Antimicrobial susceptibility profiles among commensal Mannheimia haemolytica and Pasteurella multocida isolated from apparently healthy sheep processed in California: Results from a cross-sectional pilot study

Antimicrobial resistance (AMR) is a growing challenge for the successful treatment of bacterial infections in both human and veterinary medicine. Despite the need to mitigate AMR, food-producing animal species lack adequate information on bacterial susceptibility to support antimicrobial stewardship for conditions that drive antimicrobial usage, such as ovine respiratory disease (ORD). In an effort to help address this gap, the upper respiratory tract (URT) from 620 apparently healthy sheep carcasses was sampled at slaughter in California (CA), from April through September 2021, to determine the antimicrobial susceptibility profiles of commensal Mannheimia haemolytica and Pasteurella multocida, two bacteria commonly associated with ORD. Sheep sampled in the study were selected based on origin prior to processing (out-of-state or CA) and marketing status (antibiotic-free or conventional management), two factors hypothesized to influence antimicrobial susceptibility profiles. Of the total 620 carcasses sampled, 343 had at least one isolate of M. haemolytica or P. multocida recovered, for a recovery rate of 55.3 %. The recovery rate among sampled carcasses was 46.8 % (290/620) for M. haemolytica and 15.8 % (98/620) for P. multocida. Utilizing Clinical and Laboratory Standards Institute (CLSI) clinical breakpoints as interpretive criteria, all 98 P. multocida isolates were pansusceptible to the antimicrobials tested that are labeled for use in sheep, with the exception of one isolate that classified as intermediate to tilmicosin. Of the 290 M. haemolytica isolates, the greatest resistance was found to penicillin, with 51.0 % (148) of isolates classified as intermediate and 25.2 % (73) resistant, while nine isolates were resistant to chlortetracycline and oxytetracycline and one isolate was intermediate to spectinomycin. Multidrug resistance (resistance to three or more antimicrobial classes) was not found in any isolate of either target organism. Very low to no AMR was observed across both bacterial species tested, demonstrating that M. haemolytica and P. multocida in this apparently healthy sheep population remain broadly susceptible to the antimicrobials licensed for the treatment of ORD. In addition, antimicrobial susceptibility did not differ based on origin or marketing status, as defined for this study. Sampling at slaughter provides an opportunity to collect baseline information about antimicrobial susceptibility of the commensal flora of the sheep URT that can give rise to ORD, but may not be generalizable to sheep with clinical respiratory disease or to sheep reared by non-commercial producers.