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Scientific evidence suggests that populations of wild deer can become infected with SARS-CoV-2, the coronavirus that causes COVID-19. However, it is unknown if or to what extent the disease may be transmitted between species. In November 2021, research conducted by wildlife experts and published in the peer-reviewed journal Proceedings of the National Academy of Sciences and 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. Additional studies have also shown that deer can catch SARS-CoV-2, but as CBS News reported, there is no evidence that the virus can be transmitted from deer to humans. One such study was conducted by the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service. Of the nearly 500 samples from deer in Illinois, Michigan, New York, and Pennsylvania between January 2020 and March 2021, 33% tested positive for coronavirus antibodies. Researchers at Pennsylvania State University similarly found more than 80% of white-tailed deer sampled across Iowa between December 2020 and January 2021 also tested positive for the virus. To more fully understand the science behind interspecies infection, we dug through the PNAS study, which analyzed a total of 624 serum samples collected from four U.S. states both before and after the pandemic 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. 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.) In March 2022, researchers documented a potential case of deer-to-human transmission and published the details in the preprint journal bioRxiv, as was reported by National Geographic. As we have previously reported, a preprint study is one that has not yet been peer-reviewed and as such, has not gone through the regimented evaluation by journal editors and other experts in a related field. Even so, preprint studies can offer preliminary understanding in a time when science is urgent and steady data is important in supporting public health, as has been the case during the COVID-19 pandemic. In the case of the preprint study mentioned above, researchers in Ontario identified a mutated variant of SARS-CoV-2 dubbed the WTD lineage in local deer populations. Evidence suggested that the variant may have infected a person. Subsequently, a study published in PLOS Pathogens found that white-tailed deer that had been experimentally infected with SARS-CoV-2 shed the virus for five days after inoculation. However, experts at the time noted that there wasn't a need to panic but that the findings show that the more hosts you have, the more opportunities the virus has to evolve, Arinjay Banerjee, a virologist at the University of Saskatchewan who was not involved in the study, told The New York Times. 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. The study has limitations that are worth noting. 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.
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