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SARS-CoV-2 exposure in wild white-tailed deer (Odocoileus virginianus)

Our take —

In this preprint that has not yet been peer-reviewed, researchers at the US Department of Agriculture report that 40% of white-tailed deer sampled in 2021 from four eastern states had antibodies against SARS-CoV-2. Deer are common throughout the US and are frequently in proximity to humans, so exposure may have occurred via many possible routes. It will be important to determine whether deer are transmitting the virus in the wild, the sources of exposure, and the geographic extent of infections in order to prevent the establishment of a permanent reservoir in deer populations.

Study design

Other

Study population and setting

This paper presents preliminary results of a serological surveillance program focusing on SARS-CoV-2 in white-tailed deer (Odocoileus virginianus) in the US. Between January and March 2021, 385 serum samples were collected from wild deer in Michigan, Pennsylvania, Illinois, and New York as part of routine wildlife management and disease surveillance efforts. An additional 239 serum samples collected between 2011 and 2020 from the same four states, plus New Jersey, were used as pre- and early pandemic controls to identify potential cross-reactivity of endemic coronaviruses. All samples were tested using a surrogate virus neutralization test (sVNT) that measures inhibition of binding between SARS-CoV-2 spike protein and the host ACE2 receptor. A selection of 24 samples from 2021 that were positive by sVNT (>30% inhibition) and 24 that were negative were confirmed with a virus neutralization test using live SARS-CoV-2 virus.

Summary of Main Findings

154/385 (40%) of samples from 2021 were positive by sVNT, most having ≥80% inhibition. Antibodies were detected in three samples from 2020 and one sample from 2019, while none were detected in samples from 2011-2018; percent inhibition scores for 2019 and 2020 samples were low (30.03-43.72), just above the threshold of positivity. Seropositive deer were detected in all four states in 2021, with high clustering of detections within particular counties. Samples tested with VNT produced the same number of positives (24) and negatives (24) as in the sVNT.

Study Strengths

The results are strengthened by the inclusion of matched pre-pandemic controls from the same states and counties. Confirmation of sVNT results in a subset of samples from 2021 using VNT increases confidence that the detections were not entirely due to cross-reactivity or other factors leading to false positives.

Limitations

Since the deer in this study were sampled opportunistically as part of other activities (including disease surveillance and urban removals) in only four states, the results are unlikely to be representative of the whole US deer population. It was also unclear which samples from which states were chosen for confirmation by VNT, so the seroprevalence measurements for each state may be overestimates. Without PCR or virus isolation, it is unclear whether animals were actually infectious at any point, and when infection occurred. There was also little information provided on what areas animals were sampled from (e.g., urban versus rural, nearby wastewater sources or farms), which could have provided information on how animals were exposed. Due to the limited number of samples collected later in 2020, the authors could not estimate the rate of increase in detections during the early phase of the pandemic. Finally, it was unclear if some detections by sVNT may be due to cross-reactivity with endemic coronaviruses in deer, although it is unlikely that cross-reactivity would occur in the VNT.

Value added

This study represents some of the most compelling evidence of possible spillback of SARS-CoV-2 into wild animal populations in the US. Previous experiments (https://ncrc.jhsph.edu/research/susceptibility-of-white-tailed-deer-odocoileus-virginianus-to-sars-cov-2/) had indicated that white-tailed deer were susceptible to SARS-CoV-2 infection, showed few clinical signs, and could transmit the virus to naïve contact deer. If transmission is indeed occurring in these animals in the wild, it may form a new reservoir that could continue to seed infections in the US in the future and contribute to the evolution of new SARS-CoV-2 variants.

This review was posted on: 9 August 2021