Fig 1.
The three stages of zoonotic spillover from humans to persistence in white-tailed deer.
In each stage outlined above, we describe the stage, illustrate the concept, and define the metric we use to characterize each stage across multiple scenarios of deer in wild and captive environments. We consider the introduction of SARS-CoV-2 into white-tailed deer populations through aerosolized transmission from an infected human, quantified as the Force-Of-Infection (FOIHD). Transmission occurs as an infected deer (orange circle) interacts with susceptible deer (gray circles), transmitting SARS-CoV-2 through aerosols and fluid over the course of the animal’s infectious period (γ). When the individual recovers from its infection (gold circle), it will have stemmed several secondary infections (orange circle), quantified as the basic reproductive number (R0 = 4). Depending on the magnitude of FOIHD and R0 (dashed arrows), an outbreak of infections may occur across a deer population. Average prevalence in the Fall season is averaged across daily values (dark line) and incidence proportion can be calculated through the projected fall season (dotted line). This outbreak will either persist or fade determined by the deterministic steady state of the set of ODE equations considered in this study, referred to here as equilibrium (x-axis). The image of a human hand-feeding a deer was created with the assistance of DALL-E 2.
Fig 2.
A conceptual diagram of the Susceptible-Infectious-Recovered-Susceptible (SIRS) epidemiological model used for this simulation study.
Objectives that focused on specific captive or wild scenarios had no deer-deer fence line transmissions, preventing transmission between captive or wild populations. Objective 5 focused on how fence line transmission in captive-wild systems influence outbreak dynamics on both sides of the fence.
Table 1.
Model parameter estimates for SARS-CoV-2 Susceptible-Infected-Recovered-Susceptible (SIRS) ordinary differential equations (ODE).
Fig 3.
Variation in Force-Of-Infection from humans-to-deer (FOI), probability of at least 1 human-to-deer (HtD) transmission, and basic reproductive numbers (R0) across the four scenarios considered in this study.
Human Force-Of-Infection is log10 transformed and presented as odds of HtD transmission per deer, per day. The basic reproductive number threshold between unsustained and sustained transmission from deer-to-deer is indicated with a horizontal line (R0 = 1). Box plots depict the minimum, first quartile, median, third quartile, and maximum, with outliers depicted as single points.
Table 2.
Median metrics and 80% confidence intervals for simulated SARS-CoV-2 outbreaks in white-tailed deer in four scenarios.
Fig 4.
Distributions of average prevalence, persistence probability, and incidence proportion values during the 120-day fall projection in each scenario of interest.
1000 simulations were run for each scenarioBox and whisker plots depict the minimum, first quartile, median, third quartile, and maximum, with outliers depicted as single points. Error bars for persistence represent 95% confidence intervals.
Fig 5.
The relationship between human-to-deer Force-Of-Infection and (A) average SARS-CoV-2 prevalence, (B) persistence of SARS-CoV-2, and (C) the incidence proportion during the fall, dependent on the degree of transmission from deer-to-deer (R0).
Points indicate metrics for each iteration simulated, with point color and shading indicating a particular scenario. Fitted lines indicate trends in the data, fitted with a log-normal or logistic-regression for prevalence and persistence, respectively. Transmission categories included unsustained transmission (R0 <1), low, sustained transmission (1< R0 ≤ 3), medium, sustained transmission (3< R0 ≤ 5), and high, sustained transmission (R0 > 5).
Fig 6.
Variation of average prevalence, persistence, and incidence proportion during the 120-day fall projection.
Error bars for persistence represent 95% confidence intervals. Plots are faceted by scenario, with variation in outbreak characteristics displayed for continuous introduction from humans, and various degrees of initial, single introductions with no continuous introduction from humans. Box plots depict the minimum, first quartile, median, third quartile, and maximum, with outliers depicted as single points.
Table 3.
Increases in prevalence, persistence, and incidence proportion of SARS-CoV-2 outbreaks with simulated systems with deer in captive and wild scenarios interacting across fence lines.