Domestic Spillover of Mycoplasma ovipneumoniae with Concurrent Bluetongue Virus Implicated in Bighorn Sheep (Ovis canadensis) Epizootic Pneumonia Outbreak in Badlands, South Dakota, USA.

Respiratory disease remains an important threat to bighorn sheep (BHS; Ovis canadensis), complicated by fragmented populations and environmental stressors. Significantly, BHS epizootic pneumonia can lead to all-age die-offs and is population limiting. We describe an interagency investigation of a die-off within a BHS herd in the Badlands region of South Dakota, USA from 2021-22. Minimum survey counts began declining 2 mo following a reported domestic sheep (Ovis aries) and BHS interaction and dropped from 262 to 56 BHS over 9 mo. Postmortem testing included histopathology, bacterial culture, molecular testing, and partial microbial sequencing in 13 affected BHS. Data from neighboring domestic small ruminants and historical BHS data were compared to outbreak observations. Histopathology revealed moderate to severe bronchopneumonia in many cases. Half of animals examined harbored mild to heavy lungworm burdens (5/10). Pneumonic tissue culture yielded Trueperella pyogenes (10/11), Bibersteinia trehelosi (5/11), and no live Mycoplasma spp. (0/11). Conversely, real-time PCR detected widespread Mycoplasma ovipneumoniae (12/13) for the first time in this herd, with co-occurring bluetongue virus (BTV, 4/10). Two unique M. ovipneumoniae strains were found in BHS, one directly related to nearby domestic sheep isolates. Severe epizootic pneumonia was preceded by multi-strain M. ovipneumoniae spillover, with T. pyogenes causing suppurative pneumonia. High BHS density, a moderate-severe drought, and BTV transmission compounded disease susceptibility and mortality in this herd. Strengthened collaborations are necessary to manage BHS facing multiple coinciding health threats.