A study published this week in the Proceedings of the National Academy of Sciences takes a closer look at the effects of the pandemic coronavirus on domestic dogs and cats.
The researchers found that both dogs and cats can be infected by the virus that causes Covid-19 in humans, but none of the ten animals observed in the study showed clinical symptoms like coughing, sneezing or fever. The paper also shows that while dogs don't seem to spread the disease, coronavirus-infected cats do, James Gorman reports for the New York Times.
Follow-up experiments showed that infected cats can pass the virus to other, healthy cats housed in the same room, and that cats develop a strong immune response after their infection. Even though domestic animals are rarely used as lab animals, cats’ immune response might make them a candidate for veterinary vaccine research.
"Because cats are susceptible to infection, in some cases develop disease, and develop a robust immune response following exposure, they could feasibly be used for the development of vaccines for veterinary health," Angela Bosco-Lauth, an infectious disease expert at Colorado State University and first author of the new paper, tells Nina Pullano at Inverse.
According to the study, the research began by pipetting samples of the coronavirus into the noses of the animals, which had been lightly sedated. For the three dogs and first three cats, the researchers collected the animals’ sneezed-out stuff for at least four weeks and tested it for the presence of the virus. Samples from the infected dogs didn’t hold any measurable virus, but the cats’ samples did, mostly about five to seven days after infection.
In a follow-up experiment, the scientists infected a new pair of cats with the coronavirus and then introduced a pair of healthy cats in the sick cats’ room with them. The new cats caught the virus within the five days that they shared the room with the sick cats.
All of the animals were euthanized after the tests so that the researchers could study the effect of the virus on their tissues more closely, according to the paper. The necropsies revealed mild impacts on the animals’ lungs and upper respiratory tracts, but the animals never showed symptoms during the course of the study.
“Those cats that were infected in the experiment?” Bosco-Lauth says to the New York Times. “You would never have known.”
This finding might mean that cats might be infected by the virus relatively often, but humans don’t notice because the animals don’t show symptoms. (Cats are generally skilled at hiding signs that they’re sick or in pain.)
This study looked at only three dogs and seven cats. While none of the animals involved in the study showed symptoms, there are a handful of examples of animals that did show signs of Covid-19. In April, zookeepers at the Bronx Zoo tested a tiger named Nadia for the virus after she developed a dry cough, Nora McGreevy reported for Smithsonian magazine at the time. Pet cats in the England, New York and Belgium have caught the pandemic coronavirus. A dog in the U.S. named Buddy died after catching the virus, Natasha Daly reported for National Geographic in July.
So far, there are no cases of pet-to-human transmission of the coronavirus, and human-to-pet transmission seems rare. Even in a house with 29 ferrets and two Covid-19-positive humans, none of the ferrets became ill, reports the New York Times. A paper that has not yet been published provides evidence that minks transmitted the virus to humans on a mink farm in the Netherlands.
Pet owners shouldn’t take the new study as cause for alarm.
"The risk of cat-to-human infection is considered extremely low, by us and other experts in the field, but not completely out of the question," Bosco-Lauth tells Inverse.
"We would advise pet owners to take precautions around their pets if they [the owner] develops Covid-19 disease as they could certainly spread it to their pets and from there, pets could transmit to each other or potentially to other humans or wildlife."