Assessing respiratory pathogen communities and demographic performance of bighorn sheep populations: a framework to develop management strategies for respiratory disease

Abstract

Respiratory disease (pneumonia) is a persistent challenge for bighorn sheep (Ovis canadensis) conservation as sporadic epizootics cause up to 90% mortality in affected populations and are often followed by numerous years of low juvenile recruitment attributed to lamb pneumonia. Domestic sheep (Ovis aries) and domestic goats (Capra aegagrus hircus) are the origin of the disease and asymptomatically carry respiratory pathogens that cause respiratory disease when introduced to bighorn sheep. Pathogens that have been linked to respiratory disease in bighorn sheep include several species of bacteria in the Pasteurellaceae family and another bacterial species, Mycoplasma ovipneumoniae. Despite substantial efforts by management agencies to prevent contact between bighorn sheep and domestic sheep and goats, respiratory disease epizootics continue to affect bighorn sheep populations across much of their distribution with uncertain etiology. This study sought to investigate efficacy of diagnostic protocols in detecting Pasteurellaceae and Mycoplasma ovipneumoniae and 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 regularly-used 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, and consideration of detection probability discerned that there was low confidence in negative test results for populations where the Pasteurellaceae species were not detected. 76% of study populations hosted 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 populations.