Nadia, a 4-year-old female Malayan tiger at the Bronx Zoo, has tested positive for COVID-19
Tiger at New York zoo tests positive for Covid-19
00:53 - Source: CNN
CNN  — 

The discovery that a tiger at the Bronx Zoo tested positive for the coronavirus has raised new questions about viruses that can jump from animals to humans to animals.

Jonathan Epstein

The topic is crucial since the virus that is causing the Covid-19 pandemic is believed to have originated in animals. For insight into the issue, we reached out to Jonathan Epstein, a veterinarian and disease ecologist who serves as vice president for science and outreach at the EcoHealth Alliance, a nonprofit organization, and has been involved in field and laboratory research on the sources of coronavirus. Epstein was the chief science adviser for the Smithsonian exhibit “Outbreak: Epidemics in a Connected World” which has been on display at the National Museum of Natural History since 2018.

CNN: A 4-year-old female Malayan tiger at the Bronx Zoo developed a dry cough and tested positive for the coronavirus. What possible significance could that have?

Jonathan Epstein: The confirmed tiger case is the first evidence of SARS CoV2 in a captive wildlife species – a big cat– and also suggests likely transmission from a person (likely one of the animal care staff).

This finding (other big cats at the zoo had signs of coughing, but weren’t tested) provides important information about what the disease might look like in big cats, and also lets other zoos know what could happen within their collection. To protect the health of the animals in the collection, it’s probably a good idea for zoos to have staff start wearing masks (homemade, not scarce N95s!) when working near the animals or in their enclosures.

There’s so much we don’t yet know about the virus that causes the Covid-19 pandemic, SARS CoV2. But we are learning a tremendous amount from the human epidemiological data that comes in from the US, China and every other country.

Similarly, we know little about which other species may be susceptible to infection and how this virus affects other susceptible species. Recent studies, still not yet peer reviewed, provide evidence that domestic cats are susceptible to experimental infection with SARS CoV2 and that cats sampled in Wuhan after the outbreak started had antibodies against SARS CoV2, suggesting that cats may have been infected by owners with Covid-19.

CNN: Should people be concerned about passing Covid-19 to their pets or being at risk of getting the virus from them?

Epstein: There’s currently no evidence that pet cats have transmitted or can transmit SARS CoV2 to people. It’s clear that the Covid-19 pandemic is driven by human-to-human transmission; however, there’s still a lot we don’t know, and the absence of evidence of a person being infected by a companion animal does not mean it can’t happen.

Pets do play an important role in our lives, particularly when we’re facing steep challenges such as this. The best guidance right now is from the US Centers for Disease Control and Prevention, which recommends that people who have pets and are sick should treat them like other family members and avoid contact with them.

CNN: How do coronaviruses jump from other animals to human beings?

Epstein: Prior to the emergence of SARS coronavirus in 2002, there were only four known coronaviruses in humans. None of them caused significant illness or mortality. SARS was our first known experience with a zoonotic (passed between animals and people) coronavirus – though we now know that the first four human coronaviruses also originated in animals – and one that caused both significant respiratory disease and had a case fatality rate of about 9%.

By comparison, Middle East Respiratory Syndrome (MERS) coronavirus emerged in 2012 and has a case fatality rate around 35%. There’s still uncertainty about the case fatality rate of Covid-19 because we don’t truly know how many are infected, but it’s tracking so far between 1-2%. It’s less severe than SARS or MERS, but much more easily transmitted among people.

Coronaviruses are a large family of viruses, and many mammals and birds have their own strains. Often, coronaviruses stay within a single species, but some, like SARS, MERS and now SARS CoV2 (the virus that causes Covid-19) have demonstrated the ability to infect several different animal species, including humans.

How they jump is an important question, and one which remains unanswered in terms of the current outbreak. We know that certain types of bats, called horseshoe bats, are the natural reservoir for SARS and related coronaviruses. We’ve found dozens of closely related viruses in horseshoe bats in China. Not all of them have the ability to infect humans.

Viruses, in general, have to enter a host’s cells and hijack the cell’s machinery to make more virus. To do this, each viruses must attach to a specific part of a host cell called a receptor, and then inject its genetic material (RNA in the case of coronaviruses) into the host cell for replication. It’s like a lock and key mechanism.

If the virus’s key doesn’t fit the host’s lock, it cannot bind to and enter the cell and can’t cause an infection. SARS CoV and SARS CoV2 both use the ACE2 receptor which is found in cells in the respiratory and GI tract of mammals, including people. We’re still learning which species are susceptible to SARS CoV2, but we know that bats, people, ferrets and cats are among them.