Prevalence and Antimicrobial Susceptibility of Salmonella spp. and E. coli in Market Show Swine and Cattle After the Implementation of the Veterinary Feed Directive

To combat antimicrobial resistance, the FDA implemented the Veterinary Feed Directive that mandated removal of antimicrobials from feed in 2017. The objective of the current project was to evaluate antimicrobial susceptibility profiles of Escherichia coli and Salmonella spp. isolates obtained from show animals potentially not exposed to the selective pressures of in-feed antimicrobials (Post-VFD), and compare them to similar isolates collected in 2015 (Pre-VFD). Fecal samples were analyzed for susceptibility of 14 antimicrobials utilizing a microbroth dilution technique. Samples were collected from swine (n=40) and cattle (n=40) at a statewide show in 2020. Of the 309 total Post-VFD E. coli isolates, 71.84% were resistant to at least one antimicrobial, while 80.30% of Pre-VFD isolates (n=662) were resistance to at least one. Of the 159 Post-VFD swine isolates, 80.50% were resistant to Tetracycline and 43.40% to Streptomycin; similarly, of the 330 Pre-VFD swine isolates, 96.67% exhibited resistance to Tetracycline followed by 69.90% to Sulfisoxazole. Of the 159 Post-VFD cattle isolates, 48.00% exhibited resistance to Tetracycline and 26.67% to Streptomycin; similarly of the 332 Pre-VFD isolates, 55.12% exhibited resistance to Tetracycline and 32.53% to Streptomycin. Of the 5 Post-VFD swine Salmonella isolates, 60.00% were resistant to Tetracycline. The one steer Salmonella isolate was pansusceptible. Tetracycline is minimally used in human medicine; however, it is still important in animal health. While resistance to Streptomycin was observed less often, the antimicrobial is critically important in treating human infections. A significant interaction between the main effects of sampling year and animal species impacted differences in MIC levels in 7 of the 14 antimicrobials tested. Overall, this study saw varying degrees of susceptibility across the 14 different antimicrobials tested.