In a study published May 13 in the New England Journal of Medicine, researchers from the U.S. and Japan reported that in the laboratory, cats can readily become infected with SARS-CoV-2, the virus that causes COVID-19, and may be able to pass the virus to other cats.

University of Wisconsin School of Veterinary Medicine professor of pathobiological sciences Yoshihiro Kawaoka led the study in which researchers administered to three cats SARS-CoV-2 isolated from a human patient. The following day, the researchers swabbed the nasal passages of the cats and were able to detect the virus in two of the animals, an announcement said. Within three days, they detected the virus in all of the cats.

The day after the researchers administered virus to the first three cats, they placed another cat in each of their cages. Researchers did not administer SARS-CoV-2 virus to these cats.

Each day, the researchers took nasal and rectal swabs from all six cats to assess them for the presence of the virus, the announcement said. Within two days, one of the previously uninfected cats was shedding virus, detected in the nasal swab, and within six days, all of the cats were shedding virus. None of the rectal swabs contained virus.

Each cat shed SARS-CoV-2 from their nasal passages for up to six days, the researchers said, noting that the virus was not lethal and none of the cats showed signs of illness. All of the cats ultimately cleared the virus.

"That was a major finding for us: The cats did not have symptoms," said Kawaoka, who also holds a faculty appointment at the University of Tokyo in Japan. Kawaoka additionally is helping lead an effort to create a human COVID-19 vaccine called CoroFlu.

The findings suggest that cats may be capable of becoming infected with the virus when exposed to people or other cats positive for SARS-CoV-2. It follows a study published in Science by researchers at the Chinese Academy of Agricultural Sciences that also showed that cats (and ferrets) could become infected with and potentially transmit the virus. The virus is known to be transmitted in humans through contact with respiratory droplets and saliva.

"It's something for people to keep in mind," said Peter Halfmann, a University of Wiscosin-Madison research professor who helped lead the study. "If they are quarantined in their house and are worried about passing COVID-19 to children and spouses, they should also worry about giving it to their animals."

Both researchers advise that people with symptoms of COVID-19 avoid contact with cats. They also advise cat owners to keep their pets indoors in order to limit the contact their cats have with other people and animals.

Kawaoka is concerned about the welfare of animals. The World Organization for Animal Health and the Centers for Disease Control & Prevention say there is "no justification in taking measures against companion animals that may compromise their welfare."

Humans remain the biggest risk to other humans regarding transmission of the virus. There is no evidence that cats readily transmit the virus to people, nor are there documented cases in which people have become ill with COVID-19 because of contact with cats.

There are, however, confirmed instances of cats becoming infected because of close contact with humans infected with the virus, and several large cats at the Bronx Zoo in New York had also tested positive for the virus.

For instance, according to an April 22 announcement from the U.S. Department of Agriculture, two cats in two private homes in New York tested positive for COVID-19. One was in a home with a person with a confirmed case of the viral disease. The cats showed mild signs of respiratory illness and were expected to make a full recovery.

Additional cats have also tested positive for COVID-19 after close contact with their human companions, said Sandra Newbury, director of the University of Wisconsin-Madison Shelter Medicine Program. Newbury is leading a research study in several states in the U.S. to test animal shelter cats that might have previously been exposed to human COVID-19 cases.

"Animal welfare organizations are working very hard in this crisis to maintain the human/animal bond and keep pets with their people," Newbury said. "It's a stressful time for everyone, and now, more than ever, people need the comfort and support that pets provide."

Newbury has worked with CDC and the American Veterinary Medical Assn. to develop recommendations for shelters housing potentially exposed pets, which they may do while owners are hospitalized or otherwise unable to provide care because of their illness. The University of Wisconsin-Madison study helps confirm experimentally that cats can become infected, though the risk of natural infection from exposure to SARS-CoV-2 seems to be quite low, Newbury said. Of the 22 animals the program has tested, none have had positive polymerase chain reaction tests for the virus, she added.

"Cats are still much more likely to get COVID-19 from you, rather than you get it from a cat," said Wisconsin Veterinary Diagnostic Laboratory director Keith Poulsen, who recommended that pet owners first talk to their veterinarians about whether to have their animals tested. Testing should be targeted to populations of cats and other species shown to be susceptible to the virus and virus transmission.

With respect to pets, "we're targeting companion animals in communal residences with at-risk populations, such as nursing homes and assisted living facilities," Poulsen said. "There is a delicate balance of needing more information through testing and the limited resources and clinical implications of positive tests."

The study was supported by the U.S. National Institute of Allergy & Infectious Diseases and by the Japan Agency for Medical Research & Development.