Over the past year, coronavirus vaccines have invaded billions of human arms – and the blurry butts of zoo animals on an ark. Jaguars get the jab. Bonobos are dosed. Likewise orangutans and otters, ferrets and fruit bats and of course lions and tigers and bears (oh, my God!).
Largely lagging behind, however, are two creatures that are much closer to home: domestic cats and dogs.
Pet owners have noticed.
“I get so many questions about this,” said Dr. Elizabeth Lennon, University of Pennsylvania veterinarian. “Will there be a vaccine? When will there be a vaccine? “
Technically, a pet vaccine is feasible. In fact, several research teams say they have already developed promising vaccines for cats or dogs; the recordings zoo animals receive were originally made for Dogs.
But vaccinating pets just isn’t a priority, experts said. Although dogs and cats can become infected with the virus, increasing evidence suggests that Fluffy and Fido play little to no role in its spread – and rarely become ill themselves.
“I think a vaccine is pretty unlikely for dogs and cats,” said Dr. Will Sander, a veterinarian at the University of Illinois Urbana-Champaign. “The risk of disease and disease spreading in pets is so low that any vaccine would not be worthwhile.”
A positive Pomeranian
In February 2020, a woman in Hong Kong was diagnosed with Covid-19. Two other people in her home soon tested positive for the virus, as did one unexpected household member: an elderly Pomeranian. The 17-year-old dog was the first known pet to contract the virus.
But not the last. A German shepherd dog in Hong Kong soon tested positive, as did cats in Hong Kong, Belgium and New York. The cases were extremely mild – the animals had few or no symptoms – and experts concluded that humans had transmitted the virus to the pets, not the other way around.
“To date, there have been no documented cases of the virus being transmitted to humans by dogs or cats,” said Dr. Lennon.
But the prospect of a pet pandemic sparked interest in an animal vaccine. Zoetis, a New Jersey-based veterinary drug company, started working on one when they heard about the Hong Kong Pomeranian.
“We thought, ‘Wow, this could be serious so let’s start working on a product,'” said Mahesh Kumar, Zoetis senior vice president who leads vaccine development.
By autumn 2020, Zoetis had four promising vaccine candidates, each of which produced “robust” antibody responses in cats and dogs, the company said. (The studies, which were small, were not published.)
However, as vaccine development continued, it became increasingly clear that infection in pets was unlikely to pose a serious threat to animals or humans.
In a study of 76 pets who lived 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 prone to infection than dogs, possibly for both biological and behavioral reasons.) Of the infected pets, 82.4 percent had no symptoms.
When pets get sick, they are prone to mild symptoms such as lethargy, coughing, sneezing, runny nose, or diarrhea. The animals usually recover completely without treatment, although a handful of more severe cases occasionally occur.
In addition, there is no evidence that cats or dogs transmit the virus to humans – and there is little evidence that they can easily transmit it to one another. Stray cats, for example, are much less likely to have antibodies to the virus than cats who live with humans, suggesting that the animals get the virus mostly from us and not from each other.
“It doesn’t look like cats or dogs will ever be a reservoir for this virus,” said Dr. Jeanette O’Quin, a veterinarian at Ohio State University. “We believe they couldn’t spread it from animal to animal if there weren’t any sick people around them – it wouldn’t continue to exist in their population.”
Together, these factors have convinced the experts that a pet vaccine is not necessary. In November 2020, the US Department of Agriculture, which regulates veterinary drugs, said it was not accepting applications for vaccines for cats or dogs “because the data does not suggest that such a vaccine has any value.”
Keep mink in pink
But as the pet threat receded, another problem came into focus: mink. The slender, slender mammals that are bred in large numbers have been found to be very susceptible to the virus. And not only did they die from it, they spread it among themselves and back to people.
“I think the situation in mink absolutely warrants a vaccine,” said Dr. Lennon.
So did the USDA, and in the same November notice in which the agency stated it was not considering cat or dog vaccines, it said it was open to applications for a mink vaccine.
Zoetis turned and decided to use one of his canine vaccines on mink. (Several other teams are also developing mink vaccines, and Russia has already approved vaccination for all carnivores, including mink, and has reportedly started giving it to animals.)
Studies of mink are ongoing, but when Zoetis’ work became known, zoos called. Some of their animals – including gorillas, tigers, and snow leopards – had already contracted the virus, and they wanted to give the mink vaccine a whirl. “We received a large number of inquiries,” said Dr. Kumar.
Zoetis, which decided to experimentally deliver the vaccine to zoos, has now committed to donating 26,000 doses – enough to vaccinate 13,000 animals – to zoos and wildlife sanctuaries in 14 countries.
The development results in many zoo cats, such as lions and tigers, being vaccinated while their domestic cousins are not. This is in part because these species appear to be more susceptible to the virus; some have died from infection, although the cause of death is often difficult to determine clearly.
“The big cats seem to get sicker than the house cats,” said Dr. Lennon.
In addition, zoo animals are exposed to far more people than the average domestic cat, and many are at great risk.
“I don’t want to belittle anyone’s pets,” said Dr. Sander. “I have a cat myself. But I think a lot of these animals are in high conservation status. They are very valuable genetically. And that’s why they want to try to offer the best possible protection. “
The cat vaccination calculus
While evidence so far suggests the virus isn’t a major threat to pets, there is still a lot to learn, scientists admit. It is not yet clear how often infected people transmit the virus to their pets, especially because officials don’t recommend routine tests for pets and the virus may have health effects in pets that have not yet been identified.
In a paper released earlier this month, scientists raised the possibility that the alpha variant, first identified in the UK, could cause heart infections in dogs and cats. The evidence is cumbersome, but the virus has been linked to the same problem in humans, and the link is worth investigating, experts said.
“We need to do more research in this area to see if this is a real association,” said Dr. O’Quin.
There may be individual pets who are particularly at risk from the virus. Dr. Lennon and her colleagues recently identified an immunocompromised dog that appeared to be seriously ill with the virus. Unlike most infected dogs, this one also sheds high amounts of the virus for more than a week.
“Of course that’s a case, but it really shows that Covid is not the same in all pets, just as it is not the case with all people,” said Dr. Lennon.
It is certainly possible that future research – or changes to the virus – could alter the calculations of a pet vaccine. If the virus turns out to be more common, virulent, or transmissible in dogs or cats than is currently known, it would make the case for a vaccine more compelling, scientists said. The USDA has announced that it will reassess its position if “more evidence of transmission and clinical disease” emerges on a particular species.
When that time comes, Zoetis will be ready to pick up where it left off with its pet vaccines, said Dr. Kumar. He said that if the company’s mink vaccine is licensed, veterinarians may 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 “just in case,” said James Hayward, the company’s CEO. (Like Zoetis, the company that works with Italian company Evvivax is now more focused on a mink vaccine.)
At the moment there are measures that pet owners can take to protect their animals. People who test positive for the virus should isolate themselves from their pets whenever possible or wear a face mask while taking care of them.
And of course, human vaccine 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 humans,” said Dr. O’Quin. “So please get vaccinated.”