One Health collaboration is more effective than single-sector actions at mitigating SARS-CoV-2 in deer

The emergence and global spread of SARS-CoV-2 across humans, domestic animals, and wildlife presents a complex One Health mitigation problem. The virus likely originated in Old World bats (Rhinolophidae) and has since caused a pandemic with major socioeconomic impacts. Widespread human transmission has driven spillback into multiple mammalian hosts, including domestic species, ferrets, and wildlife. Infections of farmed mink and white-tailed deer have raised particular concern due to documented animal-to-human transmission and the emergence of novel variants in these species. Surveillance since 2020 has repeatedly detected SARS-CoV-2 in deer across U.S. states, including strains closely related to contemporaneous human variants and legacy strains persisting in deer after replacement in humans. A highly divergent lineage with mutations associated with non-human hosts was identified in a human sample linked to deer contact, suggesting repeated human-to-deer transmission and sustained deer-to-deer transmission. Deer are widespread and abundant in North America, creating numerous opportunities for human–deer interactions and potential viral exchange. Governance is fragmented: wildlife management agencies oversee wild deer, agricultural agencies regulate captive deer, and public health agencies guide human health, complicating coordinated responses. To address this, the authors convened a multisector guidance committee representing wildlife management, agricultural management, and public health to frame decisions about managing SARS-CoV-2 transmission between humans and deer. The committee articulated sector-specific and shared fundamental objectives, mapped causal chains of transmission, and identified management alternatives, including One Health strategies requiring joint actions. This work aims to provide evidence-based guidance for policy and actions to mitigate aerosol-associated SARS-CoV-2 transmission risks at the human–deer interface.

Background evidence indicates that SARS-CoV-2 has spilled back from humans into multiple mammals, with particular concern for farmed mink and white-tailed deer due to evidence of animal–human transmission and variant evolution. Since 2020, detections across U.S. deer populations have shown multiple strains mirroring those circulating in nearby humans, indicating repeated human-to-deer transmission. A divergent lineage consistent with non-human host adaptation was identified in a human with deer contact in Ontario, and legacy human strains have been found persisting in deer months after being supplanted in humans, implying deer can maintain transmission. Experimental studies demonstrate deer susceptibility, upper respiratory replication, and shedding via nasal secretions, supporting direct transmission via social contact. Collectively, prior work suggests that deer populations can serve as reservoirs contributing to viral evolution and potentially reintroduction to humans, underscoring the need for a One Health approach. Key cited works include surveillance and infection studies in free-ranging deer, evidence of variant evolution in deer, and decision analysis frameworks relevant to cross-sector governance.

- Governance and problem framing: A multisector guidance committee was formed with 7 wildlife management (WM), 5 agricultural management (AM), and 2 public health (PH) representatives. Across 10 meetings (1–2 hours each), the committee defined the decision context, fundamental objectives, causal chains for human–deer and deer–deer transmission, and a suite of single-sector and joint One Health alternatives. Fundamental objectives included minimizing human infection risk, maximizing deer health, minimizing evolutionary risk (persistence), maximizing hunter satisfaction, maintaining agency authorities, maximizing public trust, and providing timely data-driven risk measures. Influence diagrams were developed to visualize links among sources of introduction/spread, alternatives, and objectives. - Transmission modes and sources: Two transmission modes were identified—direct (aerosols/fluids during social contact) and indirect (fomites/wastewater). Based on deer social behavior and experimental evidence of URT replication and nasal shedding, analyses focused on direct transmission for deer-to-deer and deer-to-human pathways; no evidence was assumed for indirect modes in this assessment. - Management alternatives: • AM sector: (1) enhance air ventilation in enclosed captive-deer settings; (2) double-fence captive facilities; (3) vaccinate captive deer with boosters. • PH sector: (1) educate the public to limit interactions with wild deer in suburban/non-hunting contexts; (2) encourage proper use of enhanced PPE during interactions with captive and wild deer. • WM sector: (1) pause permitted research involving close contact with wild deer; (2) eliminate baiting/feeding; (3) reduce wild deer densities. - Dynamic modeling: A deterministic compartmental SIRS-type model was developed linking deer harvest and SARS-CoV-2 transmission among humans, captive deer, and wild deer. Populations were closed over a 120-day fall season (September–December), with no births/deaths/immigration/emigration; harvest was modeled as random with equal likelihood across S, I, R states. Homogeneous mixing was assumed, with transmission occurring from infectious humans or deer to susceptible deer in both captive and wild settings. Deer could move from susceptible to infectious to recovered and eventually return to susceptible (waning immunity). Parameterization used empirical data and expert elicitation, targeting agro-forested Midwestern U.S. conditions. - Settings and scenarios: Four settings were defined to represent variation in deer and human densities and contact rates: rural wild (10 deer/km²; ~3.1 humans/km²), suburban wild (same deer density; ~100 humans/km² with elevated human–deer contact), captive ranch (lower-density captive with elevated human–deer contact), and captive intensive (high-density captive). Settings were combined into two interaction scenarios: (1) rural wild deer interacting with captive ranch deer and rural human populations; (2) suburban wild deer interacting with intensive captive herds and suburban human populations. - Simulation design and outputs: Baseline introduction and spread were assessed with replicate simulations sampling parameter uncertainty (200 replicates for some analyses; elsewhere 2020 replicates were run to characterize uncertainty). A population of 1000 deer per setting over 120 days was simulated with continuous hazard of human-to-deer spillover from local human infections. Key outputs included: probability of at least one human-to-deer transmission event in 120 days; basic reproduction number R0 for deer-to-deer transmission; median prevalence in deer (proxy for human risk); per capita cumulative infections in deer (proxy for deer health); and probability of persistence (fraction of runs with endemic equilibrium prevalence >0.001). Alternatives were evaluated against objectives 1–3 using consequence tables to compare performance across captive and wild herds in each scenario.

