Removal of chronic carrier ewes eliminates pneumonia in a bighorn sheep population.

Chronic pathogen carriage is one mechanism that allows diseases to persist in populations. We hypothesized that persistent or recurrent pneumonia in bighorn sheep () populations may be caused by chronic carriers of (). Our experimental approach allowed us to address a conservation need while investigating the role of chronic carriage in disease persistence.We tested our hypothesis in two bighorn sheep populations in South Dakota, USA. We identified and removed chronic carriers from the Custer State Park (treatment) population. Simultaneously, we identified carriers but did not remove them from the Rapid City population (control). We predicted removal would result in decreased pneumonia, mortality, and prevalence. Both population ranges had similar habitat and predator communities but were sufficiently isolated to preclude intermixing.We classified chronic carriers as adults that consistently tested positive for carriage over a 20-month sampling period ( = 2 in the treatment population;  = 2 in control population).We failed to detect or pneumonia in the treatment population after chronic carrier removal, while both remained in the control. Mortality hazard for lambs was reduced by 72% in the treatment population relative to the control (CI = 36%, 91%). There was also a 41% reduction in adult mortality hazard attributable to the treatment, although this was not statistically significant (CI = 82% reduction, 34% increase). : These results support the hypothesis that is a primary causative agent of persistent or recurrent respiratory disease in bighorn sheep populations and can be maintained by a few chronic carriers. Our findings provide direction for future research and management actions aimed at controlling pneumonia in wild sheep and may apply to other diseases.