During the covid-19 pandemic, cases of coronavirus infection have been detected in pets such as cats and dogs, as well as numerous animals that were in zoos, including gorillas already big felines. In the latter cases, infections have occurred even when caregivers they wore personal protective equipment.
But it was even more disturbing what happened last December, when the United States Department of Agriculture confirmed the first case of a wild animal infected with SARS-CoV-2, the virus that causes covid-19. Researchers spotted an infected wild mink in Utah near a mink farm that was affected by a COVID-19 outbreak.
Are we transmitting the virus to wildlife? And if so, what implications would it have for both animals and us?
How do viruses jump from one species to another?
We are Two scientists dedicated to the study of viruses in wildlife, and at this moment we are conducting a study which addresses the potential for the spread of SARS-CoV-2 from humans to domestic and wild animals. We scientists call “overflow” (spillover) to the phenomenon that occurs when a virus jumps from one species to another. But luckily these overflows are not common.
To infect a new species, a virus must be able to bind to a protein on a cell and penetrate that cell; and all this while trying to survive an immune system that has never been faced before. Then, and while the virus tries to avoid antibodies and other antiviral agents, it must replicate in sufficient quantity to be transmitted to the next animal.
This generally implies that the closer the contact between two species, the more likely they are to share viruses. The chimpanzee, which is the species most similar to man, you can contract numerous human viruses and fall ill. Earlier this month veterinarians at the San Diego Zoo reported that the gorilla herd at said zoo was infected with SARS-CoV-2, which points to the possibility that the virus is able to jump from humans to our closest species.
Some viruses tend to stay in a single species or similar species, while others are capable by their very nature of making much larger intraspecies jumps. The flu, for example, can infect a wide variety of animals, from sparrows to whales. And, likewise, coronaviruses are characterized by jump between species every so often.
A key question is knowing how many species and of what type SARS-CoV-2 could infect (and which of these species could help the virus to continue circulating).
The search for covid-19 in wildlife
In order for SARS-CoV-2 to spread from humans to wildlife, the animal must be exposed to a dose of the virus high enough to become infected.
The most risky situations occur when there is direct contact with humans, such as when a vet heals an injured animal. Contact between an infected person and their pet or contact between a COVID-19 infected with a farm animal also poses a risk, as these animals can act as intermediate hosts that allow the virus to spread to infect a wild species .
Another way that COVID-19 can be transmitted from humans to animals is through indirect infection, such as through sewage. Remains of covid-19 and other pathogens have been detected in this type of water, many of which are dumped without any treatment into natural areas where certain species of wildlife, such as marine mammals, may be exposed to them. It is thought that this was how some California elephant seals got the H1N1 flu during the 2009 swine flu epidemic.
To find out if SARS-CoV-2 overflows are occurring, our team at Tufts University works with veterinarians and wildlife conservators across the United States to collect and analyze samples from the animals in their care. During this time we have analyzed samples from nearly 300 wild animals of more than 20 species, and so far none (from bats to seals to coyotes) have tested positive for covid-19 in antibody tests or in swab samples. .
Our researchers have focused their activities of selective surveillance of wildlife in places where infections from domestic animals had occurred. The first confirmed case of infection with wild mink was detected while carrying out a surveillance work near a mink farm where an outbreak had occurred. And although it is not yet clear how this wild mink contracted the coronavirus, the large number of infected minks and potentially infectious particles emitted by them make us speak of a high-risk area.
Bad for animals and bad for people
When a virus jumps into a new species it sometimes mutates to more efficiently infect, replicate, and transmit within the new animal. This process is called “host adaptation.” When a virus jumps to a new host and begins this adaptation process, the results are unpredictable.
In late 2020, when SARS-CoV-2 reached a mink farm in Denmark, this experienced a number of mutations that are rare when the virus attacks humans. Some of these mutations occurred in parts of the virus that most vaccines are designed to localize. And this did not happen just once, but mutations occurred many times and independently in shoots produced in other mink farms. Although it is not yet clear what impact these mutations (if any) have on the development of the disease in humans, or even in vaccines, these are signs of host adaptation that could lead to to new variants of the virus; variants that could remain in animal hosts and re-emerge in the future.
Another risk is that SARS-CoV-2 could make animals sick. Environmentalists are especially concerned about endangered species such as the black-footed ferret, an animal similar to mink and believed to be very vulnerable to the virus.
Overflows from people to wild animals have already occurred in the past. At the end of the 20th century the Ebola virus jumped from humans to great apes and had devastating consequences for these endangered species. And more recently it has Human Respiratory Virus Detected Affecting Threatened Mountain Gorilla Populations, and that has also caused numerous deaths.
But perhaps the greatest risk to humans is that the overflow could cause the coronavirus to be stored in new animal species and in regions that have not been affected so far. This could mean that new covid-19 outbreaks could reappear in the future.
This month an article was published showing that this has already happened on a small scale in mink farms in Denmark, where the virus passed from humans to animals and then back again to humans.
Although our team has not found evidence of COVID-19 cases in wild animals in the United States, there is conclusive evidence of frequent overflows in dogs, cats and some animals that live in zoos. The finding of infected wild minks confirmed our fears. Detecting the first case in a wild animal with covid-19 was alarming, but not surprising.