What we know about the tiger with Covid-19 — and how the disease affects other animals

A tiger at the Bronx Zoo has tested positive for the coronavirus — joining a number of other animals around the world with Covid-19.

A tiger at New York City’s Bronx Zoo has tested positive for the novel coronavirus, officials at the US Department of Agriculture said Sunday, raising new questions about how the virus that causes Covid-19 spreads in animals, and whether other animals are at risk of becoming infected with the virus.

The Bronx Zoo’s tiger — a 4-year-old Malayan tiger named Nadia — is the first animal in the US and the first non-domesticated animal globally to have a confirmed Covid-19 case. At least two pets, a cat and a dog, were infected in Hong Kong; and a cat in Belgium is also believed to have had the virus. All of the pets were owned by people with confirmed Covid-19 cases.

Zoo officials believe the cat — as well as her sister, two Amur tigers (also known as Siberian tigers), and three African lions that are all exhibiting similar symptoms — may have been infected by a caretaker who has the virus but is asymptomatic, given that the zoo has been closed to the public since March 16.

“It’s the first time, to our knowledge, that a [wild] animal has gotten sick from Covid-19 from a person,” Paul Calle, chief veterinarian for the Bronx Zoo, said Sunday.

Calle added that his team took samples from Nadia that were sent to scientists and veterinarians at Cornell University, the University of Illinois, and the USDA National Veterinary Services Laboratory. All samples tested positive. Animal testing for Covid-19 requires a specialized protocol that differs from the testing done in humans. For example, testing a tiger includes placing the big cat under anesthesia; the complexity of the procedure led zoo officials to decide only one cat should be tested.

The new confirmed case is a reminder that although scientists have worked rapidly to understand the new coronavirus there is still much that isn’t known about how the virus can and cannot spread between species — and how it spreads among animals that aren’t human.

Here’s what we know — and don’t — about Nadia, and how Covid-19 spreads in animals.

What we know

• The Bronx Zoo announced a 4-year-old Malayan tiger, Nadia, tested positive for Covid-19 on Sunday
• Six other animals at the zoo — all large cats — are believed to have the coronavirus as well
• All seven animals have exhibited a dry cough and decreased appetites
• None of the animals exhibited other symptoms seen in humans, including fever or shortness of breath
• Other than the two symptoms, the zoo said Nadia and the other cats believed to be infected are “bright, alert, and interactive with their keepers”
• Neither of the dogs that tested positive in Hong Kong — a Pomeranian and a German Shepherd — exhibited any symptoms
• The infected cat in Belgium, like the zoo’s cats, did exhibit symptoms, including diarrhea, vomiting, and respiratory issues
• The Belgian cat’s case was confirmed using samples from its feces
• For her test, Nadia was sedated with general anesthesia; samples were taken from the back of her throat, nasal cavities, and trachea. Molecular testing confirmed she has Covid-19.
• The dogs and cat lived with owners with Covid-19 and are believed to have been infected by their owners
• Similarly, Nadia is believed to have been infected by a caretaker who was asymptomatic, or who cared for the cats before exhibiting symptoms
• Animals are thought to be able to infect humans with the coronavirus in some cases; as SARS expert Jonathan Epstein told Vox’s Brian Resnick, experts believe the coronavirus may have originated in an animal market in China and could have first appeared in bats
• A study from Chinese researchers at the Harbin Veterinary Research Institute in China — that has received some praise from scientists but that was done under conditions that did not approximate those of the real world, had a small sample size, and has not yet been peer reviewed — found there is community spread among cat populations
• The same study found the same is not true of dogs
• There have, however, been no confirmed cases of a pet or animal in captivity infecting a human
• Because of this, the US Department of Agriculture (USDA) has recommended coronavirus testing be done on a limited basis in animals
• Nevertheless, the Centers for Disease Control and Prevention (CDC) requests those who deal with pets and other animals wash their hands after interacting with them and that pet owners and caretakers ensure their animals maintain proper hygiene

What we don’t know

• Why one Amur tiger that lived with the Bronx Zoo’s other four infected tigers has not exhibited any symptoms — it is not clear whether this tiger is asymptomatic or does not have the coronavirus
• Why the Bronx Zoo’s other large cats have not exhibited symptoms
• Whether infected cats exhibit symptoms and dogs do not
• Whether cats really are more easily infected than dogs (as the Harbin study suggests) and if this is why a mixed-breed dog that lived with the German Shepherd tested negative for the virus
• Scientists like Linda Saif have noted other coronaviruses, like bovine CoV, can infect various species. But whether the coronavirus that causes Covid-19 can move from animal species to animal species (for instance, if it could move from a lion to an elephant) is not yet understood.
• It also is not yet clear whether transmission in animals must occur from humans — that is, whether the Bronx Zoo’s cats gave it to one another or if all exhibiting symptoms were infected by their caretaker
• Whether Covid-19 infections among animals make it more likely this coronavirus will become a seasonal one or whether animals might serve as carriers that could lead to a resurgence of the virus; this is something scientists are investigating
• Experts currently do not believe pets can transmit the virus to humans — but whether this is the case is currently poorly understood