'Substantial' portion of pets catch SARS-CoV-2, study finds

Negative PCR tests may cause underestimate of human-to-pet transmission

Published: September 17, 2020
By Lisa Wogan

Photo courtesy of Dr. Dorothee Bienzle
Dr. Dorothee Bienzle, a veterinary pathologist at the University of Guelph's Ontario Veterinary College, is a member of a research team that has found that pet cats and dogs may be more apt to catch the virus that causes COVID-19 from their owners than was previously understood.

The modest tally of cats and dogs that have tested positive in the U.S. for SARS-CoV-2, the virus that causes COVID-19, may not reflect the number of actual cases, judging from preliminary findings of a new study from the University of Guelph's Ontario Veterinary College, which suggest that a "substantial proportion" of pets can catch the virus from infected people.

The reason for the potential undercount is that the type of test predominantly used to detect the novel coronavirus, a technique called polymerase chain reaction (PCR), can miss cases that other tests will pick up, depending on when the infection was active, the researchers found.

In the Guelph study — which is not yet published or peer-reviewed but will be presented as an abstract next week at the European Society of Clinical Microbiology and Infectious Diseases (ESCMID) virtual conference on coronavirus disease — scientists detected antibodies to SARS-CoV-2 in seven out of eight cats and two out of 10 dogs.

"The evidence is enough to make us readjust our thinking about pets," Dr. Dorothee Bienzle, one of three researchers on the study, told the VIN News Service.

An official count of animals that have tested positive for the novel coronavirus in the United States, kept by the U.S. Department of Agriculture Animal and Plant Health Inspection Service, includes 21 cats and 17 dogs. The method of diagnosis in the majority of cases was by PCR.

Risk factors, susceptibility and clinical features of SARS-CoV-2 infection in animal species are not well understood. The University of Guelph study is one of at least four investigations in North America looking at the prevalence of SARS-CoV-2 in pets.

In the Guelph study, people in the area who owned a cat, dog and/or ferret and had a diagnosis of SARS-CoV-2 infection or symptoms consistent with COVID-19 within a two-week period, were invited to have their pet’s nose, throat and rectum swabbed for testing. These samples were tested for active infection using PCR. For pets in households in which people had passed the two-week window of suspected infectiousness, researchers collected a blood sample to test for antibodies, which take time to develop following exposure to a pathogen.

Researchers swabbed samples from 17 cats, 18 dogs and one ferret. All PCR results were negative except in the case of one cat, whose results were indeterminate.

Blood samples were taken from eight cats and 10 dogs. Those tests, known as serology, indicated the presence of antibodies in seven cat samples, including from the cat with an inconclusive PCR result. Two dogs also tested positive for SARS-CoV-2 antibodies.

Every cat with positive antibody results was reported to have clinical signs associated with COVID-19 around the time of its owner's infection, according to Bienzle, a professor of veterinary pathology. Two were diagnosed with pneumonia. One of the antibody-positive dogs was reported to have had an episode of respiratory disease. All of the pets recovered.

Owner surveys showed the cats in the study spent much more time in close contact with infected people, Bienzle said, which may explain at least partly why the study found more cats with antibodies than dogs.

The study population is small, Bienzle said, because of low human transmission rates in the study area. And because the study group is small, the researchers caution against extrapolating their findings to the general population in terms of the proportion of animals that are or have been infected.  Nevertheless, “these preliminary results suggest that a substantial proportion of pets in households of persons with COVID-19 may end up developing antibodies,” she said in a press release about the research.

The same gap the researchers found between PCR and antibody testing in pets exists in testing of human patients. Bienzle said PCR tests in humans are estimated to yield 30% false negatives because of the timing issue.

Due to the narrow window during which an active infection in pets can be detected — generally considered to be a matter of days — the researchers recommend using serology to assess human-to-animal transmission.

As for keeping pets safe from the virus, Bienzle recommends taking the same precautions that are advised for humans: People known to have the virus or who have symptoms consistent with an infection should keep away not only from other people, but from pets.

While COVID-19 is well-known for its potential lethality in people, its potential to kill cats and dogs is unknown. 

Another unanswered question is whether the virus can move from dogs or cats to humans. A study of infections at 16 mink farms in the Netherlands, also to be presented at the ESCMID conference, describes "the first animal to human transmissions of SARS-CoV-2 in mink farms."

As infection rates are low in Canada, the Guelph research team is shifting the scope of its work to include testing for SARS-CoV-2 antibodies in animals beyond those in COVID-19 positive households, such as populations of stray cats.

Three other university-based studies of pets living in COVID-19-positive households began with PCR tests and planned to add serology later.

Dr. Sarah Hamar, who leads the COVID-19 & Pets Research team at Texas A&M University College of Veterinary Medicine & Biomedical Sciences, said the group began collecting samples in mid-June. She told VIN News by email that the team has results for about 80 animals to date "and indeed, we are seeing more antibody-positive animals than PCR-positive animals," similar to the Guelph findings

Researchers at the University of Washington Center for One Health Research and the Washington Animal Disease Diagnostic Laboratory at Washington State University; and the Cummings School of Veterinary Medicine at Tufts University confirmed they are doing antibody tests now but do not yet have results to release.

The commercial veterinary diagnostic laboratories Idexx, Antech and Zoetis began in the spring to offer veterinarians PCR tests for their patients.

Asked whether they offer serology testing, as well, Zoetis spokesperson Christina Lood told VIN News by email, "Zoetis has developed a test to detect SARS-CoV-2 antibodies in animals that is currently in validation and not being offered commercially at this time."

She added, "We are prepared to help our veterinary customers conduct diagnostic testing for SARS-CoV-2 in animals should the USDA and CDC [U.S. Centers for Disease Control and Prevention] recommend routine, commercial testing."

Antech and Idexx were not able to respond to questions by press time.

In related news, Applied DNA Sciences, Inc., a biotechnology company based in New York, announced yesterday that it expects to begin a clinical trial on a vaccine to prevent SARS-CoV-2 infections in cats.

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