SARS-CoV-2 detected in white-tailed deer with possible zooanthroponotic spillover events

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of the coronavirus disease 2019 (COVID-19) pandemic, was first identified in Wuhan, China, in 2019. Currently, there is an increased emergence of new variants of the virus.

Study: Multiple spillovers and onward transmission of SARS-Cov-2 in free-living and captive White-tailed deer (Odocoileus virginianus). Image Credit: Tom Reichner/ ShutterstockStudy: Multiple spillovers and onward transmission of SARS-Cov-2 in free-living and captive White-tailed deer (Odocoileus virginianus). Image Credit: Tom Reichner/ Shutterstock

SARS-CoV-2 utilizes the angiotensin-converting enzyme 2 (ACE-2) receptor of the host to gain entry into the cell. ACE-2 receptors are known to be well conserved in multiple vertebrate species. Computational analyses predict high binding affinities of SARS-CoV-2 to ACE-2 receptors in many animal species, suggesting these animals could be susceptible to infection. Three species of deer are included among these animals, Père David’s deer (Elaphurus davidianus), reindeer (Rangifer tarandus), and white-tailed deer (Odocoileus virginianus).

The onset of the pandemic led to the global spread of SARS-CoV-2 among humans. This could provide opportunities for spillovers into non-human hosts. SARS-CoV-2 infections have been reported in cats, dogs, zoo animals, and farmed minks. SARS-CoV-2 infection of an animal host can result in it becoming an animal reservoir that could promote the emergence of new variants. These new variants can again spill back to humans. Such transmission has already been reported on workers on mink farms.

A new study reported on the pre-print server bioRxiv* was prompted by a recent report that suggested 40% of free-living white-tailed deer in the USA had antibodies against SARS-CoV-2. The study aimed to test the hypothesis that the infection and transmission of SARS-CoV-2 of deer occurred in nature by assaying retropharyngeal lymph node (RPLN) samples collected from free-living and captive deer in Iowa from April 2020 to January 2021.

About the study

The study included 283 retropharyngeal lymph nodes (RPLNs) samples from white-tailed deer collected by the Iowa Department of Natural Resources (DNR). Additional 60 RPLN samples that were collected in 2019 were also included to serve as the negative control. The RPLN tissues were then used to extract viral RNA, followed by detection of SARS-CoV-2 specific RNA using RT-PCR.

Furthermore, the total RNAs extracted from RPLN samples were used for whole-genome sequencing of SARS-CoV-2 and identification of new variants.

Study findings

The results indicated that out of the 283 RPLN samples, 94 tested positive for SARS-CoV-2 RNA. The first positive case of SARS-CoV-2 in deer was observed on September 28, 2020, in South Eastern Iowa. The number of positive cases of SARS-CoV-2 in deer increased from September to January. Additionally, a high viral copy number was found in deer RPLN samples. However, all the samples collected from April to August 2020 were negative for SARS-CoV-2 RNA.

The deer included in the study were either free-living on public lands or in peri-urban environments or were captive in nature or hunting preserves. The results suggested that the proportion of free-living deer that were positive for SARS-CoV-2 was much higher than the deer in captive settings. However, further studies need to be done on these results since four times more samples were collected from free-living deer than captive ones during December 2020.

The analysis of regional differences on SARS-CoV-2 positivity in deer showed temporal growth as the year progressed. The study reported ten counties having at least one positive sample. It was also observed that the largest number of samples in the collection was from a single game preserve located in South Eastern Iowa.

Furthermore, the current study identified 12 SARS-CoV-2 lineages amongst the 94 positive samples from deer. Among the 12 lineages, four were most widely distributed and represented in the deer. These four lineages were B.1.2, B.1.311, B.1, and B.1.234. Additionally, the B.1.2 lineage was most abundant in humans during that time, while the B.1.311 lineage was poorly represented. However, the second most abundant lineage in humans, B.1.565, was not represented in the sampled deer at all.

The results, therefore, showed evidence of zooanthroponotic spillovers from humans to deer and the circulation of these strains in both free-living and captive deer. The infected deer are capable of transmitting the virus to a susceptible deer three to five days post-infection. Also, the virus can be recovered from the palatine tonsils and RPLNs of infected animals for up to 21 days post-exposure.

Therefore, it can be concluded that the results of the current study suggest that deer have the potential to develop as major reservoirs for SARS-CoV-2 infections. A reservoir host can increase the mutation of the virus and its pathogenicity to the original host. Animal reservoirs can also provide refuge to the virus outside the human population, leading to a looming threat of re-emergence into humans.

Moreover, human viruses diverge due to immune pressure, while those circulating in the wild reservoirs do not. When these viruses enter again into the human population, they may infect young individuals who were not exposed to the ancestral strains. Thus, there is an urgent need for active surveillance of potential wildlife reservoirs throughout the world. It will prevent the emergence of new SARS-CoV-2 variants and multiple spillover events that can adversely affect both humans and the involved animal species.


Although the study could determine the role of white-tailed deer as animal reservoirs for SARS-CoV-2 infections, it had certain limitations. First, the RPLN samples were tested from only one state in the United States, and the sampling was not uniform. Second, the samples tested in this study were from 2020, when the Alpha and Delta variants had not yet emerged. Finally, the current study also lacked incontrovertible evidence for spill back from humans to deer.

*Important notice

bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Suchandrima Bhowmik

Written by

Suchandrima Bhowmik

Suchandrima has a Bachelor of Science (B.Sc.) degree in Microbiology and a Master of Science (M.Sc.) degree in Microbiology from the University of Calcutta, India. The study of health and diseases was always very important to her. In addition to Microbiology, she also gained extensive knowledge in Biochemistry, Immunology, Medical Microbiology, Metabolism, and Biotechnology as part of her master's degree.


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