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In the spring of 2020, when the novel coronavirus infiltrated the twin cities, Hinh Ly couldn’t stop thinking about cats and dogs.
Dr. Ly, a veterinary and biomedical researcher at the University of Minnesota, knew that humans were the main cause of the pandemic. But he also knew that many people loved kissing and cuddling their pets, in illness and health. He asked himself: How transferable was SARS-CoV-2 to mankind’s best friends?
In March 2020, Dr. Ly reported that two dogs in Hong Kong had received positive PCR tests for the virus. However, these tests require the virus to actively replicate and therefore only reveal active infections. Wiping the snouts of many pets appeared to Dr. Ly considered too time consuming to figure out how easily the animals could become infected.
So he proposed to his wife, Yuying Liang, a researcher in the same department who runs the laboratory with him, an idea to test cats and dogs for antibodies that would reveal a previous infection with the virus. “I had the idea, but she’s the boss,” said Dr. Ly.
The results of these antibody tests, recently published in the journal Virulence, suggest that domestic cats are more susceptible to SARS-CoV-2 infection than dogs.
Fortunately, infected cats show mild symptoms at best. “I’m still a bit surprised that cats get infected so easily and yet rarely show signs of disease,” said Dr. Angela Bosco-Lauth, a biomedical researcher at the College of Veterinary Medicine and Biomedical Sciences at Colorado State University who was not involved in the study did the research.
And there is still no evidence that infected cats or dogs pose a risk to humans, said Dr. Jonathan Runstadler, a virologist at Tufts University’s Cummings School of Veterinary Medicine who has studied how the coronavirus is affecting animals but not on the new work.
The new study supports recent research that it may be “fairly common” in households where people test positive for SARS-CoV-2, potentially including cats and dogs, said Dr. Runstadler.
To test for antibodies from pets, the Minnesota researchers needed the animals’ serum, the part of the blood that contains antibodies. Dr. Ly turned to Dr. Daniel Heinrich, the director of the clinical pathology laboratory at the university’s veterinary center. (Dr. Henrich is also the author of the new study.) Every day, pets passed the center and their blood was tested for a myriad of reasons, including “annual checkups, unrelated diseases, inappropriate peeing on the wall,” said Dr. Ly.
These samples are usually discarded. But dr. Heinrich asked pet owners to use the serum anonymously in the study, and the researchers received their first handful of samples in April.
The researchers first examined about 100 samples and found that about 5 percent of the cat serum contained coronavirus antibodies, while almost none of the dog serum did. To be sure, Dr. Ly hundreds more samples drawn from blood drawn in April, May and June as Covid cases rose in the area.
In the end, the scientists found that 8 percent of the cats carried antibodies to the coronavirus while less than 1 percent of the dogs did, suggesting that cats were more prone to infection.
Since the pet owners gave their consent anonymously, the researchers could not understand which person could have transmitted the virus to the different cats and dogs. It is also unclear whether the infected domestic cats lived indoors or outdoors, or how the virus could be transmitted from cat to cat, said Dr. Ly.
Researchers don’t know why cats seem more vulnerable than dogs. One possibility concerns ACE2, a protein on the cell surface that is a receptor for the coronavirus. The genetic sequence of the human ACE2 protein is much more similar to the equivalent sequence in cats than in dogs.
But animal behavior could also play a role. A recent study that found similar results – that cats are more easily infected with the coronavirus than dogs – found that cats often prefer to sleep on beds rather than dogs. “Maybe it’s because we’re hugging the cats more,” speculated Dr. Ly. “Maybe we’ll kiss the cats more.”
Dr. Bosco-Lauth said she believes pets “are unlikely to contribute to the SARS-COV-2 epidemiology in the long term”. But there is still no way of knowing for sure.
For people who tested positive for Covid-19, Dr. Ly, to distance yourself not only from humans but also from cats and dogs. “You can’t hug her,” he said.
Dr. Ly and Dr. Liang have neither cats nor dogs in their own house. They have a tank with guppies that seem pretty safe from the coronavirus at the moment.