Over the past year, coronavirus vaccines have gone into billions of human arms — and into the fuzzy haunches of an ark’s worth of zoo animals. Jaguars are getting the jab. Bonobos are being dosed. So are orangutans and otters, ferrets and fruit bats, and, of course, lions and tigers and bears (oh, my!).
Largely left behind, however, are two creatures much closer to home: domestic cats and dogs.
Pet owners have noticed.
“I get so many questions about this issue,” Dr. Elizabeth Lennon, a veterinarian at the University of Pennsylvania, said. “Will there be a vaccine? When will there be a vaccine?”
Technically, a pet vaccine is feasible. In fact, several research teams say that they have already developed promising cat or dog vaccines; the shots that zoo animals are receiving were initially designed for dogs.
But vaccinating pets is simply not a priority, experts said. Although dogs and cats can catch the virus, a growing body of evidence suggests that Fluffy and Fido play little to no role in its spread — and rarely fall ill themselves.
“A vaccine is quite unlikely, I think, for dogs and cats,” Dr. Will Sander, a veterinarian at the University of Illinois Urbana-Champaign, said. “The risk of disease spread and illness in pets is so low that any vaccine would not be worth giving.”
A positive Pomeranian
But not the last. A German shepherd in Hong Kong soon tested positive, too, as did cats in Hong Kong, Belgium and New York. The cases were exceedingly mild — the animals had few or no symptoms — and experts concluded that humans had spread the virus to the pets, rather than vice versa.
“To date, there hasn’t been any documented cases of dogs or cats spreading the virus to people,” Dr. Lennon said.
But the prospect of a pet pandemic sparked interested in an animal vaccine. Zoetis, a veterinary pharmaceutical company based in New Jersey, began working on one as soon as they heard about the Hong Kong Pomeranian.
“We figured, ‘Wow, this could become serious, so let’s start working on a product,’” Mahesh Kumar, a senior vice president at Zoetis who leads vaccine development, said.
By the fall of 2020, Zoetis had four promising candidates for a vaccine, each of which elicited “robust” antibody responses in cats and dogs, the company announced. (The studies, which were small, have not been published.)
But as vaccine development progressed, it became increasingly apparent that the infection of pets was unlikely to pose a serious threat to animals or people.
In one study of 76 pets living with people who had the virus, 17.6 percent of cats and 1.7 percent of dogs also tested positive. (Studies have consistently shown that cats are more susceptible to infection than dogs, perhaps for both biological and behavioral reasons.) Of the infected pets, 82.4 percent had no symptoms.
When pets do fall ill, they tend to have mild symptoms, which may include lethargy, coughing, sneezing, runny noses or diarrhea. The animals typically make full recoveries without treatment, although a handful of more severe cases do occur occasionally.
Moreover, there is no evidence that cats or dogs spread the virus to humans — and there are few signs that they readily transmit it among themselves. Stray cats, for instance, are much less likely to have antibodies to the virus than cats that live with people, suggesting that the animals are largely getting the virus from us, rather than from each other.
“It doesn’t look like cats or dogs would ever be a reservoir for this virus,” Dr. Jeanette O’Quin, a veterinarian at Ohio State University, said. “We believe that if there weren’t sick people around them, they would not be able to continue spreading it from animal to animal — it would not continue to exist in their population.”
Together, these factors convinced experts that a vaccine for pets was not necessary. In November 2020, the U.S. Department of Agriculture, which regulates veterinary medicines, said that it was not accepting any applications for cat or dog vaccines “because data do not indicate such a vaccine would have value.”
Keeping mink in the pink
But as the pet threat was receding, another problem was coming into focus: mink. The sleek, svelte mammals, which are farmed in large numbers, turned out to be highly susceptible to the virus. And not only were they dying from it, they were spreading it to each other and back to humans.
“I think that the situation in mink absolutely warrants a vaccine,” Dr. Lennon said.
The U.S.D.A. thought so, too, and in the same November notice in which the agency said it was not considering cat or dog vaccines, it declared itself open to applications for a mink vaccine.
Zoetis pivoted, deciding to repurpose one of its dog vaccines for mink ones. (Several other teams are also developing mink vaccines, and Russia has already approved a shot for all carnivores, including mink, and has reportedly started administering it to animals.)
Studies in mink are ongoing, but when word got out about Zoetis’s work, zoos came calling. Some of their animals — including gorillas, tigers and snow leopards — had already caught the virus, and they wanted to give the mink vaccine a whirl. “We got a huge number of requests,” Dr. Kumar said.
Zoetis, which decided to supply the vaccine to zoos on an experimental basis, has now committed to donating 26,000 doses — enough to vaccinate 13,000 animals — to zoos and animal sanctuaries in 14 countries.
The development means that many zoo-dwelling cats, like lions and tigers, are getting vaccinated, while their domestic cousins are not. In part, that’s because these species appear to be more susceptible to the virus; some have died after becoming infected, although the cause of death is often difficult to conclusively determine.
“The big cats seem to be getting sicker than the house cats,” Dr. Lennon said.
Moreover, zoo animals are exposed to many more people than the average house cat, and many are highly endangered.
“I don’t want to diminish anybody’s pets,” Dr. Sander said. “I have a cat myself. But I think a lot of those animals are high conservation status. They’re genetically very valuable. And so they want to try and provide the best protection possible.”
The cat vaccine calculus
Although the evidence so far suggests that the virus is not a major threat to pets, there is a lot left to learn, scientists acknowledge. It is still not clear how frequently infected humans pass the virus to their pets, especially because officials do not recommend routine testing for companion animals, and the virus may have health effects in pets that have not yet been identified.
In a paper published earlier this month, scientists raised the possibility that the Alpha variant, which was first identified in Britain, might cause heart inflammation in dogs and cats. The evidence is circumstantial, but the virus has been linked to the same problem in humans and the connection is worth exploring, experts said.
“We need to do more research in this area to find out if this is a real association,” Dr. O’Quin said.
There may be individual pets who are at especially high risk from the virus. Dr. Lennon and her colleagues recently identified an immunocompromised dog who appeared to become severely ill from the virus. Unlike most infected dogs, this one also shed high levels of the virus for more than a week.
“Of course, that’s one case, but it really does illustrate that Covid isn’t the same in all pets, just like it isn’t in all people,” Dr. Lennon said.
It is certainly possible that future research — or changes in the virus — could change the calculus on a pet vaccine. If the virus turns out to be more prevalent, virulent or transmissible in dogs or cats than is currently known, that would make the case for a vaccine more compelling, scientists said. The U.S.D.A. has said that it may re-evaluate its position if “more evidence of transmission and clinical disease” emerges in a particular species.
If that time comes, Zoetis is prepared to pick up where it left off with its pet vaccines, Dr. Kumar said. He said that if the company’s mink vaccine is licensed, veterinarians might be able to use it off-label in the event of an unexpected outbreak in cats or dogs.
Applied DNA Sciences, a New York-based biotech company, has also developed a promising cat vaccine “as a ‘just in case,’” James Hayward, the company’s chief executive, said. (Like Zoetis, the company, which is working in partnership with the Italian company Evvivax, is now more focused on a mink vaccine.)
For now, there are steps that pet owners can take to protect their animals. People who test positive for the virus should isolate away from their pets, if possible, or wear a face mask while caring for them.
And, of course, a vaccine for humans is now widely available in the United States. “The best way to prevent SARS-CoV-2 in our pets is to prevent the disease in people,” Dr. O’Quin said. “So please get vaccinated.”
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