Not only did humans transmit Covid-19 to deer, but deer might have returned the favor, a new study published in Nature Communications on July 10 reports. Researchers from the University of Missouri, the USDA, and the CDC collected 8,830 nasal or oral swabs from free-ranging whitetails from November 2021 to April 2022, including hunter-harvested deer. Across those samples, which came from 26 states in the Northeast, Southeast, and Midwest, plus Washington D.C., humans likely transmitted Covid-19 to deer at least 109 times.
As a result of those 109 transmissions, 39 deer then likely transmitted Covid-19 to other deer. Beyond that, deer might have transmitted Covid-19 back to humans on three separate occasions, twice in North Carolina and once in Massachusetts. The authors of the study qualify this by referring to the cases as “potential” instances of spillover back to humans, but this is some of the most concrete evidence of deer-to-human transmission to emerge yet.
In each instance of potential deer-to-human disease transfer, the nucleotide sequences of the SARS-CoV-2 strains found in sick humans was 99.93 percent or more identical to strains found in tested whitetail deer. But the evidence doesn’t stop there; the makeup of these viral strains didn’t show up in any public database of recorded strains sourced from humans. In other words, whitetail deer (and one random lion from a zoo in North Carolina) provided the closest match to the strain these individuals were infected with.
What complicates these results is the fact that, when researchers contacted some of these people, they didn’t report having any close contact to deer or to the zoo where the lion resided in the prior month. It would be one thing if they were all avid deer hunters who just came off a successful 2021 season. But they weren’t.
This opens up a host of questions. How did Covid-19 jump from deer to humans if those humans weren’t in close contact with deer? Did whitetail deer transmit Covid-19 to the family dog, who then passed it on to these people? The mystery of whitetail deer-to-human transmission of Covid-19 remains.
This isn’t the first time a whitetail deer-to-human transmission event has popped up in published research. The same conclusion arose from a study originally published in bioRxiv, a “preprint” research journal that allows scientists to “make their findings immediately available to the scientific community to receive feedback on draft manuscripts before they are submitted to journals.” Researchers discovered that an Ontario resident had contracted the same “highly divergent” strain of Covid-19 that showed up in 17 tested whitetails in the area. Unlike the Massachusetts and North Carolina cases, that individual had been in close contact with multiple deer throughout the 2021 hunting season.
While the unedited, non-peer-reviewed nature of the study raised eyebrows when Canadian researchers first published it in February 2022, it was later accepted and published nine months later by Nature Microbiology, a research journal that boasts a “rigorous peer-review process.”