entirely possible that they contract COVID-19 from other cats or humans, then pass it back to humans. Although we aren’t sure that cats are a reservoir for the virus, this small study (and other evidence from China) seems to indicate that many have been infected, we don’t have much evidence of whether cats can pass the virus back to humans.
From my perspective, I would recommend that you keep your pet cat socially isolated along with yourself and your family. Even though there are hundreds of environmental reasons why cats ought to be kept in the house, the risk that they can bring COVID-19 into your house from outside seems to be credible and troubling.
Of course, much more data needs to be gathered before we consider more serious and unthinkable recommendations about cats and COVID-19. We might also need to quickly develop a vaccine for cats, although it might be just slightly different than the human version.
This bears watching.
Update 23 April 2020
News reports have stated that two cats in New York state were found to have contracted the SARS-CoV-2 virus and showed symptoms of the disease. One cat seemed to have contracted it from their human companion, who tested positive for COVID-19. Another cat in that same household tested negative.
The other cat became ill about a week after a person in its household had a short respiratory illness, though it was not confirmed to be COVID-19. It is an outdoor cat, so it might have come in contact with another infected animal or person in the area.
In both cases, the cats did not appear to pass the disease back to other human companions in the household.
Both cats’ symptoms included coughing and slightly runny noses. Public health agencies in New York are recommending that any human companions who have COVID-19 restrict their contact with any of their pets as much as possible. That includes petting, snuggling, and other contacts. Moreover, they should wear a face-covering while caring for their pets.
I want to reiterate that if you have an outdoor cat, who are destroying our songbird population, stop. Keep them inside, no matter how much you think that your feline ought to be free to roam the country. It might save your life.
And monitor them for respiratory diseases – if they exhibit symptoms that indicate COVID-19, you may want to isolate the animal from anyone who could be susceptible to the worst complications of the disease.
We don’t know if cats are going to be considered a reservoir for the disease. We don’t know if cats can pass the disease to humans. We don’t know if this is a serious matter or extremely rare. Just be aware, and please keep your cats inside!
- Wertheim JO, Chu DK, Peiris JS, Kosakovsky Pond SL, Poon LL. A case for the ancient origin of coronaviruses. J Virol. 2013;87(12):7039–7045. doi:10.1128/JVI.03273-12