In a recent study posted to the Research Square* preprint server, researchers explored the epidemiological factors of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in white-tailed deer.
Study: Epidemiological Dynamics of SARS-CoV-2 in White-tailed Deer. Image Credit: TonyCampbell/Shutterstock.com
*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
Background
SARS-CoV-2 was detected in white-tailed deer (WTD) in 2020. In 2021, there was proof of regional SARS-CoV-2 spread in WTD through human-to-deer and deer-to-deer routes.
WTD populations could become reservoir hosts for SARS-CoV-2, leading to the likelihood of variant persistence, the emergence of new variants, and spillback into human populations due to endemic transmission.
During a pandemic, cross-species transmission can cause significant global health consequences. Understanding the causes of zoonotic pathogens is crucial to avoid such consequences.
About the study
In the present study, researchers measured the prevalence of SARS-CoV-2 infection in North American WTD.
Between October 2021 and September 2022, 10,722 samples of WTD were obtained from 27 states and Washington, DC. Samples of WTD have been collected postmortem from various sources, including management events, hunter-harvest samples, and opportunistic samples from deaths like roadkill. The state departments of natural resources collected these samples.
Department of Agriculture-Animal and Plant Health Inspection Service (USDA-APHIS), Wildlife Services, and other agencies were involved. The study recorded management type and deer-specific metrics such as sex and age class.
Samples were collected from the nose or mouth and analyzed for SARS-CoV-2 viral ribonucleic acid (RNA) using real-time reverse-transcriptase-polymerase chain reaction (rRT-PCR).
The team evaluated human density for all counties. The proportion of land in each county that can support WTD populations was determined with the Gap Analysis Project (GAP) WTD species distribution model.
The New York Times repository of SARS-CoV-2 cases was used to calculate the weekly SARS-CoV-2 death rate in humans in each county.
Results
Between October 2021 and March 2022, 10,217 oral and nasal swab specimens from WTD were tested across 27 states and Washington, DC. 13% of the samples tested positive for SARS-CoV-2 viral RNA.
This viral RNA was more prevalent in males than females, despite similar sample sizes being collected from both genders. Detection rates were comparable in juvenile and adult groups, although the sampling was biased toward adults.
A significant interaction between sex and management method was observed, but no significant effects were found for age class or swab type. Male deer had higher infection rates than females in samples obtained by agency management, but the difference was less significant for male deer obtained by hunters.
Human population density and deer habitat have a slightly positive effect but are not statistically significant. The prevalence of WTD is expected to rise from 10% to 15% between October 2021 and March 2022 as human population density increases from 10 to 100 people per square kilometer.
The prevalence of WTD is also expected to increase from 10% to 15% between October 2021 and March 2022 as the proportion of WTD habitat increases.
According to the model, there was a correlation between the prevalence of SARS-CoV-2 in WTD and the infection rate in humans. The model employed the lagged human SARS-CoV-2 mortality rate as a proxy indicator for infection.
The model suggests that the likelihood of WTD prevalence rises by 13% with each additional 11 human fatalities per 100,000 county residents. The model also revealed that approximately 10% of positive WTD detected between October 2021 and March 2022 resulted from human infection pressure.
Between October 2021 and March 2022, the average prevalence rates were higher on the East Coast compared to the Mid- and South-West regions.
The mean county-level apparent prevalence showed more extreme values than time-averaged estimates for counties having low sample sizes. The peak prevalence across the range of WTD assessed showed spatial variation.
Conclusion
The study findings showed that SARS-CoV-2 was found in white-tailed deer populations in most counties where they are present. Peak infection prevalence time estimates were comparable across multiple sampled counties, indicating that human-to-deer spillover transmission may have been widespread.
Quantifying disease dynamics and risk variables is crucial for developing effective strategies to curb, monitor, and control zoonotic diseases. Surveillance data can be analyzed using models to estimate disease prevalence across space and time, which can help fill in gaps in data collection.
Analyzing reconstructed disease trajectories spatially can help identify areas affected by disease and may be at higher risk for future outbreaks.
In conclusion, similar peak infection prevalence teams across various counties provide evidence of human-to-deer transmission. However, the widespread estimates of local epidemiological reproduction numbers suggest that deer-to-deer transmission is also likely. The studies model estimated that 10% of infected WTD were from human infection pressure.
*Important notice: medRxiv 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.
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