Whether pets such as cats and dogs can get COVID-19 is a subject of emerging controversy. Just today, a preprint* originally released on March 31 suggesting pets can be infected was published in Science. On the other hand, a new preprint was released today suggesting domestic pets under typical domestic circumstances are not being infected.
A variety of anecdotes have circulated suggesting animals can be infected. Tim Ferriss seemed to suggest his dog got infected at the end of an interview with former US Surgeon General Vivek Murthy, author of the new book, Together: The Healing Power of Connection in a Sometimes Lonely World. More recently, a tiger at the Bronx Zoo tested positive.
Let's take a closer look.
Experimental Infection of Cats, Dogs, and Ferrets
In the paper published in Science today, researchers experimentally infected various animals, including common pet and farm animals, through either the nose or throat, in some cases tested the animals' abilities to transmit the virus between each other, and after a period of time euthanized the animals and tested a wide variety of organs for signs of infection.
Whether ferrets were infected through the nose or throat, the virus replicated only in the upper respiratory tract: in the nose, soft palate (the back of the roof of the mouth), and the tonsils. No evidence of the virus was found in their lungs, heart, liver, spleen, kidneys, pancreas, small intestine, or brain, though low levels were found in some of the stool samples. One out of three ferrets developed fever and loss of appetite 10-12 days after infection. They had immune cells known as neutrophils and macrophages in their lungs, mild inflammation of the bronchi (the passages that lead from the windpipe to the lungs), and severe inflammation of the blood vessels and surrounding tissues.
In young adult outbred cats aged 6-9 months, the virus infected the soft palate, tonsils, trachea (“windpipe”), lungs, and small intestines. The virus was consistently found in their stool. Three cats were exposed to an inoculated partner by being kept in an adjacent cage separated by a double-layered net, and one of them became infected. This suggests that young adult cats can not only become infected but can also transmit the disease to other cats through the air.
In kittens aged 70-100 days, the pattern of infection was similar and they had “massive lesions” in the tissues of their nose, windpipe, and lungs.
In 3-month-old beagles, some of the dogs had some virus detected in their stool and developed antibodies to the virus, but none of them had evidence of viral replication in any of their internal organs.
Pigs, chickens, and ducks couldn't be infected.
14% of Cats in Pet Hospitals and Animal Shelters Got Infected in Wuhan
In a preprint released on April 3, researchers reported collecting serum samples from 102 cats in Wuhan's animal shelters and pet hospitals. Fifteen cats had antibodies to SARS-CoV-2, the coronavirus that causes COVID-19. Six were stray cats, and three were from owners who were confirmed as COVID-19 cases.
When the serum was mixed with SARS-COV-2-infected cells, the samples from cats with COVID-19-confirmed owners were most effective at neutralizing the virus, while some of the samples from the stray cats were not able to neutralize the virus despite the presence of antibodies. This suggests that exposure to their infected owners allowed the cats to develop more specific and more effective antibodies.
The authors did not report the breed or ages of the cats, which is important since the Science paper discussed above only looked at young cats, and the kittens seemed to have greater damage done to their tissues than the young adults. They also did not report details of how the animals interacted with humans and how they may have been exposed.
Cats and Dogs in Close Contact with French COVID-19 Owners
In a preprint released today, researchers looked at whether cats and dogs had been infected after being in close contact with COVID-19-confirmed owners.
Among 18 owners of 9 cats and 12 dogs who were members of a community associated with a French veterinary campus, 11 developed symptoms of COVID-19 and 2 got tested and were confirmed.
All of them lived in the same room with their pets. All of the cats and one-third of the dogs shared their owners' beds. 78% of cat owners and 92% of dog owners accepted face and hand licking.
The cats were all domestic European Shorthair cats between 6 months and 6.5 years old, with an average age of 3.3 years. 6 of the dogs were crossbred and 6 belonged to breeds including Labrador Retriever, Shetland Sheepdog, Belgian Malinois, and White Swiss Shepherd. The dogs ranged in age from 4 months to 8 years, with an average age of 2.7 years.
For the most part the animals were not sick, but three of the cats had respiratory or digestive signs that could be considered consistent with COVID-19.
No antibodies to the virus were found in the blood of any of the animals, and nasal and rectal swabs failed to show evidence of any infections.
Comparing the Results of the Three Studies
If we take the 14.7% infection rate in Wuhan cats as a benchmark, we would only expect to find one infected cat among the nine tested in the French study. Under these assumptions, the sample size was just too low in the French study to protect against a false negative.
On the other hand, we know virtually nothing about the degree of exposure of the Wuhan cats. We know the owners of the French study were co-sleeping and most were accepting face and hand licking. This might make the data in the Science paper a better benchmark, where one in three cats were infected by staying in close quarters with experimentally inoculated cats. In that case we would expect three of the cats to have gotten sick in the French paper. Indeed, exactly three cats had respiratory and digestive signs, but none tested positive.
Nevertheless, we still have some problems with the numbers:
In the Science paper, only three cats were paired up with an inoculated partner, and only one got sick. The sample size is too small to determine the rate of transmission.
In the French paper, only two of eleven owners got tested for COVID-19. As I reported here and here, the likelihood someone with flu-like symptoms has COVID-19 is close to 5%, and this is increased to probably somewhere between 16 and 62% if they also lose their sense of smell or taste. The French paper did not report any of the specific symptoms the pet owners had, so it's not clear how many of the nine owners with symptoms and no testing actually had COVID-19.
With that said, it is possible the three cats with respiratory and digestive problems had very mild cases of COVID-19 that did not generate antibodies, and by the time their tests were taken their viral loads were too low to be detected.
Furthermore, the Science paper used kittens and young adult cats, and the older cats in the French study may be less vulnerable to infection.
The Bottom Line
Right now it appears that dogs have a very slight vulnerability to infection that may not be very consequential, while young adult cats are capable of getting mild infections and spreading the virus, and kittens are capable of getting very sick. This has only been shown under experimental conditions where the animals are directly inoculated through the nose or throat.
Almost nothing is known about how different breeds differ in their vulnerability to infection, and it remains unclear whether cats can be infected just from sharing rooms and licking the face or hands of an infected owner.
It does seem wise to protect kittens from any humans known to be sick with COVID-19, and to keep your face and hands well-washed regardless of whether you believe you have the virus if you have a kitten or young cat that licks them. Ferrets should be similarly protected. It is less clear whether these precautions need to be taken around dogs, and completely unknown whether they should be taken around most other pets.
However, given that good hygiene is already an important part of preventing spread between humans, it seems best to regard any pets as just another member of the household to be protected during this time.
Stay safe,
Chris
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Disclaimer
I am not a medical doctor and this is not medical advice. I have a PhD in Nutritional Sciences and my expertise is in conducting and interpreting research related to my field. Please consult your physician before doing anything for prevention or treatment of COVID-19, and please seek the help of a physician immediately if you believe you may have COVID-19.
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*Footnotes
* The term “preprint” is often used in these updates. Preprints are studies destined for peer-reviewed journals that have yet to be peer-reviewed. Because COVID-19 is such a rapidly evolving disease and peer-review takes so long, most of the information circulating about the disease comes from preprints.