Evaluation of subtherapeutic use of the antibiotics apramycin and carbadox on the prevalence of antimicrobial-resistant Salmonella infection in swine

The antibiotics apramycin and carbadox were fed to growing swine, and the prevalence of Salmonella isolates that are resistant to apramycin and related aminoglycoside antibiotics was examined. Three hundred twelve Salmonella-positive pigs raised on one of five farms in an integrated swine operation and slaughtered at a central plant were used. All farms fed carbadox during the grower phase, and two farms administered apramycin during the first 21 days of age. Ileocolic lymph nodes and cecal contents were sampled at slaughter. One hundred of the 312 pigs were randomly selected to examine apramycin- and carbadox-resistant Salmonella infection, while all 312 pigs were used to evaluate the association between apramycin exposure and infection with Salmonella organisms resistant to amikacin, gentamicin, kanamycin, and streptomycin. Antimicrobial resistance was determined using disk diffusion and breakpoint concentrations. Apramycin treatment appeared to have little effect on apramycin- (12.5 versus 20.9%) or streptomycin- (76.4 versus 73.5%) resistant Salmonella isolates when averaged across farms and compared to control animals. Feeding carbadox resulted in carbadox-resistant Salmonella infection in only 5.3% of the isolates on one farm. The prevalence of amikacin-, gentamicin-, and kanamycin-resistant Salmonella isolates on farms feeding apramycin and carbadox were 0, 0, and 1.8%, respectively. Serogroup B was the most prevalent serogroup isolated, followed by C1 and E1. Apramycin and carbadox treatment did not appear to have any effect on the serogroup isolated. Subtherapeutic use of carbadox and apramycin did not appear to increase the prevalence of antimicrobial-resistant Salmonella in market-age swine.