The novel coronavirus appears to have somehow jumped from humans to wild deer in some parts of the United States.
In the northeast corner of the nation, a recent federal survey found neutralizing antibodies for SARS-CoV-2 in 40 percent of all white-tailed deer (Odocoileus virginianus) that were sampled.
In the state of Michigan alone, 67 percent of free-ranging deer showed immune markers for the coronavirus in their bloodwork.
It's the first evidence of widespread exposure to SARS-CoV-2 in wild animals, and while the preprint study still needs to be verified and peer-reviewed, the findings are cause for concern.
While none of the deer showed adverse health effects, the presence of specific antibodies in their blood suggests they recently fought off the virus.
By silently harboring and spreading this pathogen, scientists worry deer populations are allowing SARS-CoV-2 to adapt and evolve into new strains – ones that could possibly re-infect humans years down the road with even greater transmissibility and severity than before.
After all, white-tailed deer in the US cross paths with our species a lot, whether it be from fieldwork, conservation work, feeding, hunting, or human wastewater, providing a perfect pathway for a virus to spread back and forth.
"The geographic distribution of this species encompasses most of North America and these animals are particularly abundant near urban population centers located in the eastern US," the authors write in their paper.
"Moreover, white-tailed deer can form social groups, a contact structure with the potential to support the intraspecies transmission of multiple pathogens."
Ever since the global pandemic first started, scientists have been worried about the novel coronavirus jumping from humans to another species of animal, known as zoonotic spillback.
Last year, for instance, an outbreak among farmed minks led to a massive cull of livestock in Europe and the United States. But unlike captive animals, infections among wild animals are not so easily controlled.
That's why scientists are so concerned by the recent findings. If SARS-CoV-2 can indeed find refuge in the wild, it could make eradication extremely difficult. If the virus adapts among another species and then reinfects humans, our vaccines might be far less effective in the future.
Recently, in Utah, a seemingly healthy wild mink tested positive for SARS-CoV-2, becoming the first free-ranging animal to pick up the virus. As scientists predicted, however, that was probably just the tip of the iceberg. Now, it seems apparent the virus has taken off among wild deer as well.
These free-ranging animals will need to be tested for viral RNA if we want to be absolutely sure that they are providing a reservoir for the novel coronavirus, but the presence of antibodies in their blood suggests they have somehow been exposed.
Previous studies in the laboratory have shown white-tailed deer are highly susceptible to SARS-CoV-2, and that one infected individual of this species can infect another.
This new survey suggests a similar spread could be happening in the wild, although more research is needed to figure out how that's happening.
The team had access to 385 wild white-tailed deer serum samples from January to March 2021, as well as 239 archived samples from 2011 to 2020, which they tested for SARS-CoV-2 antibodies.
Before the onset of the pandemic in 2019, government researchers found no immune markers for the SARS-CoV-2 virus in the bloodwork of wild deer. After the pandemic began, however, these antibodies began to pop up more and more.
In 2020, specific blood proteins for SARS-CoV-2 were found among three deer. Within the first three months of this year, however, nearly half of all 385 blood samples taken from deer in Michigan, Pennsylvania, Illinois, and New York showed the same neutralizing antibodies.
How these deer were exposed to the virus in the first place is still unclear. It could have jumped directly from humans, or it could have been passed from livestock or wild animals that came into contact with us and then onto white-tailed deer.
As such, officials in the US are calling for greater wildlife surveillance, especially among predators and scavengers that regularly interact with deer.
"If there is a common source of exposure for the deer, then likely the same source can expose other animals," virologist Arinjay Banerjee from the University of Saskatchewan, who wasn't involved with the study, told Nature.
SARS-CoV-2 may be spilling into the wild faster than we can mop it up.
The study was published in bioRxiv.