COVID-19 is more prevalent in animals than we thought

We view COVID-19 as a human pandemic, but it’s much more than that. The disease-causing virus, SARS-CoV-2, can infect a wide and growing range of animals, captive and wild.

So far, the virus has been detected in more than 100 domestic cats and dogs, as well as tigers, lions, gorillas, snow leopards, otters and spotted hyenas in captivity, according to the US Department of Agriculture. Zoo staff in the United States have recorded a single positive case in a binturong, coati, cougar, domestic ferret, fishing cat, lynx, mandrill and squirrel monkey.

In the United States, only three wildlife species – mink, mule deer and white-tailed deer – have tested positive, according to the USDA. Cases have been detected elsewhere in the world in wild black-tailed marmosets, large hairy armadillos and a leopard.

But testing on wild animals is infrequent, and COVID-19 has likely affected many other species, which emerging research is beginning to show. “I think the spread to wild animals is much wider than previously thought,” says Joseph Hoyt, a disease ecologist at Virginia Tech.

How does SARS-CoV-2 infect so many species and what are the impacts?

The receiver connection

A major reason lies in a complicated receptor present in all mammals, called ACE-2. This receptor plays an important role in the regulation of blood pressure and other physiological functions.

Once the SARS-CoV-2 spike protein enters the body, it begins to infect host cells by binding to the ACE-2 receptor, which is prevalent in the upper respiratory tract and sinuses of humans and many other mammals.

The physical structure of the ACE-2 receptor varies relatively little across vertebrate species compared to other similar proteins, says Craig Wilen, a virologist at Yale University. Even so, there are enough small variations that scientists initially thought some mammals would be very unlikely to be infected.

But that way of thinking has changed as animals originally thought to be less susceptible have proven anything but. It now appears that many, if not most, mammalian ACE-2 receptors are sensitive and do not represent a limiting factor for the virus.

“It looks like it’s pretty good…even if it’s not a perfect match,” says Rick Bushman, a professor at the University of Pennsylvania School of Medicine who studies host-microbe interactions.

Instead, there are likely many other factors at play that determine vulnerability, the details of which remain almost entirely unknown.

A wide range

We already know the virus can infect and spread in wild mink and white-tailed deer – and for both species there is at least one verified case in which the virus jumped from humans to animals and back to humans. Besides mink, domestic ferrets and golden hamsters also seem to easily transmit the virus to each other in captivity.

In addition to previously listed animals, an upcoming study published ahead of print in BioRxiv has identified probable cases of SARS-CoV-2 infection in deer mice, raccoons, opossums, gray squirrels, footed mice white, striped skunks, etc.

Carla Finkielstein, co-author of the paper, along with Hoyt and conservation biologist Amanda Goldberg, were surprised when they first found evidence of SARS-CoV-2 infection in Virginia opossums.

“We were worried because that means he’s jumping” to distant mammals, Finkielstein says. “Possums are biologically very different from us,” Goldberg adds.

Opossums are marsupials that give birth to bee-sized young, which suckle on teats in their mother’s pouches. Marsupials diverged from placental mammals – which include many common mammals – more than 150 million years ago.

If SARS-CoV-2 can infect opossums, they reasoned, chances are it could infect a wide variety of mammals. Indeed, the team found signs of antibodies against SARS-CoV-2 in significant percentages of six urban wildlife species in southwestern Virginia. They also obtained positive PCR results – which indicate but do not prove infection – in two of these species and four others, including red foxes and bobcats.

Another recently submitted paper also found signs of the pathogen infecting 17% of New York sewer rats tested. And a small percentage of wild Connecticut white-footed mice have also been infected, according to research by Yale University doctoral student Rebecca Earnest.

Infection Questions

But how are wild animals such as deer exposed to the virus?

The question remains unanswered, but there are theories. Wildlife could become infected by coming into close contact with human waste or sewage, or by inhaling the virus near people. Exposure can also occur from interactions with pets such as cats and dogs – or deer in captivity – which may carry the virus.

But “I think everyone agrees…no one knows,” Bushman says.

Regardless of how white-tailed deer are exposed, it often happens. A 2021 study suggested that more than a third of deer in the Northeastern and Midwestern United States had been exposed. Another paper found that the virus had entered deer at least four times separately from humans, yet a third study found the virus passed back to a single human in Canada. (Read more: American wild deer found with coronavirus antibodies.)

One of the reasons animal infections are important is that they represent new reservoirs for the virus, where it can be maintained and acquire new mutations that could theoretically help it spread better if it finds its way back. towards the man.

“More transmission across more species is not something we want to see,” Earnest says.

Overlooked problem

SARS-CoV-2’s ability to infect wildlife amounts to a hidden panzootic — the animal version of an epidemic — with effects almost entirely unknown, Finkielstein says.

Infected animals often appear to show mild symptoms, but experts know next to nothing about how different variants of the virus affect most animals. Sometimes the infections are fatal. The virus appears to kill a small percentage of infected mink, and three snow leopards have died due to complications from COVID-19 at the Lincoln Children’s Zoo in Nebraska.

Wilen warns that we don’t really know how sick animals can get in the wild. He cites the example of the chimpanzee simian immunodeficiency virus (SIVcpz), which jumped into humans and transformed into HIV-1. SIV was long thought to cause only mild symptoms in chimpanzees, but research has finally determined that the virus can cause an AIDS-like disease in animals, which usually shortens their lifespan.

The effects of viruses are particularly difficult to study in wild animals, especially at an ecological level, adds Hoyt.

“We don’t know the consequences of this for wildlife,” agrees Finkielstein. “That’s another aspect that has been largely ignored.”

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