Outcomes of Selective Removals for Control of Pneumonia in a Bighorn Sheep Metapopulation

Pneumonia is a pervasive, population-limiting disease of bighorn sheep (Ovis canadensis) with limited options for management. We conducted a selective removal experiment in two regions (north and south) of the Hells Canyon bighorn sheep metapopulation to test the hypothesis that pneumonia is maintained in bighorn sheep populations by chronic carriers of the bacterium Mycoplasma ovipneumoniae. We detected M. ovipneumoniae in 83 adults over 11 years across seven study populations. We removed five carriers of M. ovipneumoniae and nine non-carriers from two treatment populations in northern Hells Canyon and 15 chronic carriers from a treatment population in southern Hells Canyon. We did not remove any sheep from four control populations. Local elimination of M. ovipneumoniae in the two northern treatment populations within a year after removals was indicated by no further detection of the pathogen, waning antibody levels, and lack of antibody in animals born after removals. Elimination in treatment populations was followed by fadeout of M. ovipneumoniae in the four adjacent control populations over the next 4 years without any further removals. Selective removals were associated with a decline in prevalence but did not eliminate M. ovipneumoniae in the southern treatment population. Clearance of infection led to nearly doubling of survival over the first 4 months of life, a 74% increase in recruitment to 7-10 months of age, and an increase in the average annual rate of population growth from 1% to 12%. The results of this experiment provide support for a focus on carriers of M. ovipneumoniae for mitigating low lamb recruitment associated with pneumonia-induced mortality observed in many bighorn sheep populations across North America. However, mixed outcomes indicate that a better understanding of infection persistence and fadeout could increase the effectiveness of management interventions.