Although the exact origins of the COVID-19 pandemic aren't clear, most researchers estimate the virus made the jump from bats to pangolins before infecting humans. Now, scientists worry the virus could make the jump from humans back into wild animal populations.
If COVID-19 managed to infect and spread among wild animals, it could pose a threat to endangered species. As well, wild animal populations could serve as a reservoir for further virus evolution and a source of future human outbreaks.
So far, scientists have documented human-to-animal coronavirus spread on a mink farm and at the zoo, where several tigers and lions were infected.
At home, humans have transmitted the virus to domestic cats and dogs. Some semi-feral cats in Wuhan and the Netherlands have also tested positive for antibodies triggered by a coronavirus infection.
"There have not been any reports yet of actual wildlife being infected with [the coronavirus]," lead study author Sophie Gryseels told UPI in an email.
"We hope this is because it has actually not happened yet, but then again, there is not much surveillance going on of healthy wildlife for [coronavirus] infections, so if it had happened already, we might not know about it," said Gryseels, a biologist at the University of Antwerp in Belgium.
As well, infected animals have exhibited only mild symptoms, but Gryseels said it's possible the disease takes a more serious course among other animal species.
Though COVID-19 has yet to have grave consequences for animal populations, Gryseels suggests the threat of human-to-animal transmission is real and significant.
"We know of several mammal species that are about as susceptible to [the coronavirus] as humans are, like ferrets, mink, hamsters, North American deer mice, tigers and macaques and a few other species," she said. "When they are experimentally inoculated with [the coronavirus], or in some cases accidentally infected by human caretakers, the infection takes off easily, and they can further transmit the virus to co-housed animals."
Gryseels and her colleagues hope their paper will inspire caretakers, scientists and others who interact with wild and captive animals to take extra precautions.
Researchers suggest the same safety precautions that can help slow human transmission -- hand washing, mask wearing and social distancing -- can help prevent human-to-animal transmission.
For most people, the risk of human-to-animal COVID-19 transmission is minimal.
"Luckily for us the mammal species that humans probably have the most interactions with in total in global terms, and would thus seem to be most likely to catch the virus if they were biologically susceptible, are house mice and brown and black rats -- who luckily don't seem to be susceptible," Gryseels said.
While researchers expect humans to eventually develop herd immunity against COVID-19, via a combination of infection-triggered immunity and vaccination, other animal species might not be so lucky.
The combination of susceptibility and short lifespan could leave some species especially vulnerable to COVID-19, including the North American deer mouse, the bank vole in Europe, macaques in Asia and stray cat populations all over the world.
"The main worry then is that, like in human populations, the virus could just continue to spread without stop," Gryseels said. "Especially in populations where animals have a short life span and high reproductive rate, such as in rodents, we worry that the virus can persist for a long time, as there are continuously new, susceptible animals being born that are naive for the virus."