They said pets couldn’t get the coronavirus, so how did a tiger test positive? Vets explain.

Nine lives ago (in February), when the world gawked at photos of Chinese cats wearing masks to ward off the coronavirus, veterinarians elsewhere were quick with reassurance that pets were unlikely to get sick.

The vets are still saying that. But with the news Sunday that a Bronx Zoo tiger had tested positive for the virus, the infected-pet scenario no longer seems quite so far-fetched.

In addition to Nadia, the 4-year-old Malayan tiger with a confirmed infection, six other big cats at the zoo in New York had dry coughs and were presumed infected — prompting the Philadelphia Zoo and others to reexamine their safety measures and scientists to reevaluate the available research. There isn’t much, though a recent Chinese study suggests that domestic cats can become infected if deliberately exposed to high doses of the coronavirus in a laboratory. (Dogs and birds, no).

In between fielding queries from researchers and other zoos, the Bronx Zoo also attempted to quell a social-media backlash from people who wondered why a tiger could get tested when some humans are left in the dark. Answer: Tests of Nadia’s nasal swabs were performed at two veterinary laboratories, which would not be allowed to process human samples.

And before you imagine a lurid sequel to the Netflix series Tiger King, rest assured that the big gal was sedated before someone inserted what must have been a very large Q-tip.

Bronx Zoo officials say the big cats got the virus from a human employee who was infected but had no symptoms. The animals — including Nadia’s sister Azul, two Amur tigers, and three African lions — all are expected to recover.

The tiger tale is a reminder that viruses can jump from humans to animals as well as the reverse — a biological phenomenon that has been going on for untold thousands of years. Which, after all, is how this whole thing got started back in December. Scientific consensus remains that the coronavirus jumped to humans at a live-animal market in Wuhan, China, likely with bats involved in some way, though the exact route of transmission remains unclear.

» READ MORE: How viruses 'jump' from animals to humans

A disease transmitted from animals to humans is called a zoonosis, which does not mean it happened in a zoo, though the two words have the same origin.

A disease that goes in the other direction is sometimes called reverse zoonosis, but to a biologist, that is a meaningless distinction. We’re all animals, and a virus does not care which way it goes, so long as it gets to invade the host’s cells and make millions of copies of itself.

Less clear is what conditions enable these transfers in each case, said Shelley C. Rankin, professor of clinical microbiology at the University of Pennsylvania School of Veterinary Medicine.

To cause an infection, a virus must have the right kind of proteins on its surface to latch on to a “receptor” — a binding site on the cells of the host animal. The coronavirus does this with a protein shaped like a spike, which, unfortunately for us, is very closely matched to a receptor in the human respiratory system.

But that kind of close match is not enough to cause illness, Rankin said. Once it gets inside, the virus also must be genetically equipped to take over the “machinery” of the host cell to make copies. For the process to continue after that, the new viruses have to be good at escaping and getting themselves transmitted to the next host.

Ordinarily, the odds are against all these steps lining up so that a virus can jump from one kind of animal to another. The receptors and other necessary structures in human cells are different enough from those in other animals.

Yet it happened with the tigers.

Karen A. Terio got the call from Bronx Zoo officials April 1. As the chief of the zoological pathology program at the University of Illinois at Urbana-Champaign, she had written a textbook chapter on illnesses in big cats, so zoo officials wanted to know if she and her colleagues could test one of their tigers for the coronavirus.

They could. Nadia’s samples arrived in Illinois two days later, and the test came back positive. Duplicate samples sent to Cornell University yielded the same result.

For additional confirmation, one of Terio’s colleagues left at 3 a.m. the next day to drive the tiger’s samples to a USDA veterinary lab in Ames, Iowa. Same outcome.

That means the receptor in the tiger’s airways was close enough to its human counterpart that the virus was able to latch on, Terio said.

Bronx Zoo officials say there is no evidence that the tigers could spread the virus further, though employees are wearing protective equipment to safeguard other animals. The zoo closed March 16, and Terio said that when it reopens, there is no reason that visitors should be worried about getting sick from a big cat.

Those fences along the animal enclosures? Talk about social distancing!

“You’re never going to be that close to these animals,” she said.

At the Philadelphia Zoo, staff started taking precautions a month ago with bats and primates, the species that were thought to be most likely to become infected with the coronavirus, chief operating officer Andy Baker said. That meant wearing protective equipment and eliminating close interactions between zookeepers and animals unless absolutely necessary.

Two weeks ago, similar measures were implemented for those working with the zoo’s big cats and its red river hogs. The news from the Bronx prompted yet another review, with protective measures added for those taking care of bears, red pandas, and otters, despite no evidence those species are at risk, he said.

What about pets? Millions of Americans have cats, and there are no confirmed reports in this country that anyone with COVID-19 has transmitted it to a pet, much less the reverse. A handful of cats may have been infected by their owners in China, but the evidence has not yet been published, said Rankin, the Penn veterinarian.

It could mean tigers are somehow more susceptible to infection than their housebound counterparts, but the evidence is not there yet.

“The problem with a lot of this is things are moving so quickly, we’re learning on the fly," Terio said. “We just don’t know.”

For now, the U.S. Centers for Disease Control and Prevention recommends that sick people limit contact with animals. Get a family member to take on pet-care duties, if possible. Wash your hands. And, while this goes against the instincts of many pet owners, the agency adds this advice for those who are infected:

“Avoid contact with your pet, including petting, snuggling, being kissed or licked, and sharing food.”

Don’t expect to get your pet tested, unless you have the virus and the animal is clearly sick. Even then, a test may be discouraged, as the chemicals used are the same as those needed to test humans, Rankin said.

“All of the resources really should be directed toward human medicine,” she said.

To date, the bulk of the evidence suggests that cats, dogs, and other pets are at little risk from the coronavirus.

And for one pair of zoo animals, the virus may have had a positive effect, the New York Times reported. At a zoo in Hong Kong, two giant pandas mated successfully this month, apparently for the first time in 13 years of living together. The zoo had been closed since late January, so there were no crowds of human onlookers for the momentous event.

Perhaps, the theory goes, the animals just needed a bit of privacy.