Late last year, as the coronavirus surged across the United Kingdom, Dr. Luca Ferasin and his colleagues started noticing an uptick in patients with symptoms of myocarditis, or heart inflammation.
The condition is a rare side effect of the mRNA Covid-19 vaccines, most commonly seen in men under 30. It can also be caused by infection with the virus itself.
But these patients weren’t humans; they were cats and dogs.
“These were dogs and cats that were depressed, lethargic, they lost appetite,” said Ferasin, a veterinary cardiologist at The Ralph Veterinary Referral Centre in Buckinghamshire, England. “And they had either difficulty breathing because of accumulation of fluid in their lungs due to the heart disease, or they were fainting because of an underlying abnormal heart rhythm.”
Before December, about 1.5 percent of pets referred to The Ralph were diagnosed with myocarditis, he said. But in the period between December and March, that number jumped dramatically, increasing to 12.5 percent of pets with confirmed myocarditis.
Ferasin and his colleagues later found out that many of the pets’ owners had either tested positive for Covid or had symptoms of the disease within three to six weeks of their pets becoming ill. That information, coupled with the fact that the timing coincided with the surge in cases driven by the alpha variant of the virus in the U.K., prompted the researchers to test the pets for SARS-CoV-2.
Ferasin detailed the rise in Covid-induced myocarditis cases in pets in a report published Friday in the journal Veterinary Record.
Of 11 animals, two cats and one dog tested positive for the alpha variant of the virus, and two additional cats and an additional dog tested positive for Covid antibodies. The remaining five animals tested negative for antibodies and the virus. None of the animals tested had symptoms of a respiratory infection or any other typical signs of Covid, but all of them had myocarditis, Ferasin said.
Because The Ralph only sees cardiac patients, the researchers can’t say whether dogs and cats may develop typical Covid symptoms in other cases of infection.
It’s also unknown if veterinarians in general practices are seeing more cases of myocarditis caused by SARS-CoV-2, said Margaret Hosie, a veterinary virologist at the MRC-University of Glasgow Centre for Virus Research.
But maybe that’s because those vets don’t know it’s a possibility, said Hosie, who was not involved with the research. Reports like this will help general practice vets become aware of Covid-induced myocarditis in pets, so they’ll know to ask about Covid exposure and test for it.
All of the pets in this study recovered after supportive treatment with supplemental oxygen and diuretics to help remove fluid from the lungs, with the exception of one cat with persistent abnormal heart rhythm, in which case the owners decided to euthanize the animal. None of the animals were treated with antiviral medications, Ferasin said.
There have been other cases of pet cats and dogs around the world testing positive for other variants of Covid, including the delta variant, but there’s no evidence to date that the other variants cause similar heart issues in pets. In addition, Ferasin said the rate of pets with myocarditis referred to The Ralph has returned to its pre-Covid level of 1 to 2 percent.
As a precaution, Ferasin and Hosie both advised pet owners with Covid to avoid contact with their pets, just as they would with other humans.
Download the NBC News app for full coverage of the Covid-19 pandemic
“If it is not possible to get someone else to look after their pet, they should consider wearing a mask when preparing their food to minimize the likelihood of infecting them,” Hosie said.
Several studies to date show the virus is transmitted from people to their cats and dogs, but not vice versa. “So, people shouldn’t panic” if their pets start showing signs of illness, Ferasin said.
So far, it seems Covid doesn’t cause severe problems in animals; most recover quite quickly, Hosie said.
Nonetheless, it’s important to study Covid in pets because it’s possible they could be a viral reservoir that allows the virus to mutate in a way that causes more severe disease in humans, she added.
“Obviously we’re focused on preventing human-to-human transmission just now, because that’s crucial,” she said. “But if we were to take our eye off other species, we could be storing up problems in the future.”