Soon after the new coronavirus started spreading around the globe, reports emerged of cases in animals — pet cats in Hong Kong, tigers in a New York City zoo and mink on farms in the Netherlands. Now researchers are urgently trying to discover which species can catch the virus, and whether they can pass it to people.

So far, there have been only two reported cases of animals — both mink — passing the virus SARS-CoV-2 to people. At the moment, the chance of contracting the disease from an infected animal is negligible compared to the risk of catching it from an ill person, say researchers.

But as the numbers of infected people fall and restrictions on movement ease, infected animals might have the potential to spark new outbreaks. Researchers are calling for extensive sampling of pets, livestock and wildlife to improve understanding of the risk.

The virus could be spreading undetected in some animals that we don’t know about, says Joanne Santini, a microbiologist at University College London. “We just don’t have enough data,” she says.

Several scientists worry that the virus could end up passing back and forth between animals and people. This could really frustrate efforts to control the pandemic, says Arjan Stegeman, a veterinary epidemiologist at Utrecht University in the Netherlands. “We need to take actions now to prevent that from happening,” he says.

At-risk animals

Roughly a dozen animals are known to be susceptible to the virus. Several species, including pet dogs and cats, captive lions and tigers, and farmed mink, almost certainly caught the virus from people. That probably means that related canids, felids and mustelids — the group that includes mink, weasels, badgers, martens and wolverines — could also be susceptible, but so far, no one has checked, says Jürgen Richt, a veterinary virologist at Kansas State University in Manhattan.

Hamsters , rabbits and common marmosets are also susceptible, according to laboratory experiments in which the animals were deliberately infected. Experiments in pigs, ducks and chickens show ² that they are not susceptible, but there have been no studies of other livestock animals, such as cows, sheep and horses. “If SARS-CoV-2 becomes established in wildlife or other species that have close contact with livestock, then this increases the possibility for interspecies transmission,” says Linda Saif, a virologist at the Ohio State University in Wooster.

More studies should assess the susceptibility of various species and whether they can infect other animals, says Richt. Cats, ferrets, hamsters ¹ and horseshoe bats were all able to pass the coronavirus to animals of the same species in the lab, and mink living in close quarters on Dutch farms have passed the infection between them.

But the fact that an animal can infect another of the same species doesn’t necessarily mean that it can infect people, says Saif. To assess this risk, researchers need a better understanding of the amount of virus to which a person needs to be exposed before they get infected, she says.

Animals that shed large amounts of virus and come into close contact with people should be watched closely, says Martin Beer, a virologist at the Federal Research Institute for Animal Health in Riems, Germany.

Mink farms

The infections at Dutch mink farms suggest that some animals can infect people. At least two dozen mink at four farms in the province of North Brabant have been infected with SARS-CoV-2, with some developing pneumonia and dying.

Looking at genomes from mink and people at two farms, Stegeman and his colleagues found that people working with the animals had probably passed the virus to some of them, which spread it to other mink. The results were posted on bioRxiv on 18 May ³ .

Further unpublished genomic analysis suggests that a person on one of the farms could have been infected by the mink, says Stegeman. That person seems to have gotten infected after starting to work with the animals, he says, so their infection probably came from the mink, rather than the other way around. The person’s viral genome was also more closely related to those found in the mink than to sequences from other infected people in the Netherlands, including those living near the farm.

But Saif, who hasn’t seen the genomes, says it is very difficult to prove direction of transmission. Finding closely related viruses, combined with other circumstantial evidence, such as the timeline of exposure and disease emergence can help, but “it will be challenging to directly prove animal-to-human,” she says.

Still, the possibility should not be excluded, says Soren Alexandersen, director of the Geelong Centre for Emerging Infectious Diseases in Australia. And the virus could be spreading undetected on other mink farms across Europe, North America and Asia, says Alexandersen.

Multiple hosts

It is not uncommon for pathogens to jump between species, making it difficult to control their spread. SARS-CoV-2 most likely originated in bats , but researchers do not know whether other animals were involved in its journey to people. The 2009 pandemic H1N1 influenza virus originated in pigs, jumped to people, spread worldwide and then passed back to pigs. The virus continues to circulate in the animals , where it has combined with other flu viruses to create new variants that have crossed over to people, says Stegeman.

Several scientists also worry that SARS-CoV-2 could jump back and forth between cats and people, because the animals often roam between households. Although cats can infect other felines, so far there have been no reports of cats infecting people.

Asisa Volz, a veterinary virologist at the University of Veterinary Medicine Hannover in Germany, plans to investigate whether cats spread the virus in a retirement home in Bavaria where residents separated from infected individuals still became ill. A cat there was found to have traces of RNA from the coronavirus, which suggests it could have been shedding virus as it roamed the facility. Volz and Beer will test the facility’s cats for antibodies against the virus and study the chronology of events to see whether the felines were a source of infection.

Stegeman also plans to test cats living with people who have had COVID-19 in the Netherlands. If it turns out that cats can pass the virus to people, he says, it would become even more difficult to control the spread.

“The establishment of a pandemic virus in animal populations can be critical and should be always taken into consideration,” says Beer.