Antimicrobial activity in vitro against Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni from cattle with naturally occurring bovine respiratory disease and its association to clinical outcome

Antimicrobial resistance is frequently blamed for clinical failures of treatment or control of bovine respiratory disease (BRD). Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, and Mycoplasma bovis are generally recognized as the four main bacterial pathogens associated with BRD. This study used data from randomized, controlled, clinical trials that recorded qualitative classification of clinical outcomes (success, or failure) and of results of antimicrobial susceptibility testing (AST; classified as resistant, or not resistant to florfenicol, gamithromycin, tildipirosin, or tulathromycin). Association of results in vitro to clinical outcome for treatment or control of naturally occurring BRD (1 319 calves) was quantitatively evaluated. Clinical outcome was not significantly (P = 0.4643) associated (Fisher’s exact test) with qualitative results of AST in vitro for pathogens that were not exposed to antimicrobial medication in vivo (971 head of cattle). Clinical outcome was significantly (P < 0.0001) associated with qualitative results of AST in vitro for pathogens that were exposed to antimicrobial medication in vivo (348 head of cattle). For pathogens not exposed to antimicrobial medication in vivo, 1.85 % (95 % confidence interval [CI] = 0.38 to 5.32) of clinical failures were attributable to antimicrobial resistance. For pathogens exposed to antimicrobial medication in vivo, 51.72 % (95 % CI = 32.53 to 70.55) of clinical failures were attributable to antimicrobial resistance. In conclusion, antimicrobial resistance of bacterial pathogens associated with BRD and not exposed to antimicrobial medication in vivo is a quantitatively minor cause of clinical failure for treatment or control of naturally occurring BRD.