Cross-sectional study of antimicrobial-resistant Salmonella in market-age swine

Abstract

There is increasing concern about the widespread use of antibiotics in agriculture and veterinary medicine and the role that this plays in emerging microbial resistance. To investigate this problem, we examined antimicrobial resistance levels among Salmonella isolates recovered from the lymph nodes and cecal contents of market-age swine at slaughter. These swine were raised on five farms in an integrated Texas swine operation. We tested two hypotheses: 1) the somatic serogroup or serous of the Salmonella isolate is associated with its antimicrobial resistance status; and 2) exposure to subtherapeutic doses of apramycin in the first 21 days post-birth is associated with antimicrobial-resistant Salmonella infection among market-age swine. Antimicrobial resistance was determined by disk diffusion using 13 antibiotics common in the treatment of disease in human and veterinary medicine. Three hundred and sixty-five Salmonella isolates were analyzed. Using breakpoint concentrations, 100 of these Salmonella isolates were tested for resistance to apramycin and barbados, antibiotics administered subtherapeutically on the farms. Upon examination of the 365 Salmonella isolates, we found that lymph node and cecal isolates differed in their frequency of resistance to penicillin G (p=0.03) and sulfisoxazole (p<0.01). Resistance to chlortetracycline, penicillin G, streptomycin and sulfisoxazole was prevalent (>19.2%). We detected a significant association between the somatic serogroup and samovar of the Salmonella isolate and resistance to penicillin G, streptomycin and sulfisoxazole. This association differed based on the antibiotic of interest and the source of the isolate. Although the prevalence of apramycin-resistant Salmonella infection was greater among swine that received apramycin in the first 21 days post-birth than among those that did not, this difference was not significant (odds ratio (OR)=2.3, 95% confidence interval (CI): 0.8 - 7.1). Streptomycin-resistant Salmonella infection also was more prevalent among pigs that received apramycin, but this difference was nonsignificant after adjusting for somatic serogroup (OR=1.7, 95% CI: 0.8 - 3.4) and samovar (OR=1.3, 95% CI: 0.6 - 2.8). These findings suggest that resistance to a given antibiotic is associated with the source and the somatic serogroup and serovar of the Salmonella isolate.