Imperfect Tests, Pervasive Pathogens, and Variable Demographic Performance: Thoughts on Managing Bighorn Sheep Respiratory Disease

Respiratory disease (pneumonia) has been a persistent challenge for bighorn sheep (Ovis canadensis) conservation and its cause has been attributed to numerous bacteria including Mycoplasma ovipneumoniae and several Pasteurellaceae family species. This study sought to investigate efficacy of diagnostic protocols in detecting Pasteurellaceae and Mycoplasma ovipneumoniae, generate sampling recommendations for different protocols, assess the distribution of these disease agents among 17 bighorn sheep populations in Montana and Wyoming, and evaluate what associations existed between detection of these agents and demographic performance of bighorn sheep populations. Analysis of replicate samples from individual bighorn sheep revealed that detection probability for regularlyused diagnostic protocols was generally low (<50%) for Pasteurellaceae and was high (>70%) for Mycoplasma ovipneumoniae, suggesting that routine pathogen sampling likely mischaracterizes respiratory pathogen communities. Power analyses found that most pathogen species could be detected with 80% confidence at the population-level by conducting regularly-used protocols multiple times per animal. Each pathogen species was detected in over half of the study populations, but after accounting for detection probability there was low confidence in negative test results for populations where Pasteurellaceae species were not detected. Seventy-six percent of study populations hosted both Mycoplasma ovipneumoniae and Pasteurellaceae pathogens, yet a number of these populations were estimated to have positive population growth rates and recruitment rates greater than 30%. Overall, the results of this work suggest that bighorn sheep respiratory disease may be mitigated by manipulating population characteristics and respiratory disease epizootics could be caused by pathogens already resident in bighorn sheep population

Respiratory pathogens and their association with population performance in Montana and Wyoming bighorn sheep populations.

Respiratory disease caused by Mycoplasma ovipneumoniae and Pasteurellaceae poses a formidable challenge for bighorn sheep (Ovis canadensis) conservation. All-age epizootics can cause 10-90% mortality and are typically followed by multiple years of enzootic disease in lambs that hinders post-epizootic recovery of populations. The relative frequencies at which these epizootics are caused by the introduction of novel pathogens or expression of historic pathogens that have become resident in the populations is unknown. Our primary objectives were to determine how commonly the pathogens associated with respiratory disease are hosted by bighorn sheep populations and assess demographic characteristics of populations with respect to the presence of different pathogens. We sampled 22 bighorn sheep populations across Montana and Wyoming, USA for Mycoplasma ovipneumoniae and Pasteurellaceae and used data from management agencies to characterize the disease history and demographics of these populations. We tested for associations between lamb:ewe ratios and the presence of different respiratory pathogen species. All study populations hosted Pasteurellaceae and 17 (77%) hosted Mycoplasma ovipneumoniae. Average lamb:ewe ratios for individual populations where both Mycoplasma ovipneumoniae and Pasteurellaceae were detected ranged from 0.14 to 0.40. However, average lamb:ewe ratios were higher in populations where Mycoplasma ovipneumoniae was not detected (0.37, 95% CI: 0.27-0.51) than in populations where it was detected (0.25, 95% CI: 0.21-0.30). These findings suggest that respiratory pathogens are commonly hosted by bighorn sheep populations and often reduce recruitment rates; however ecological factors may interact with the pathogens to determine population-level effects. Elucidation of such factors could provide insights for management approaches that alleviate the effects of respiratory pathogens in bighorn sheep. Nevertheless, minimizing the introduction of novel pathogens from domestic sheep and goats remains imperative to bighorn sheep conservation

Assessing respiratory pathogen communities in bighorn sheep populations: Sampling realities, challenges, and improvements

