The impact of antimicrobial use regulations on antimicrobial resistance among isolates from bovine samples submitted to a veterinary diagnostic laboratory in Central New York.

In recognition that antimicrobial resistance in human pathogens may stem from antimicrobial use in agricultural settings, the United States Food and Drug Administration (FDA) ordered restrictions on antimicrobial usage (AMU) in food-producing animals. In 2012 the FDA restricted the extra-label use of third-generation cephalosporins, and in 2017 the FDA mandated veterinary oversight for the use of antimicrobials in the feed and water of food-producing animals and eliminated production-related uses. However, the impact of these restrictions on the antimicrobial resistance status of important pathogens, such as , remains unclear. To address this gap in knowledge, we analyzed veterinary diagnostic laboratory data on 2413 isolates from submitted bovine samples. We fitted logistic regression models to evaluate changes in proportions of antimicrobial-resistant isolates, and we used accelerated failure time (AFT) models to determine changes in minimum inhibitory concentration (MIC) values. Our analysis revealed the 2012 AMU restriction to be associated with a decrease in the odds of resistance to chlortetracycline (OR = 0.49; 95 % CI = 0.28-0.86), oxytetracycline (OR = 0.47; 95 % CI = 0.27-0.82), and neomycin (OR = 0.45; 95 % CI = 0.25-0.80). Furthermore, we found significant decreases in MIC values for chlortetracycline (CR = 0.74; 95 % CI = 0.62-0.87) and oxytetracycline (CR = 0.64; 95 % CI = 0.56-0.73) for the same AMU restriction. We found a significant association between the 2017 AMU restriction and decreased odds of resistance to florfenicol (OR = 0.28; 95 % CI = 0.09-0.92). serotype was an important predictor of resistance to all antimicrobials assessed via logistic regression or AFT models. Overall, our study suggests that in the region served by the laboratory, AMU restrictions have either had no detectable effect or are associated with decreasing AMR and MIC trends for isolated from bovine samples, depending on the antimicrobial.