Modelling management strategies for chronic disease in wildlife: Predictions for the control of respiratory disease in bighorn sheep

Abstract

1. Controlling persistent infectious disease in wildlife populations is an ongoing challenge for wildlife managers and conservationists worldwide, and chronic diseases in particular remain a pernicious problem.2. Here, we develop a dynamic pathogen transmission model capturing key features of Mycoplasma ovipneumoniae infection, a major cause of population declines in North American bighorn sheep Ovis canadensis. We explore the effects of model assumptions and parameter values on disease dynamics, including density-versus frequency-dependent transmission, the inclusion of a carrier class versus a longer infectious period, host survival rates, disease-induced mortality and recovery rates and the epidemic growth rate. Along the way, we estimate the basic reproductive ratio, R 0 , for M. ovipneumoniae in bighorn sheep to fall between approximately 1.36 and 1.74.3. We apply the model to compare efficacies across a suite of management actions following an epidemic, including test-and-remove, depopulation-andreintroduction, range expansion, herd augmentation and density reduction. 4. Our results suggest that test-and-remove, depopulation-and-reintroduction and range expansion could help persistently infected bighorn sheep herds recovery following an epidemic. By contrast, augmentation could lead to worse outcomes than those expected in the absence of management. Other management actions that improve host survival or reduce disease-induced mortality are also likely to improve population size and persistence of chronically infected herds. Synthesis and applications. Dynamic transmission models like the one employedhere offer a structured, logical approach for exploring hypotheses, planning field experiments and designing adaptive management. We find that management strategies that removed infected animals or isolated them within a structured metapopulation were most successful at facilitating herd recovery from a low-prevalence, chronic pathogen. Ideally, models like ours should operate iteratively with field experiments to triangulate on better approaches for managing wildlife diseases.

Bighorn sheep show similar in‐host responses to the same pathogen strain in two contrasting environments

“…Managers planning test‐and‐remove efforts surrounding new disease events should proceed with caution within the epidemic's first few months to avoid removing animals who might eventually recover (especially if the disease‐induced mortality burden of the event among adults is low). Lastly, data on incubation period, timing of seroconversion, and timing of clinical signs have direct utility for epidemiological models that help guide management (e.g., Almberg et al, 2021 ). We hope these data can aid research and management going forward.…”

“…Researchers have speculated that environmental context could have important bearing on the severity of M. ovipneumoniae spillover events (Butler et al, 2018 ; Manlove et al, 2014 ), and these speculations are supported by several comparative studies documenting variation in bighorn sheep disease severity across environments (e.g., Butler et al, 2018 ). There is evidence that herd mixing patterns may constrain disease burden in lambs (Manlove et al, 2014 ) and could therefore play a role in herd recovery (Almberg et al, 2021 ; Dugovich et al, 2017 ; Lula et al, 2020 ). Environmental context could also effect epidemiological patterns.…”

“…There is evidence that herd mixing patterns may constrain disease burden in lambs (Manlove et al, 2014) and could therefore play a role in herd recovery (Almberg et al, 2021;Dugovich et al, 2017;Lula et al, 2020). Environmental context could also effect epidemiological patterns.…”

Demographic uncertainty and disease risk influence climate‐informed management of an alpine species

“…We quantified risk for 29 extant populations of bighorn sheep that overlap mountain goat herds, 58 extant populations of mountain goats, and up to 5 additional mountain goat populations (as specified by each AMS). At the time of analysis, 1 population of bighorn sheep was sympatric with a mountain goat population that was known to carry pneumonia‐associated pathogens, while 24 mountain goat populations were sympatric with bighorn sheep that were known to carry pneumonia‐associated pathogens (Almberg et al 2016, 2018, 2019). Therefore, 1–29 bighorn sheep populations were exposed to pneumonia‐associated pathogens from mountain goats, and 24–58 mountain goat populations were currently harboring or exposed to pathogens from bighorn sheep.…”

Model Integrated Disease Management to facilitate effective translatable solutions for wildlife disease issues

“…The adoption of epidemiological models can thus support and guide in situ management actions a priori (e.g. Mycoplasma ovipneumoniae , Almberg et al, 2022). On‐ground disease management efforts in wildlife are the practical application of model‐derived management recommendations, which may reveal unidentified or underappreciated variables relevant to management (e.g.…”