Document Type

Thesis - Open Access

Award Date

2026

Degree Name

Master of Science (MS)

Department / School

Natural Resource Management

First Advisor

William Severud

Abstract

Mycoplasma ovipneumoniae (M. ovi) decimates bighorn sheep (Ovis canadensis) populations through acute die-offs followed by years or decades of failed lamb recruitment. Test-and-remove management has emerged as the primary intervention to restore population growth, and this thesis quantifies the demographic costs of endemic M. ovi infection and evaluates the diagnostic tools upon which this strategy depends. I quantified vital rates across five bighorn sheep subherds (four infected, one uninfected) on the Pine Ridge Reservation (2023–2025), supplemented by pre-outbreak data (2017–2018) from the North Unit of Badlands National Park. Infected subherds declined (λ = 0.965, 95% CrI: 0.813–1.039) while the uninfected Palmer Creek subherd grew (λ = 1.199, 95% CrI: 0.936–1.285), driven predominantly by 90-day lamb survival of 8.1% (95% CI: 2.7–24.0%) in infected versus 90.9% (95% CI: 79.7–100%) in uninfected subherds (p < 0.001). Life table response experiment (LTRE) decomposition confirmed that lamb survival contributed 95.9% of the observed difference in λ, with a 96.1% posterior probability that infected λ was lower than uninfected λ. Piecewise exponential modeling identified a discrete window of infection-associated mortality in lambs, based on the rate ratio (RR), between days 22–70 of life (RR = 29.30, 95% CI: 10.16–84.43, p < 0.001), consistent with pneumonia-associated mortality in other enzootic populations. Known-fate adult survival did not differ between carriers (80.0%, 95% CI: 58.7–100.0%) and non-shedders (85.0%, 95% CI: 70.7–100.0%) of M. ovi, or between M. ovi infected (93.4%, 95% CI: 86.5–100.0%) and uninfected (90.2%, 95% CI: 78.2–100.0%) subherds, indicating that carriers are not selectively removed through natural mortality. Under sustained transmission, a proportion of surviving lambs likely acquire and maintain infection, continually replenishing the carrier pool and making natural disease clearance improbable in high-prevalence populations. To evaluate interlaboratory diagnostic consistency, I compared Washington Animal Disease Diagnostic Laboratory (WADDL) and Montana State University (MSU) using 105 paired samples from 76 bighorn sheep and found exclusively unidirectional discordance, with all discordant samples classified positive by MSU and negative by WADDL. MSU classified 7 of 12 WADDL indeterminate samples as positive, suggesting that indeterminate results may warrant treatment as presumptive positives for carrier identification during test-and-remove operations, though interlaboratory diagnostic variation cannot be excluded. Among individuals with serial paired tests, discordance between labs resulted in 25% (7 of 28) being assigned a different carriage classification (e.g., chronic, intermittent, cleared, or non-shedder) depending on whether WADDL or MSU results were used, directly affecting which animals would be prioritized for removal under the test-and-remove protocol. I also evaluated concordance between pooled pre-lambing qPCR data and subherd-level lamb die-offs, both to assess the prospective predictive value of carrier detection and to identify cases where complete lamb failure despite negative qPCR results suggested undetected carriage. At MSU cycle threshold (Ct) values ≤28.5 under an assumed chronic carriage framework, MSU achieved perfect predictive accuracy (AUC = 1.000), while WADDL failed to detect two carriers in subherds that subsequently experienced 100% lamb mortality. Current test-and-remove guidance indicates that intermittent carriers do not require removal and previously negative animals do not require retesting. However, intermittent carriers in this study preceded subherd-level lamb die-offs, and previously negative animals subsequently tested positive upon retesting, indicating that both assumptions are insufficient for achieving subherd clearance. These findings demonstrate that M. ovi creates a demographic trap resolvable through test-and-remove management, but only if diagnostic protocols are sensitive enough to detect all carrier types, underscoring the need for interlaboratory collaboration to ensure reliable carrier identification.

Publisher

South Dakota State University

Additional Files
Final Supplementary.pdf (403 kB)
Supplementary Figures & Tables
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Rights Statement

In Copyright