Association between farm-level antimicrobial usage and antimicrobial resistance in <i>Escherichia coli</i> isolates from healthy swine in ten farms in Japan

We examined the antimicrobial resistance of 1,493 Escherichia coli isolates from 440 swine fecal samples (211 and 229 samples were obtained from the sow and fattening groups, respectively) on ten farms and analyzed the association between the antimicrobial resistance rate and the annual antimicrobial usage at the farm level adjusted by the daily defined dose in Japan. For most antimicrobial agents, the rate of resistance was greater on farms with higher levels of antimicrobial use, suggesting that the amount of the corresponding antimicrobial class used at the farm level may have affected the resistance rates. Some antimicrobial resistance was positively associated with the use of the corresponding or structurally related antimicrobial agents: resistance to tetracycline in the sow group and resistance to chloramphenicol, streptomycin, kanamycin, sulfonamide-trimethoprim, and nalidixic acid in the fattening group. In addition, for many antimicrobial-resistant strains, an association was found with the use of single or multiple antimicrobial classes other than the corresponding class: resistance to ampicillin and kanamycin in the sow group and resistance to ampicillin, cefazolin, cefoxitin, cefotaxime, tetracycline, chloramphenicol, streptomycin, kanamycin, gentamicin and nalidixic acid in the fattening group. In the case of the association with the noncorresponding class, most of the E. coli isolates were multidrug resistant, suggesting that there is a high possibility of coselection on farms and that reducing the use of multiple antimicrobial agents may be effective at decreasing antimicrobial resistance.