How widespread 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 concerns about the virus spreading through animal populations.

The findings, published this month, come as further reports of Covid in pets and captured animals appear. Lincoln Children’s Zoo in Nebraska lost three rare snow leopards due to the disease earlier this month.

Although the Sars-Cov-2 virus causes few or no symptoms in most non-human animals, some scientists fear that wildlife could become a reservoir of infection – and possibly a viral mutation – that could be transmitted back to humans. It could also threaten endangered species.

How widespread is Covid in animals?

The World Organization for Animal Health, known as the OIE, has recorded 598 outbreaks of Covid in animals affecting 14 species in 30 countries by the end of October. Almost all of them involved captured animals in close contact with humans – with farmed mink in Denmark being the hardest hit.

Map showing epidemics of Covid-19 in animals

No one knows how general Covid is in the wild because there has been little testing. The two new studies of white-tailed deer in the United States are the first to show widespread infection in wild populations.

One article published in PNAS by scientists at the U.S. Department of Agriculture found antibodies to Sars-Cov-2 in 40 percent of wild white-tailed deer across four U.S. states earlier this year.

A second study, led by Penn State University and not yet published, detected the virus directly by PCR testing in 80 percent of samples taken in Iowa from the same species – the most abundant North American deer with about 30 million individuals.

“We were all very surprised and confused by the high proportion of deer testing positive,” said Katriina Willgert of Cambridge University’s Disease Dynamics Unit, who participated in the Penn State study. “The genetic and geographic data suggest multiple transfusions of infected humans and then widespread deer-to-deer transmission.”

Which species are most susceptible to infection?

Sars-Cov-2 originated from bats and may have moved to humans through an as yet unidentified animal carrier. A combined rate of natural distribution and laboratory experiments indicate that the virus can infect most mammals, although unlike influenza it does not infect birds.

Carelessness varies considerably and is unpredictable between species. “We are very fortunate that studies show that the main farm animals – pigs, cattle and sheep – are quite resistant to infection, as opposed to white-tailed deer,” said Keith Hamilton, head of OIE on readiness and resilience. “It would be a disaster if they were also very susceptible.”

Cats, especially large cat species such as lions, tigers and leopards, appear to be more sensitive than dogs, said Ken Smith, a professor of pathology at the Royal Veterinary College in London.

Snow leopard
Snow leopard. Large cat species appear to be more sensitive to Covid than dogs © Cindy Ord / Getty Images

What can humans do to prevent the spread of coronavirus among animals and humans?

Although the only clear evidence of transmission of coronaviruses from animals back to humans came during Denmark’s intense outbreak among minks last year, experts advise humans to avoid contact with pets while they or their animals show Covid symptoms.

The discovery of a widespread infection among deer shows that hunters must take precautions such as wearing gloves when handling corpses, said Alastair Ward, head of biological sciences at Hull University in the UK.

Vaccines are available to protect animals against Covid. Zoos in the United States are inoculating a growing menagerie of species by stings developed specifically for non-human recipients. Vulnerable large cats are a favorite target.

Zoetis, the animal health company invented by Pfizer, has developed a vaccine based on pike protein Sars-Cov-2 that can be adapted for a wide range of species. Raised minks are inoculated on a large scale but Covid vaccines are generally not available for pets.

Could animals be a constant 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, transmission of diseases in wildlife has considerable implications for human health,” said Graeme Shannon, a zoologist at Bangor University in Wales.

“The findings worry that deer could be a reservoir of Sars-Cov-2. Not only could this easily infect many animals, but also, more anxiously, it would flow back to humans,” he added. “Any reinfection of wild reservoirs could hinder our long-standing efforts to fight and suppress the disease.”

A possible threat is that as the virus spreads within an animal reservoir, more dangerous strains could appear and then transfer into humans, although this is not a predicted conclusion. Development that optimizes suitability 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 within the Iowa deer were the same as those circulating at the time within the human population. But it may not have been time for the virus to evolve in these animals.

Another concern is whether Covid will disappear from the deer or continue to be transmitted between them indefinitely. Suresh Kuchipudi, lead author of the Penn State article, appealed for more evidence.

“The research underscores the critical need to urgently implement surveillance programs to monitor the spread of Sars-Cov-2 within deer and other acceptable wildlife species and to put in place methods to mitigate possible ebb,” he said.

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