Establishment of disk diffusion national epidemiological cut-off values for Pasteurella multocida and Mannheimia haemolytica isolated from bovine respiratory diseases and characteristics of antibiograms in Japan.

Antimicrobial resistance is a global public health threat, and accurate antimicrobial susceptibility testing is crucial for guiding appropriate antimicrobial use. Although the disk diffusion method is widely used in livestock, standardized breakpoints for veterinary pathogens are lacking. Bovine respiratory disease, a major cause of economic loss in calves, is commonly associated with Pasteurella multocida and Mannheimia haemolytica. This study aimed to establish inhibition zone diameter cut-off values for disk diffusion and to create an antibiogram for these pathogens. The cut-off values were calculated based on minimum inhibitory concentration (MIC) reference values: the Clinical and Laboratory Standards Institute clinical breakpoints for penicillin and tetracycline; the European Committee on Antimicrobial Susceptibility Testing epidemiological cut-off values for kanamycin and tilmicosin (P. multocida); and MIC cut-off values calculated from this study for cefazolin, marbofloxacin, thiamphenicol, and tylosin. The cut-off values for penicillin and tetracycline provide useful guidance for clinical drug selection, whereas caution is required when interpreting the results for the other agents evaluated because their cut-off values were based on epidemiological cut-offs rather than clinical breakpoints. Re-evaluation of previously reported cut-off values for kanamycin, cefazolin, and tetracycline using a larger number of isolates provided more robust thresholds that were less susceptible to strain-specific variation. The antibiogram revealed low susceptibility rates for some agents. Because antimicrobial use is associated with the development of resistance, these findings indicate the need for prudent antimicrobial stewardship and continued surveillance of resistance in livestock pathogens.