by Humans spread the virus that causes Covid-19 more than 100 times to wild white-tailed deer in the United States in late 2021 and early 2022, according to a new study from the Animal and Plant Health Inspection Service of the United States Department of Agriculture. The infection spread widely among the deer population, and in at least three cases, researchers suspect humans contracted the virus from deer.
The study also found that many coronavirus lines such as Alpha, Delta and Omicron continued to circulate in deer after they left the human population.
If the virus continues to circulate in deer, as it does in humans, the study suggests these animals could become a long-term reservoir, allowing the virus to hide and develop new and potentially more dangerous mutations.
Scientists fear that those viruses could then bounce back into the human population and cause another serious wave, as they did when Omicron arrived seemingly out of nowhere. While that scenario is worrying, it is still largely theoretical.
View animal reservoirs
Experts say a wildlife reservoir would be a bigger problem if we had managed to control the virus in humans. But the infection is still spreading among humans and will most likely mutate in us because we are the favored hosts.
“The more species it contains and the more transmission there is, the greater the risk of new variants. However, it’s hard to say whether deer are a major risk in the grand scheme of things right now,” said Scott Weese, a veterinarian who studies animal-to-human transmission at the University of Guelph in Canada.
The U.S. government plans to continue and expand its research on animals to track how the virus moves through animal populations, according to a USDA press release announcing the study results. The study was published Monday in the journal Nature Communications.
“We prefer not to have it in deer, to avoid another potential mechanism for variant origination and to avoid exposure to even more animal species,” said Weese, who was not involved in the new study.
But there’s little we can do to stop it from spreading now, he said. Scientists sometimes vaccinate wild animal populations to prevent the spread of disease, but it’s an expensive business.
Just as there is no vaccine that completely prevents transmission of Covid-19 infections between humans, there is no vaccine that prevents transmission in animals.
Why deer?
Deer and humans have strikingly similar ACE2 receptors, the gateways the virus uses to break into cells.
For the study, scientists spent months collecting nearly 9,000 respiratory swabs from wild deer across 26 states and the District of Columbia. Those smears yielded nearly 400 viral sequences from 34 lines of the virus that causes Covid-19. They were able to closely compare those sequences to find viruses in humans that closely resembled those in deer.
In 109 cases, researchers were able to show that deer were infected with human viruses.
Contact between deer and humans
In at least three cases, human infections cataloged in databases closely matched the genetic sequences of viruses previously carried by deer, leading researchers to suspect that those humans could have contracted their infections from the animals.
Deer have become commonplace in the US, even in urban settings. Population studies have estimated the U.S. deer population at 30 million animals, which may forage for food in human waste or drink from contaminated sewage. Humans can come into direct contact with deer while feeding or hunting, or through feces. Domestic cats that spend time outdoors can also act as a go-between of sorts, Weese says, catching the virus outside and bringing it home.
Cats are also susceptible to the virus, as are many other species that are farmed — such as mink — or kept in zoos.
The U.S. Centers for Disease Control and Prevention says a person’s risk of contracting Covid-19 from pets or wild animals is low. The agency advises that certain groups, such as hunters, take special precautions to further lower their risk.
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