When a tiger tests positive for the novel coronavirus, the immediate question is: What about other cats?

Nadia, a four-year-old Malayan tiger who had a dry cough and a slight loss of appetite, tested positive for the virus that has caused a human pandemic, the Bronx Zoo reported on Sunday.

She is doing well, according to Dr Paul Calle, the Bronx Zoo’s chief veterinarian. So are three other tigers and three lions that show the same symptoms. And, he said, neither Nadia’s infection nor early scientific reports from China of infections among domestic cats should make cat owners fear for their pets, or fear that the cats may pass the virus to humans.

“None of them actually ever acted terribly sick,” Dr Calle said on Monday of the zoo’s infected cats. But there are many respiratory ailments specific to cats, and the zoo anaesthetised Nadia, took samples and sent them for testing to veterinary colleges at Cornell and the University of Illinois, and then to the National Veterinary Services Laboratory, for confirmation. The results came back positive.

The test is not the same as the one used for humans, Dr Calle said, so testing the tiger did not interfere with human testing. “You cannot send human samples to the veterinary laboratory, and you cannot send animal tests to the human laboratories,” he said, “so there is no competition for testing between these very different situations”.

Dr Calle noted that there had been several experiments in which domestic cats were inoculated with large amounts of the coronavirus, but “that does not replicate what is happening in people’s homes around the world”. The amount of virus the cats were given, directly into the nose, was quite high.

He added, “If cats were generally susceptible, there would have been lots of reports in the preceding months about that.”

Karen A. Terio is the chief of the Zoological Pathology Program at the University of Illinois veterinary college, where tests for the tiger were done. “Given the number of people in this country that have been infected with the virus and have become ill, and the number of people in this country that own domestic cats,” she said, “it seems fairly improbable that cats are an important source of the virus for people if the first case we’re diagnosing it in is a tiger.”

Confirmed human cases are well over one million worldwide, and they are still increasing. There has been one report from Belgium of a cat that had symptoms of the coronavirus, but the report was unclear, Dr Calle said. Hong Kong also reported that one cat tested positive for the coronavirus.

A study of feral cats in Wuhan, not yet peer reviewed, showed that some had antibodies to the coronavirus, indicating some level of exposure to the virus and some response by their immune systems. But the cats were not ill when tested.

None of this is too surprising, according to Dr Jonathan Epstein of the environmental health non-profit EcoHealth Alliance, who said laboratory experiments showed that cats were susceptible to infection with Sars, another coronavirus.

However, Dr Calle said it was important to keep things in perspective. “There’s no evidence anywhere, other than the initial spillover, that any animal has infected any person anywhere,” he said.

The US agriculture department, the World Organisation for Animal Health and the American Veterinary Medical Association all say on their websites that there is so far no evidence that domestic animals can pass on an infection to people. But they all advise that people who are sick should take the same precautions about contact with their pets that they would with humans.

“There’s still a lot we don’t know,” Dr Terio said. “We’re all trying to play catch-up and learn about this virus in real time, as things are happening.”

Dr Epstein emphasised that it was still unclear what level of infection cats might have and whether they might transmit it to one another, which has been suggested.

He said the worldwide pandemic was being driven by human-to-human transmission, but advised, as other experts do, to “treat cats as other family members.” “There’s no evidence yet that cats can transmit this virus to people,” he said.

“But you don’t want to take this chance in the absence of information.”

Scientists at various labs are looking at animal susceptibility, both in terms of pets and with an eye to what animals could be used in laboratory studies. The same preliminary, unreviewed study that found cats susceptible in the laboratory also found that the virus reproduces poorly in dogs, pigs, chickens and ducks.

Among animals that may be used in laboratory tests, the novel coronavirus infects genetically engineered mice as well as some monkeys. Chimpanzee sanctuaries in the US have stopped tours and reduced staff members’ interactions with their animals in case apes may be vulnerable, too.

Ferrets are yet another potential laboratory animal. A report accepted for publication in Cell Host & Microbe documents that ferrets both become infected and pass the virus on to one another, showing some symptoms similar to those of humans, such as a fever, lethargy and coughing. All the animals recovered, however.

Jae Jung, a microbiologist at the Keck School of Medicine at the University of Southern California, was one of the authors on that paper. He said that mice are likely to be on the front lines of testing, particularly for new drugs or vaccines.

Established drugs or vaccines that are similar to previous ones used in humans may go straight to human trials.

In genetically engineered mice, the virus appears in all cells, and the symptoms of the mice are not similar to those of humans. Monkeys, Dr Jung said, are close to humans, but they can be used only in small numbers.

Ferrets are particularly useful, because, like monkeys, they do not need to be genetically engineered, but they are easier to raise in a lab than monkeys and the structure of their breathing system is similar in some ways to that of humans.

They become infected in the lungs, as humans do.

Ferrets have been used in influenza research and in research on Sars, partly because they cough and, Dr Jung found, can pass the virus to one another. They may be useful to study how the virus is transmitted.

Dr Jung said the ferrets showed a fever and lethargy and “occasionally coughing”. Around Day 10 or 12 of infection, he said, “they all recovered”.

The primary scientific importance of understanding how the virus acts in animals is to learn more about it for human prevention and treatment. But the knowledge can also have an impact on the animals themselves.

Public health agencies and those concerned with animal welfare worry about a backlash against pets if people are worried that their animals might become infected.

The World Organisation for Animal Health urges people not to turn against their pets, saying, “There is no justification in taking measures against companion animals which may compromise their welfare.”

Although shelters in the US reported an increase in adoptions as many states and localities required social isolation, Meredith Ayan, the executive director of the animal rescue non-profit SPCA International, said her organisation had received anecdotal reports of increased numbers of stray dogs in Italy and Hubei Province, in China.

None of the reports were from systematic surveys, and it was not clear what the reason for the increase might have been. Owners could have been in hospitals or died. People may have lost work and been unable to feed their animals.

As for the big cats at the Bronx Zoo, Dr. Calle said, “All of the tigers and lions only had mild illness and they’re all showing progressive signs of recovery, and we’re expecting them to make a full recovery.”