Animals are more likely to contract COVID-19 than expected

The COVID-19 pandemic is often thought to be unique to humans, but it’s not that simple: the disease-causing virus, SARS-CoV-2, can also infect animals, both in captivity and in the wild.

According to the United States Department of Agriculture (USDA), the virus has so far been detected in more than 100 domestic cats and dogs, as well as tigers, lions, gorillas, snow leopards, captive-bred otters and spotted hyenas. In addition, US zoos have recorded a single positive case in binturong, coati, cougar, house ferret, raccoon cat, lynx, mandrill and squirrel monkey.

Also according to the USDA, only three wildlife species have tested positive in the United States: mink, mule deer and white-tailed deer. Other cases have been identified in other parts of the world, such as in populations of melanura oustitis and large hairy armadillos, and one case has been identified in the leopard.

However, testing in wild animals is rare, and as recent scientific research suggests, COVID-19 has likely affected other species. “I think the spread of the virus in wildlife is greater than previously thought,” says Joseph Hoyt, a disease ecologist at Virginia Polytechnic Institute and State University.

How can SARS-CoV-2 infect such a wide variety of species and what are the consequences?

THE ROLE OF THE ACE2 RECEIVER

ACE2, a complex cell receptor found in all mammals, appears to play a central role in these infections. It is particularly involved in the regulation of blood pressure and other physiological functions.

Once the SARS-CoV-2 spike protein enters the body, it begins to infect the host cell by coupling to the ACE2 receptor found in the upper airways and sinuses of humans, but also many other mammals.

According to Craig Wilen, a virologist at Yale University, the physical structure of the ACE2 receptor varies relatively little between vertebrate species, compared to other proteins of the same type. Yet there are variations small enough for scientists to think that some mammals have a very low chance of being infected.

This hypothesis has since evolved, as animals, thought to be less likely to contract the virus, have proven to be highly susceptible to contamination. It now appears that, in mammals, many, if not all, ACE2 receptors are also sensitive to SARS-CoV-2, and thus are not a rate-limiting factor for the virus.

“The compatibility looks pretty good … although it’s not perfect,” says Rick Bushman, a professor at the University of Pennsylvania School of Medicine who studies host-microbe interactions.

Many other factors could well characterize the vulnerability to COVID-19, which we still don’t know how to explain at this moment.

MANY SPECIES ARE INTERESTED

We already know that the virus can infect mink and white-tailed deer and spread through those populations. For each of these species there has been at least one confirmed case of virus transfer from humans to animals and then from animals to humans. In addition to mink, ferrets and pet golden hamsters appear to transmit the virus easily.

(Read: COVID-19: Did US public health authorities try to crack down on an animal-to-human viral spillover?)

In addition to the animals listed above, a pre-print study published in BioRxiv identified probable cases of SARS-CoV-2 infections in deer mice, raccoons, opossums, gray squirrels, white-footed mice and striped skunks. In Europe, a study of bank voles revealed that this rodent was unable to contaminate other species if infected.

Study co-author Carla Finkielstein, conservation biologist Amanda Goldberg and Joseph Hoyt were amazed to find evidence of SARS-CoV-2 infection in Virginia opossums.

“We were concerned because that meant the virus was hopping between species” and transferring between unrelated mammalian species, says Finkielstein. “Opossums are biologically very different from us,” Goldberg adds.

Opossums are marsupials that give birth to young the size of bees, which drink their mother’s milk from the pouch. Marsupials split from the placental mammal group (which includes many common mammals) over 150 million years ago.

If SARS-CoV-2 can infect opossums, it is likely it could infect a huge variety of mammals. In fact, the team of researchers found large amounts of antibodies to SARS-CoV-2 in six species of urban wildlife in southwestern Virginia. PCR tests (which imply, but do not prove infection) came back positive in two of these species, as well as four others, including red fox and lynx.

Another recently presented study mentions traces of pathogens in 17% of New York rats tested. In Connecticut, a small percentage of wild white-footed mice are also infected, according to research by Yale University doctoral student Rebecca Earnest.

THE UNCERTAIN ORIGIN OF THE INFECTION

How wild animals such as deer are exposed to the virus remains to be seen.

We still don’t have the answer to this question, but theories have been proposed. Wild animals could become infected through close contact with our waste or sewage or by inhaling the virus when near humans. Exposure could also occur through interactions with pets such as cats and dogs (or captive deer), which can be carriers of the virus.

“Everyone agrees… that nobody knows,” Bushman explains.

Whatever the reason, deer are frequently exposed to the virus. A 2021 study suggested that more than a third of deer in the northeastern and midwestern United States were exposed to it; another study showed that a deer had been contaminated at least four times by humans; and a third study found a man had been infected by a deer in Canada.

(Read: COVID-19: Antibodies have been detected in wild deer.)

These infections in animals need to be monitored, as they can act as new reservoirs for the virus and thus allow it to survive and undergo new mutations that could theoretically help it spread more easily in humans.

“There shouldn’t be more broadcasts, in more species,” Earnest says.

A NEGLECTED PROBLEM

According to Finkielstein, the ability of SARS-CoV-2 to infect wild animals corresponds to a panzootic (the equivalent of a pandemic in animals) of which we are completely ignorant of the details.

Infected animals often appear to show mild symptoms, but experts know next to nothing about how the various variants might affect them. Sometimes the infection proves fatal. The virus appears to cause the death of a small percentage of infected mink, and three snow leopards have died of COVID-19-related complications at the Lincoln Children’s Zoo in Nebraska.

Craig Wilen points out that we don’t really know how sick animals are in the wild. He cites as an example the chimpanzee simian immunodeficiency virus (cSIV) which, when transmitted to humans, became HIV-1. Scientists have long believed that SIV causes only mild symptoms in chimpanzees, but research has shown that the virus can cause AIDS-like conditions in animals, which would cause their life expectancy to decrease.

The effects of viruses are particularly difficult to study in wild animals, especially on an ecological scale, Hoyt says.

“We don’t know what the consequences of these infections are for wildlife,” says Finkielstein. “This is another aspect that has been very little studied. »

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