How common is Covid in animals and what are the risks to humans?
Two studies showing high levels of Covid-19 infection among wild deer in the United States have renewed concern that the virus is spreading through animal populations.
The results, released this month, come as more reports of Covid in pets and captive animals emerge. Lincoln Children’s Zoo in Nebraska lost three rare snow leopards to the disease earlier this month.
Although the Sars-Cov-2 virus causes few or no symptoms in most non-human animals, some researchers fear that wildlife may become a reservoir of infection – and possibly viral mutation – that can be transmitted back to humans. It can also threaten endangered species.
How widespread is Covid in animals?
The World Organization for Animal Health, known as the OIE, has recorded 598 Covid outbreaks in animals affecting 14 species in 30 countries by the end of October. Almost everyone has involved animals in captivity in close contact with humans – with farmed mink in Denmark being hardest hit.
No one knows how widespread Covid is in wildlife because there have been few tests. The two new studies of white-tailed deer in the United States are the first to show widespread infection in wild populations.
A paper published in PNAS by researchers at the U.S. Department of Agriculture found antibodies to Sars-Cov-2 in 40 percent of the wild white-tailed deer in four U.S. states early in the year.
Another study, led by Penn State University and not yet published, detected the virus directly through PCR testing in 80 percent of samples taken in Iowa from the same species – the most widespread North American deer with an estimated 30 million individuals.
“We were all very surprised and amazed at the high percentage of deer testing positive,” said Katriina Willgert of the Cambridge University Disease Dynamics Unit, which participated in the Penn State study. “The genetic and geographical data suggest more transmission from infected humans and then widespread transmission from deer to deer.”
Which species are most susceptible to infection?
Sars-Cov-2 is derived from bats and may have been transmitted to humans via a still unidentified intermediary between animals. Combined evidence from natural transmission and laboratory tests suggests that the virus can infect most mammals, even though, unlike influenza, it does not infect birds.
Susceptibility varies significantly and unpredictably between species. “We are very fortunate that studies show that the main livestock – pigs, cattle and sheep – are quite resistant to infection, unlike white-tailed deer,” said Keith Hamilton, OIE’s head of preparedness and resilience. “It would be a disaster if they were also very receptive.”
Cats, especially large cat species such as lions, tigers and leopards, appear to be more susceptible than dogs, says Ken Smith, professor of pathology at the Royal Veterinary College London.
What can people do to prevent the spread of coronavirus between animals and humans?
Although the only clear evidence that coronavirus is transmitted from animals back to humans came during Denmark’s intensive outbreak among mink last year, experts advise people to avoid contact with pets while they or their animals show covid symptoms.
The discovery of widespread deer infection shows that hunters should take precautions such as wearing gloves when handling carcasses, said Alastair Ward, head of biological sciences at Hull University in the UK.
Vaccines are becoming available to protect animals from Covid. Zoos in the United States inoculate a growing menagerie of species with jabs developed specifically for non-human recipients. Vulnerable big cats are a favorite target.
Zoetis, the animal health company developed by Pfizer, has developed a vaccine based on Sars-Cov-2 spike proteins, which can be adapted to a wide range of species. Farmed mink are vaccinated to a significant extent, but Covid vaccines are generally not available for pets.
Can animals be a permanent reservoir of infection – and a source of new mutations?
U.S. studies of white-tailed deer have worried biologists. “Although experimental work suggests that infected deer tend to have no symptoms, wildlife transmission has significant consequences for human health,” said Graeme Shannon, a zoologist at Bangor University in Wales.
“The results raise concerns that deer could be a reservoir for Sars-Cov-2. Not only could this easily infect a large number of animals, but also, more worryingly, it would infect humans,” he added. “wildlife reservoirs can complicate our long-term efforts to combat and suppress the disease.”
A potential threat is that as the virus spreads in an animal reservoir, more dangerous strains can emerge and then move into humans, even if it is not a matter of course. Evolution that optimizes fitness for another species will not necessarily make the virus more effective at infecting humans. New varieties appeared in mink and spread to humans in November 2020, although none proved to be unusually transmissible or virulent in human populations.
The Sars-Cov-2 variants identified in the Iowa deer were the same as those circulating at that time in the human population. But there may not have been time for the virus to develop in these animals.
Another concern is whether Covid will disappear from the deer or continue to transmit indefinitely among them. Suresh Kuchipudi, lead author of the Penn State newspaper, appealed for more evidence.
“The study highlights the critical need to immediately implement monitoring programs to monitor Sars-Cov-2 spread within deer and other susceptible animal species and introduce methods to mitigate potential spills,” he said.
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