Serotype and anti-microbial resistance trends among bovine Salmonella isolates from samples submitted to a veterinary diagnostic laboratory in central New York, 2007-2021

Aims: Salmonella enterica is a leading cause of acute enteritis in people, and dairy cattle are an important reservoir of this pathogen. The objective of this study was to analyse serotype and anti-microbial resistance trends of Salmonella isolated from dairy cattle in the United States between 2007 and 2021.Methods and Results: We collected data for bovine Salmonella isolates obtained from samples submitted to Cornell University's Animal Health Diagnostic Center (AHDC). We analysed 5114 isolates for serotype trends, and a subset of 2521 isolates tested for anti-microbial susceptibility were analysed for resistance trends. The most frequently identified serotypes were Salmonella Cerro, Dublin, Typhimurium, Montevideo, 4,[5],12:i:-, and Newport. Among these serotypes, a Cochran-Armitage trend test determined there was a significant increase in the proportion of isolates serotyped as Salmonella Dublin (p < 0.0001) and Montevideo (p < 0.0001) over time. There was a significant decrease in the proportion of isolates serotyped as Salmonella Cerro (p < 0.0001), Typhimurium (p < 0.0001), and Newport (p < 0.0001). For the anti-microbial resistance (AMR) analysis, we found an overall increase in the proportion of multi-drug-resistant isolates over time (p = 0.009). There was a significant increase in the proportion of isolates resistant to ampicillin (p = 0.007), florfenicol (p = 0.0002), and ceftiofur (p < 0.0001) and a marginal increase in resistance to enrofloxacin (p = 0.05). There was a significant decrease in the proportion of isolates resistant to spectinomycin (p = 0.0002), trimethoprim/sulphamethoxazole (p = 0.01), sulphadimethoxine (p = 0.003), neomycin (p < 0.0001), and gentamicin (p = 0.0002).Conclusions: Our results provide evidence of an increase in resistance to key anti-microbial agents, although the observed trends were driven by the sharp increase in the proportion of Salmonella Dublin isolates over time.