The Ralph

Photo courtesy of Ralph Veterinary Referral Centre

A sudden spike in cases of myocarditis seen at this veterinary specialty hospital outside London spurred research into a potential link between the virus that causes COVID-19 and heart problems in cats and dogs.

The international veterinary community has been put on alert by new British research that has drawn a link between a more virulent form of COVID-19 and heart problems in cats and dogs.

Experts in infectious disease and the study's lead author caution, however, that the research has not established the coronavirus variant as a cause of the cardiac illness. The paper, not yet peer-reviewed, was posted March 18 on biorxiv.org, which distributes unpublished preprints of research in the life sciences.

"There will be a lot of skepticism about our study; I've got no doubt about that," Dr. Luca Ferasin, a cardiology specialist at the Ralph Veterinary Referral Centre in Marlow, near London, told the VIN News Service.

"We just want to send out a message to veterinarians that they should be aware of this potential link. So be vigilant."

The research came about when Ferasin and others at the referral center noticed a spike in cases of myocarditis, or inflammation of the heart muscle, among patients between December and February. None of the patients had a previous history of heart disease, and all displayed similar clinical signs, including acute onset lethargy, lack of appetite and shallow breathing.

Some of the pets' owners had experienced COVID-19 respiratory symptoms three to six weeks before the pets became ill, and had tested positive for the virus.

The incidence of myocarditis in dogs and cats at the center historically was zero to two per month, or 1.4% of all cases referred to the hospital. It jumped to 12.8% between December and February, including eight cases in January and 10 in February. The surge occurred at the same time a more transmissible mutation of SARS-CoV-2, the virus behind COVID-19 — discovered in the United Kingdom and known as B.1.1.7 — was spreading rapidly among humans.

Dr. Luca Ferasin

Photo courtesy of Ralph Veterinary Referral Centre

Dr. Luca Ferasin, a cardiologist, agrees with skeptics that research he co-authored does not prove SARS-CoV-2 causes heart problems in pets, but he believes the association bears monitoring.

To see if there was a connection, the veterinarians, assisted by virologists at the University of Montpellier in France, tested 11 animals with myocarditis for SARS-CoV-2. Six of the 11 tested positive for the B.1.1.7 variant: four cats and two dogs.

Three of the six positive tests were determined via rectal swab on animals that showed clear clinical signs, though all oropharyngeal and nasal swabs came up negative. The other three positive readings came via serology testing for antibodies carried out two to six weeks after the patients developed signs of myocarditis.

Of the five animals that tested negative, Ferasin said, two actually had tested positive for antibodies but the readings weren't particularly high, so they were counted as negatives as a precaution. He said the veterinarians would have liked to have tested more animals but were operating with limited resources.

The number of myocarditis cases identified at the clinic began to decline in March, down to six, correlating with a fall in human COVID-19 cases in Britain amid lockdowns and a vaccine rollout. "The coincidence of the curve of myocarditis cases and the curve of COVID is highly compelling, in my opinion, so I don't think we can just ignore it," Ferasin said.

Dr. Julian Drewe, a veterinary epidemiologist at the Royal Veterinary College in London, disagrees. "This paper does not present compelling evidence," he told VIN News via email. "It is not possible to infer causation here."

Drewe said the sample size was "very small" and noted that the animals were not all tested at the same time or at the same stage of clinical-sign progression. Nor were animals without heart disease tested for SARS-CoV-2 — so there was no comparison group.

Furthermore, Drewe said, "Animals were seen at a referral hospital, which means they are unlikely to be representative of the general pet population."

Dr. Scott Weese, an infectious diseases veterinarian at the University of Guelph's Ontario Veterinary College, also warned practitioners not to get carried away with the study's findings. At the same time, he said they shouldn't be ignored.

"Clinical observations of abnormal patterns are important," Weese wrote in his blog, Worms and Germs. "Most often, they end up being nothing remarkable — just random changes in normal events, or coincidences. Sometimes, though, they're an early warning that something is going on. So, they are worth investigating."

Weese said six of 11 tested animals having some degree of evidence of exposure to SARS-CoV-2 seems like a fairly high proportion but added that it's not far off numbers seen more broadly in animals from COVID-19-positive households.

"So this does not provide any evidence of a link between SARS-CoV-2 infection in pets and myocarditis," Weese said. "Nonetheless, I wouldn't dismiss it, and we should look into it more."