Observational study on antimicrobial resistance in and isolates from Ontario calf samples submitted to a diagnostic laboratory from 2007 to 2020.

The objectives of this study were to i) describe and isolates; ii) investigate the temporal trends in antimicrobial resistance (AMR) profiles; and iii) evaluate the impact of season and age on these AMR profiles from diagnostic and post-mortem samples in Ontario calves ≤ 2-months-old submitted from 2007 to 2020 to the Animal Health Laboratory in Guelph, Ontario, Canada. Antimicrobial susceptibility testing results were measured by the Kirby-Bauer disk diffusion method. A total of 1291 isolates with AMR profiles were obtained from calves, with ( = 434) and ( = 378) being the most common bacteria characterized for AMR. For , 79% of isolates tested showed a positive result in F5/K99, whereas for isolates, . Typhimurium (33%) and . Dublin (22%) were the 2 most common serotypes identified. Multivariable logistic regression models were built to evaluate AMR profiles for ( = 414) and ( = 357) to each antimicrobial tested. Most isolates (91%) and isolates (97%) were resistant to at least one of the antimicrobials tested. In general, and had higher odds of resistance in calves aged ≥ 2 wk compared to 1-week-old calves, and little difference was seen in the level of resistance over the years observed or between seasons in most of the antimicrobials tested. Prospective research should investigate potential risk factors for the development of AMR in calves examples being antimicrobial use and farm management practices.