• Mariana Fonseca
• Luke C. Heider
• Henrik Stryhn
• J T. McClure
• Daniella Rizzo
• Simon Dufour
• Herman W. Barkema
• David F. Kelton
• David Renaud
• Javier Sanchez

Escherichia coli can be a reservoir of antimicrobial resistance (AMR) genes and are commonly used as indicators of AMR on farms. There is a debate on the role of the antimicrobial route of administration on AMR. The objectives of this study were to 1) investigate the phenotypic prevalence of AMR in E. coli, and 2) explore the association between the route of antimicrobial use (AMU) and AMR in E. coli.

Pooled fecal samples from 5 animals from each group (calves, heifers, lactating cows) and a manure storage sample were collected from 140 dairy herds. Antimicrobial use rate was calculated as a defined course dose per population time (DCD/100 animals-year) and separated into intramammary and systemic use. Antimicrobial susceptibility testing was conducted using microbroth dilution methods. A multivariate, multivariable logistic model was built with resistance to several antimicrobials as the outcome.

Overall, 24.5% (266/1086) of the E. coli isolates were resistant to at least one antimicrobial. Resistance to third-generation cephalosporins, fluoroquinolones, and carbapenems was 2.2, 1.4, and 0.1%, respectively. Twenty-seven percent of the isolates were resistant to tetracycline. The proportion of multidrug-resistant isolates was 15.0%. Median intramammary and systemic AMU DCD/100 animal-years were 79.9 and 22.7, respectively. Intramammary AMU was not associated with AMR, but systemic use was. For instance, for an IQR increase in the systemic AMU, the odds of AMR increased in 19%.

The resistance pattern was comparable to previous studies on dairy cattle in Canada. AMR was associated with the systemic AMU but not with intramammary administration.