- Human-to-deer spillover probability over 120 days (population of 1000 deer): rural wild median 0.01 (95% PI: 2.2×10^-25–0.25); suburban wild 0.20 (95% PI: 0.02–0.96); captive ranch 0.55 (95% PI: 0.03–0.99); captive intensive 0.87 (95% PI: 0.08–1.0). - Deer-to-deer transmission (R0 medians): intensive captive 6.24 (95% PI: 0.23–144.0); captive ranch 1.53 (95% PI: 0.73–3.21); rural and suburban wild 0.74 (95% PI: 0.35–1.3), with skewed distributions occasionally >1.0 in wild settings. - Scenario outcomes: Intensive captive herds interacting with captive deer and humans had the highest prevalence (median ~37%; 95% PI: 0.00–107.9), while rural wild herds interacting with ranch captive deer and humans had the lowest (median ~3%; 95% PI: 0.00–11.10%). Per capita cumulative infections were highest in intensive captive herds (median 1.46; 95% PI: 0.0001–3.72) and lowest in rural wild deer (median 0.168; 95% PI: 0.20–0.231). Probability of persistence over 120 days: intensive captive 97% of simulations; rural wild 64%. - Effectiveness of single-sector alternatives: Vaccination with boosters in captive deer (AM sector) most effectively reduced per capita infections and persistence in captive settings, with spillover benefits to sympatric wild deer. WM actions eliminating baiting/feeding and reducing wild deer densities were effective at lowering prevalence and cumulative infections in wild deer but had little effect in captive deer. PH sector education/PPE interventions were evaluated; proper PPE use was emphasized as an important mitigation during close encounters. - One Health vs single-sector: The integrated One Health alternative requiring coordinated actions by AM, WM, and PH agencies was the only strategy that eliminated SARS-CoV-2 transmission in captive deer and substantially reduced transmission in wild deer. Single-sector actions provided partial reductions but underperformed relative to the coordinated One Health approach.

The study demonstrates that coordinated One Health strategies can achieve greater reductions in SARS-CoV-2 transmission across human, captive deer, and wild deer interfaces than any single-sector intervention. Decision analysis facilitated cross-agency dialogue, clarified objectives, and supported transparent evaluation of trade-offs, helping agencies align management actions with health outcomes across sectors. The results underscore the high transmission potential in intensive captive settings (high R0, high persistence) and comparatively lower, though sometimes self-sustaining, transmission in wild settings depending on contact and density conditions. From a management perspective, captive-deer vaccination with boosters emerged as a key AM action that directly reduces disease burden in captivity and indirectly benefits nearby wild populations. WM measures that reduce contacts in wild deer (density reduction, eliminating baiting/feeding) effectively mitigate spread in wild herds, consistent with broader epidemiological insights on contact-driven transmission. PH guidance, including education and consistent PPE use during close human–deer interactions, is an important layer of protection. However, the integrated One Health strategy outperformed these individual measures by acting concurrently on multiple transmission pathways. Implementation challenges include fragmented authorities, differing mandates and decision processes across agencies, resource commitments, and potential misalignment of perceived costs and benefits. Despite these hurdles, the findings argue for sustained cross-sector coordination to track evolving risks, share data, and adapt interventions, particularly in regions with high deer densities and substantial human–deer contact.

This work provides quantitative evidence that coordinated One Health actions surpass single-sector interventions in mitigating SARS-CoV-2 transmission in white-tailed deer, eliminating transmission in captive settings and substantially reducing it in wild herds. The study contributes a structured decision-making framework, an influence-diagram-guided set of management alternatives, and a deterministic SIRS model parameterized for realistic human–deer contact scenarios. These tools can inform policy, guide recommendations, and support enforcement strategies across wildlife, agricultural, and public health agencies. Future research should extend evaluation beyond the three disease-related objectives to include social and governance objectives (e.g., hunter participation, public trust, agency authority), incorporate site-specific conditions and constraints, and refine ecological and epidemiological parameters. Modeling enhancements could include demographic processes, heterogeneous mixing, movement between herds, longer time horizons to capture evolution, and systematic assessment of compliance and implementation feasibility.

- Coordination and governance: Effective One Health implementation requires multi-agency coordination across disjoint authorities, mandates, and funding mechanisms, which may be difficult to achieve and sustain. Benefits and costs may be unevenly distributed among agencies, affecting buy-in. - Scope of objectives: Only three disease-related objectives (prevalence, cumulative infections, persistence) were quantitatively evaluated; other fundamental objectives (e.g., hunter satisfaction, public trust, agency authority, timely data) were not operationalized in this analysis. - Model assumptions: Closed populations over 120 days with no demography, homogeneous mixing, and random harvest may not reflect real-world heterogeneity in contacts, movement, and age structure. The focus on direct transmission excludes potential indirect pathways (e.g., fomites, wastewater), which were assumed unsupported for this assessment. - Parameter uncertainty and generalizability: Parameterization relied partly on expert elicitation for contact rates and setting definitions. Results were tailored to agro-forested Midwestern U.S. conditions and may require localization for other regions. Some reported replicate counts differ across sections (e.g., 200 vs. 2020), reflecting potential reporting inconsistencies. - Practical constraints and compliance: Effectiveness of WM actions (e.g., eliminating baiting/feeding, reducing densities) can be limited by compliance and feasibility, especially in areas with high hunting participation. - Vaccine availability: While vaccination/boosting in captive deer was highly effective in simulations, there is currently no deer-specific SARS-CoV-2 vaccine; use of broader-spectrum vaccines may require experimental authority and regulatory considerations.