<div><p>Respiratory disease has been a persistent problem for the recovery of bighorn sheep (<i>Ovis canadensis</i>), but has uncertain etiology. The disease has been attributed to several bacterial pathogens including <i>Mycoplasma ovipneumoniae</i> and <i>Pasteurellaceae</i> pathogens belonging to the <i>Mannheimia</i>, <i>Bibersteinia</i>, and <i>Pasteurella</i> genera. We estimated detection probability for these pathogens using protocols with diagnostic tests offered by a fee-for-service laboratory and not offered by a fee-for-service laboratory. We conducted 2861 diagnostic tests on swab samples collected from 476 bighorn sheep captured across Montana and Wyoming to gain inferences regarding detection probability, pathogen prevalence, and the power of different sampling methodologies to detect pathogens in bighorn sheep populations. Estimated detection probability using fee-for-service protocols was less than 0.50 for all <i>Pasteurellaceae</i> and 0.73 for <i>Mycoplasma ovipneumoniae</i>. Non-fee-for-service <i>Pasteurellaceae</i> protocols had higher detection probabilities, but no single protocol increased detection probability of all <i>Pasteurellaceae</i> pathogens to greater than 0.50. At least one protocol resulted in an estimated detection probability of 0.80 for each pathogen except <i>Mannheimia haemolytica</i>, for which the highest detection probability was 0.45. In general, the power to detect <i>Pasteurellaceae</i> pathogens at low prevalence in populations was low unless many animals were sampled or replicate samples were collected per animal. Imperfect detection also resulted in low precision when estimating prevalence for any pathogen. Low and variable detection probabilities for respiratory pathogens using live-sampling protocols may lead to inaccurate conclusions regarding pathogen community dynamics and causes of bighorn sheep respiratory disease epizootics. We recommend that agencies collect multiples samples per animal for <i>Pasteurellaceae</i> detection, and one sample for <i>Mycoplasma ovipneumoniae</i> detection from at least 30 individuals to reliably detect both <i>Pasteurellaceae</i> and <i>Mycoplasma ovipneumoniae</i> at the population-level. Availability of PCR diagnostic tests to wildlife management agencies would improve the ability to reliably detect <i>Pasteurellaceae</i> in bighorn sheep populations.</p></div

Power to detect five respiratory pathogens in bighorn sheep populations using different diagnostic protocols.

<p>For all panels, each curve illustrates the power to detect each pathogen at the population-level (y-axis) given the number of animals sampled from a population (x-axis) when the specified protocol was conducted once per animal. Within each panel, protocols where detection probability was estimated at zero or one are not displayed. The horizontal dashed-gray line across each panel represents adequate (i.e., 80%) detection power.</p

Summary of diagnostic protocols used in this study to detect respiratory pathogens in bighorn sheep.

<p>Summary of diagnostic protocols used in this study to detect respiratory pathogens in bighorn sheep.</p

Power to detect five respiratory pathogens in bighorn sheep populations using the TSB protocols.

<p>Variability in power to detect pathogens at the population level is shown as it relates to different pathogens, population size, pathogen prevalence, number of animals sampled, and number of times protocols are conducted per animal. For all panels, each curve illustrates the power to detect each pathogen (y-axis) given the number of animals sampled from a population (x-axis). The horizontal dashed-gray line across each panel represents adequate (i.e., 80%) detection power. A. Variability in the power to detect each pathogen when the TSB protocol is conducted once per animal at three levels of pathogen prevalence in a population of 100. B. Effects of conducting TSB protocol multiple times per animal on power to detect two pathogens with either relatively high or relatively low detection probability in a population of 100 animals with 10% pathogen prevalence. C. Effect of population size on power to detect two pathogens with either relatively high or relatively low detection probability in a population of 100 animals with 10% pathogen prevalence.</p

Estimated prevalence and 95% confidence intervals for four respiratory pathogens in four bighorn sheep populations.

<p>Prevalence estimates obtained where protocols only used culture tests to detect pathogens are indicated by open symbols, those obtained where protocols used a combination of culture and PCR tests are shown as gray symbols and those obtained where only PCR tests were used are shown by black symbols. The mean numbers of protocols conducted per animal are shown across the x-axis of each panel. Naïve prevalence estimates (the proportion of animals a pathogen was detected in for a given sampling occasion) are indicated with gray asterisks. Prevalence estimates for <i>Mannheimia haemolytica</i> or <i>Mannheimia spp</i>. are for beta hemolytic or leukotoxigenic strains. 95% confidence intervals were inestimable when detection probability was estimated at 0 or 1.</p

Estimated detection probabilities and 95% confidence intervals for five respiratory pathogens in bighorn sheep.

<p>One set of protocols was used to detect the four <i>Pasteurellaceae</i> organisms (shaded) and a separate set of protocols was used to detect <i>Mycoplasma ovipneumoniae</i> (not shaded). Detection probabilities for <i>Mannheimia haemolytica</i>, <i>Mannheimia spp</i>., and <i>Bibersteinia trehalosi</i> are for beta hemolytic or leukotoxigenic strains. Protocols that used fee-for-service diagnostic tests are indicated with an asterisk (*) in the legend and above the upper confidence limit. The total number of samples assessed using each protocol is indicated in the legend.</p