On Sunday, the Bronx Zoo announced that a four-year-old Malayan tiger, Nadia, had contracted the coronavirus, the first wild animal in captivity to test positive in the world. In addition to Nadia, her sister Azul, two Amur tigers, and three African lions also developed symptoms for COVID-19. The zoo does not know who passed the virus to the cats, but says it was likely an asymptomatic or pre-symptomatic keeper.
“There is no one person who we’re suspicious of, and there is no person who worked with the cats who’s tested positive,” the zoo’s chief veterinarian, Paul Calle, said on Tuesday. Keepers don’t go into the enclosures with tigers, but in the course or caring for the dangerous animals they do interact with them through barriers, Calle said.
In late March, Nadia, who lives in the zoo’s Tiger Mountain exhibit, appeared to be sick. She wasn’t finishing her daily allotment of meat, and had developed a dry cough. What does a dry cough sound like from a tiger? “Like a dry cough from a tiger,” said Calle. “A cough like any other animal would have. There was some wheezing, no sneezing, and no discharge from the eyes or nose.”
Nadia was anesthetized while the zoo’s vets ran a battery of standard tests on her, which included x-rays, an ultrasound, and blood tests. They also tested her for respiratory diseases common in cats, then sent samples for coronavirus testing to veterinary labs at Cornell University, the University of Illinois, and the National Veterinary Services Laboratory. (Tiger tests differ from human tests; Nadia did not receive preferential treatment like politicians, influencers, or NBA players.) By the time the test came back positive, other cats had begun displaying similar COVID-19 symptoms.
“None of the cats got particularly sick,” Calle said, “and we expect them all to make full recoveries.”
The zoo has been closed to the public since March 16, but a staff of about 300 essential employees continues to care for the park’s 6,000 animals. Keepers now practice social distancing with the big cats, no longer getting close to the felines when feeding them treats. Though no other animals have contracted the coronavirus, Calle is most concerned about the zoo’s primates, especially gorillas, as they are closely related to people. “They’re susceptible to the common cold and the cold sore virus and influenza,” said Calle. “What we had not anticipated was that this infection would spread to the cats.”
Zoo pathologists around the world are constantly monitoring animals for infectious diseases, and some have gone so far as to argue that captive animals provide a public good, as sentinels for emerging zoonotic threats. “Zoos in large metropolitan areas are perfect long-term epidemiological surveillance sites,” said Tracey McNamara, a professor of pathology at Western University of Health Sciences College of Veterinary Medicine.
In the summer of 1999, McNamara was the Bronx Zoo’s chief pathologist when she noticed neurologically-damaged crows dropping into exhibits at the zoo. McNamara soon realized that the virus killing crows — and a pair of captive-bred Chilean flamingos at the zoo — was likely the same mysterious form of encephalitis hospitalizing New Yorkers at the time. What was eventually identified as West Nile virus spread to animals throughout the zoo, including a cormorant, an Asian pheasant, rhinoceroses, snow leopards, and elephants.
It is still unclear which animals are vulnerable to the coronavirus. Ferrets may pass the virus to each other. A domestic cat in Belgium has reportedly tested positive for COVID-19, and initial studies have found that the virus reproduces in dogs, pigs, chickens, and ducks. A bat-infected wild animal is thought to have spread the virus to the first human, but there is no evidence to suggest that domestic animals can pass the virus to humans.
“We know that coronavirus can be found in animals, now the only question is which ones,” said MacNamara. “We’ve had nothing but announcements saying, ‘Nothing to see here! Nothing to be worried about!’ I heard the same thing during West Nile virus. The bottom line is you need facts. Until the studies are performed, you can’t make any statements.”