Resistance to antimicrobials used at hatchery level among clinical Escherichia coli isolates from Ontario broilers.

Antimicrobials are used for prophylactic purposes in some flocks because chicks are susceptible to pathogenic bacterial infection at the hatchery stage. The objectives of this study were to assess temporal changes in resistance to ceftiofur, gentamicin, and spectinomycin in Escherichia coli clinical isolates from Ontario broiler chickens between 2008 and 2015, to determine whether stage of production, year and season are predictors of resistance, and whether published data on antimicrobial use help to interpret the temporal patterns in resistance. Logistic regression revealed that stage of production, year, and season were significant predictors of resistance to all 3 antimicrobials. Resistance to ceftiofur fluctuated over time, with a trend towards decreasing resistance between 2013 to 2015; resistance to gentamicin and spectinomycin increased over the study period, with significantly high resistance clusters identified from 2013 to 2015. Comparisons with published hatchery-level antimicrobial use data suggest that these trends may reflect changes in use of ceftiofur and spectinomycin in Ontario.