A tiger at the Bronx Zoo tested positive for the new coronavirus, a first that experts said underscores how much remains unknown about the virus and how it affects animals.

Evidence suggests that COVID-19, the disease caused by the new coronavirus, may have first infected humans via transmission at an animal market in Wuhan, China. Veterinary experts said more research is needed to understand the threat of a pet infecting its owner, even if it appears low.

"This is an important case because it is a natural infection," said Karen Terio, chief of the Zoological Pathology Program at the University of Illinois' College of Veterinary Medicine. "But a tiger is different than a domestic cat, and I can't emphasize that enough."

In "the first case of its kind," a caretaker is believed to have transmitted the virus to Nadia, a 4-year-old Malayan tiger at the Bronx Zoo, the U.S. Food and Drug Administration said in a statement Sunday.

Nadia and several other tigers and lions at the zoo began showing respiratory symptoms, and the tiger was tested after meeting the necessary criteria for animal testing and getting approval from animal and public health officials, Terio said.

Leyi Wang, a veterinary virologist at the University of Illinois who studies coronaviruses in animals and received the test sample, said genetic tests showed the virus in the cat matched more than 99% to the virus in humans.

"Basically, it's the same virus," he said.

Can pets get the coronavirus?

Even before the confirmed case of COVID-19 in the tiger, the U.S. Centers for Disease Control and Prevention recommended that sick people limit contact with their pets if possible.

With tigers, "there's a degree of social distancing that you automatically do," Terio said. It's possible the virus was transmitted via respiratory droplets or through fomites, such as the animal's food, but the exact means of transmission is unknown, she said.

In Hong Kong, at least two dogs tested positive for the virus after close exposure with their owners who were sick with COVID-19.

"We don't know whether any pets in the U.S. have a similar coronavirus or not. So there's still more work needed to be done on the different animal species in terms of surveillance," Wang said.

Terio said, "If you think about the number of people in this country and around the world that have been infected with this virus and have gotten very ill with this virus, and if you think about the number of people who have pet cats, the fact that we're seeing the first case in a tiger, to me, suggests that there's probably some differences in how even the different cat species respond to the virus."

Terio explained that viral proteins and genetic differences in receptors that a virus uses to attack a person or animal differ between species, and those differences could cause the virus to be more severe in one species than another.

The ACE-2 receptor, or angiotensin-converting enzyme 2, is the enzyme in humans the virus uses as an entryway. ACE-2 receptors are similar in felines and humans, Terio said, but there are differences. And it's not the same among all cats.

"There's all sorts of different steps between getting exposed to a virus and developing disease that are going to depend on species," Terio said. "And a tiger is a different species than a domestic cat and in a completely different family than dogs, so there's a lot of potential differences and reasons for why you might see something in one species versus another." 

Could pets be infected but asymptotic, like some people?

This is unknown and more research is needed, Terio said.

Some early studies in China suggested it's possible that cats were infected but their immune systems fought off disease. "Whether that's applicable across species is unknown. We also don't necessarily know – maybe they did get mildly sick and then they recovered. We don't know," Terio said.

At the Bronx Zoo, Nadia, her sister Azul, two Amur tigers and three African lions all developed a dry cough but are expected to recover. The cats experienced some decreased appetite, though they have been otherwise healthy.

"It is not known how this disease will develop in big cats since different species can react differently to novel infections, but we will continue to monitor them closely and anticipate full recoveries," the zoo said in a statement. 

Other tigers and big cats at the zoo have not shown any symptoms, the zoo said.

Can pets give owners coronavirus?

There's been no evidence that suggests pets in the USA could spread the infection to people, but this also needs more research, Terio and Wang said.

Some initial research in China suggested that pangolins may be the animal that allowed the virus to cross over from bats to humans, but the exact source remains unknown.

Terio said that given how different those species are from most pets in the USA,  the level of concern around pets potentially giving their owners the virus is low.

"But that's what we know right now today, and that may change," she said.

In studying other coronaviruses in animals, Wang said there's less evidence of transmission from animals to humans, but "I do worry. It potentially could happen."

Can pets get tested? 

Just because the tiger at the Bronx Zoo was tested doesn't mean pets will be, too, Terio said.

"This isn't like we're able to test everybody's domestic cats," she said. "The testing is very specific, and it's very stringent rules as to who and what samples are able to be tested."

At the Bronx Zoo, only one of the sickened animals was tested because testing requires the animals be put under general anesthesia, which poses a risk for their health.

Wang said his lab, along with a lab at Cornell University, received the test samples, and both got the same positive result over the weekend. The tests were sent to the USDA National Veterinary Services Laboratories in Ames, Iowa, where the results were confirmed.

The CDC says on its website, "At this time, testing of animals for COVID-19 is not recommended."

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