Context: These findings were published in July in bioRxiv, a preprint server hosted by the nonprofit research institution Cold Spring Harbor Laboratory. This means that the findings have not gone through a rigorous peer-review process, so until peer-reviewed publication has occurred, we are leaving this fact check unrated. Although the study indicated that certain white-tailed deer populations were exposed to SARS-CoV-2, an expert told Snopes that data should not be extrapolated to represent the prevalence of viral antibodies in deer populations as a whole.
Research conducted by wildlife experts and published in July 2021 found that the blood plasma of white-tailed deer, an abundant species present in every continental U.S. state but Alaska, showed evidence of coronavirus antibodies.
A total of 624 serum samples from four U.S. states were collected both before and after the pandemic was evaluated for possible SARS-CoV-2 exposure. Forty percent of deer sampled during the study period tested positive for viral antibodies.
Antibody prevalence does not necessarily correlate directly to infection (more on that later), but the researchers noted that their findings suggest that the virus may be spilling over from human to wildlife populations.
The findings were first reported by National Geographic on Aug. 2 and were subsequently covered by a number of national media outlets, including Smithsonian Magazine and Science. Before taking a closer look at the study, it is important to note that the findings were published in bioRxiv in July, a preprint server hosted by the nonprofit research institution Cold Spring Harbor Laboratory. This means that the findings have not gone through a rigorous peer-reviewed process and until publication in a reputable journal has occurred, we are leaving this claim unrated.
Crossover of SARS-CoV-2 from Human to Animal Populations
SARS-CoV-2 and other coronaviruses are known to infect domestic and wild animal species. Throughout the COVID-19 pandemic, evidence suggested that the virus was transmitted from humans to captive tigers and gorillas, as well as domestic dogs and cats. This, the study authors suggest, presents the possibility that new species of animals could become reservoirs of SARS-CoV-2 to “maintain, disseminate, and drive novel evolution of this virus” in a process known as “spillback,” or reverse zoonosis. And of particular concern are wildlife species that are abundant and live near close urban areas.
“Reverse zoonosis could lead to the establishment of novel wildlife reservoirs outside of southeast Asia, a potential that poses significant risks to both human and animal health,” wrote the authors. “Besides health impacts to wildlife, persistent infections in a novel host could lead to adaptation, strain evolution, and re-emergence of strains with altered transmissibility, pathogenicity, and vaccine escape.”
White-tailed deer are one of the multiple species endemic to the U.S. that are potentially susceptible to SARS-CoV-2 not only because of their close relation to densely populated human areas but also because they form social groups. (Previous research published in the Journal of Virology showed that deer shed the virus through the nasal secretion and feces.)
The ‘Gold Standard’ of Testing for Infectious Diseases
Researchers working under the National Wildlife Disease Program (NWDP), a collaboration with the U.S. Department of Agriculture (USDA), Animal Plant Health Inspection Service (APHIS), and the Wildlife Services, regularly conduct wildlife surveillance in search of potentially harmful-to-humans pathogens. In January 2021, NWDP launched a pilot program in which researchers opportunistically collected serum samples, or blood plasma, as part of wildlife management activities that including events like urban removal.
In the first three months of the year, 385 samples were collected from Michigan, Pennsylvania, Illinois, and New York, and compared against 239 samples collected between 2011 and 2020 from the same states, including New Jersey. Researchers used a process known as serosurveillance, a study that tests antibody levels against infectious diseases and is considered by the National Center for Immunization Research and Surveillance to be the “gold standard” for testing past infection or vaccination against infectious diseases.
The total number of deer with antibodies present varied from state to state; Illinois saw the lowest at 7% while the highest recorded was 67% in Michigan. Of the 152 samples collected in 2021, 40% contained coronavirus antibodies. Three samples from January 2020 and one from 2019 also showed evidence of coronavirus antibodies. However, no samples from 2011 to 2018 tested positive.
In addition to its not having been published in a peer-reviewed journal, the study has limitations that are worth noting.
Examining Possible Routes of Transmission
White-tail deer are in every U.S. state but Alaska, but only deer populations in four states were included in the research. While the findings provide a baseline characterization for possible infections in other parts of the country, it does not necessarily mean all states could see antibody rates as high as 40% of individuals. Furthermore, it is possible that the deer were showing antibodies for other coronaviruses, such as SARS.
In an email to Snopes, Thomas Deliberto, a wildlife biologist with the USDA-APHIS Wildlife Services National Wildlife Center, emphasized that although the study indicated that certain white-tailed deer populations were exposed to SARS-CoV-2, the data should not be extrapolated to represent the prevalence of viral antibodies in deer populations as a whole.
“More research is needed in order to determine how the deer were exposed to the virus and potential impacts, if any, to overall deer populations, other wildlife, and people,” said Deliberto.
Regardless of the limitations, the researchers note that white-tailed deer in the populations assessed were exposed to a coronavirus — most likely the one responsible for COVID-19 — probably through human contact. Deer encounter humans conducting field research, conservation work, hunting, and wildlife tourism, to name a few points of possible contact. Previous research showed that SARS-CoV-2 outbreaks in farmed mink were a direct transmission from infected humans to the animals, as was also the case in many of the infected animals that were found in zoos.
Though largely inconclusive, some research also shows that there is also the possibility that deer could have come into contact with the virus by way of contaminated water sources.
But the Centers for Disease Control and Prevention maintains that the risk of animals spreading SARS-CoV-2 to people is low.
“At this time, there is no evidence that animals play a significant role in spreading SARS-CoV-2, the virus that causes COVID-19, to people. More studies are needed to understand if and how different animals could be affected by SARS-CoV-2,” wrote the health agency.
“Some coronaviruses that infect animals can be spread to people then spread between people, but this is rare.”
And while antibody prevalence suggests infection, without a direct test, the researchers cannot confirm infection based on the data. Rather, they conclude that the findings emphasize a need for continued and expanded wildlife surveillance not only in deer, but also in predators and scavengers that come into contact with them.