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

## TL;DR

This study examines how antimicrobial use regulations in agriculture affected resistance in Salmonella from cows in New York, finding some reductions in resistance for specific antibiotics.

## Contribution

The study provides empirical evidence on the impact of FDA antimicrobial use restrictions on Salmonella resistance in bovine samples.

## Key findings

- The 2012 AMU restriction was linked to decreased odds of resistance to chlortetracycline, oxytetracycline, and neomycin.
- MIC values for chlortetracycline and oxytetracycline decreased significantly after the 2012 AMU restriction.
- The 2017 AMU restriction was associated with reduced odds of resistance to florfenicol.

## Abstract

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 Salmonella, remains unclear. To address this gap in knowledge, we analyzed veterinary diagnostic laboratory data on 2413 Salmonella 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). Salmonella 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 Salmonella isolated from bovine samples, depending on the antimicrobial.