In a recent study posted to the bioRxiv* pre-print server, researchers performed a longitudinal study to examine the neutralizing antibody kinetics of a naturally-severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infected white-tailed deer (WTD) population for around 16 months.
The wild WTD, Odocoileus virginianus, has the potential to be a natural SARS-CoV-2 reservoir. However, it is unknown how long natural infection persists in WTD, and the precise duration of the persistence of neutralizing antibodies in this species is also unknown. In the WTD species found across Midwestern states and Texas in the United States (US), studies have documented ~30-40% SARS-CoV-2 seroprevalence. Experimental studies have also shown that WTD lung cells are permissive to SARS-CoV-2 infection. Therefore, they transmit the virus vertically and through contact.
Field investigations have frequently reported spillover events from humans to WTD, subsequently deer-to-deer SARS-CoV-2 transmission. Studies detected several SARS-CoV-2 lineages in WTD that have not infected humans, raising concerns of zoonotic spillover to humans and the emergence of novel SARS-CoV-2 variants. In one of their previous studies, the researchers sampled 36 WTD from a deer facility in Central Texas in September 2021. Over 90% of these samples were seropositive for SARS-CoV-2, twice that of wild WTD species. The confined environment of this deer facility facilitated SARS-CoV-2 transmission.
About the study
In the present study, researchers used archived serum samples of 21 WTD of the same deer facility in Texas to determine the time of their initial SARS-CoV-2 exposure and the duration of antibody persistence in these animals. Notably, the team collected one more blood sample and swab sample on March 4, 2022, for the current prospective study.
They tested swab samples of the test animals using quantitative reverse transcriptase-polymerase chain reaction (RT-qPCR) that amplifies the SARS-CoV-2 RNA-dependent RNA polymerase (RdRp) gene. Likewise, the team tested the presence of SARS-CoV-2 in blood samples using plaque reduction neutralization tests (PRNT90) across seven-time points between November 9, 2020, and March 4, 2022.
The PRNT90 results showed that all 21 female WTDs from the captive cervid facility in central Texas tested seropositive in January 2021. This confirmed that SARS-CoV-2 infection and subsequent seroconversion occurred between 16 December 2020 and 27 January 2021 in these animals. Over 90% of these animals had detectable neutralizing antibodies for SARS-CoV-2, indicating that anti-SARS-CoV-2 antibodies in these animals persisted for at least 13 months.
The researchers observed a gradual decline in the geometric mean of endpoint PRNT90 titers, from 299.6 in January 2021 to 254 in March 2021. The subsequent decline rate in PRNT90 titers was rapid and attained a figure of 66.5 by July 2021. Yet again, the decline rate slowed to 46.2 in September 2021 and 36.7 in March 2022. Notably, Deer-10 had a four-fold increase in PRNT90 titers between January and March 2021.
The study data indicate that the WTD is a key species for a sustainable enzootic SARS-CoV-2 transmission cycle. All 21 deer turned seropositive on January 27, 2021, indicating the occurrence of an initial single exposure event rather than multiple exposure events over time. Moreover, the neutralizing antibodies persisted for at least 402 days in this naturally-infected WTD population. Later, the authors observed a continuous decrease in neutralizing antibody titers over 13 months. However, consistent with previous reports, the RT-qPCR results from respiratory and rectal swabs of all deer were negative between September 2021 and March 2022.
The current data expands prior estimates based on experimental infection studies. Studies have not quantified the threshold of protective neutralizing antibody endpoint titers for SARS-CoV-2 in WTD. Therefore, high endpoint titers do not necessitate protection from reinfection in deer, although the study findings showed that neutralizing antibodies persisted over 13 months in WTD. Moreover, Deer-006 and Deer-011 had a four-to-eight-fold temporary increase in neutralizing antibody titers over the study period. Thus, like vaccinated humans, sequential exposures to SARS-CoV-2 resulted in anamnestic responses in deer. It is noteworthy that anamnestic responses confound the rate of neutralizing antibody decay in some cases.
To conclude, the study findings strengthen an understanding of the natural infection kinetics in WTD. Future research should determine whether anti-SARS-CoV-2 neutralizing antibodies in WTD can prevent another infection and viral transmission by infected WTDs, given these species, found abundantly in nature, have the unique potential to serve as animal hosts of SARS-CoV-2.
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.
- Sarah Anne Hamer, Chase M Nunez, Christopher Roundy, Wendy Tang, Logan F Thomas, Jack J Richison III, Jamie Benn, Lisa Auckland, Terry Hensley, Walter E Cook, Alex Pauvolid-Correa, Gabriel Hamer. (2022). Persistence of SARS-CoV-2 neutralizing antibodies longer than 13 months in naturally-infected, captive white-tailed deer (Odocoileus virginianus), Texas. bioRxiv. doi: https://doi.org/10.1101/2022.07.19.500662 https://www.biorxiv.org/content/10.1101/2022.07.19.500662v1