Long-term carriage of Mycoplasma ovipneumoniae in bighorn sheep and implications for test-and-remove protocols

Abstract

Mitigating disease in wild populations is challenging, especially for endemic infections. Removal of infected individuals can be a viable strategy but requires careful consideration of protocols, desired outcomes, and allowable loss. Pneumonia has devastated bighorn sheep populations in North America for decades, presenting persistent management and conservation challenges. Animals that are chronically infected with Mycoplasma ovipneumoniae (M. ovi) are thought to be responsible for the persistence of M. ovi in a population and removing chronic carriers (i.e., test and remove) has been a promising management tool to mitigate pneumonia. Although selective removal of chronic carriers has the potential to reduce pathogen prevalence, patterns of M. ovi carriage are not well understood, which can hinder designing the most effective test-and-remove protocols. Using data across 8 years and four populations of bighorn sheep, we aim to understand patterns of pathogen carriage across an animal's life and to evaluate removal outcomes of hypothetical test-and-remove scenarios to help inform protocols of test-and-remove efforts. Pathogen prevalence was seasonal—animals were 1.62 (SE = 0.39) times more likely to test positive for M. ovi in spring than in autumn. Further, an animal that tested positive in autumn was more likely to test positive again the following season compared with an animal that tested positive in spring, which has implications for when to test animals during a test-and-remove effort if the goal is to identify animals that consistently test positive (i.e., chronic carriers). On average, an animal that tested positive for M. ovi had a 21%–55% chance of testing positive again the following year, indicating chronic carriage. Hypothetical test-and-remove scenarios illustrated that the use of a single positive test for removal criteria could increase the number of animals removed by 12 or 23 percentage points compared with scenarios focusing removal on only chronic carriers, depending on the timing of testing. Overall, we highlight the consequences for animal removals based on the timing of testing and defining removal criteria to help inform test-and-remove efforts.