ASSOCIATIONS AMONG ANTIMICROBIAL DRUG TREATMENTS AND ANTIMICROBIAL RESISTANCE OF FECAL ESCHERICHIA-COLI OF SWINE ON 34 FARROW-TO-FINISH FARMS IN ONTARIO, CANADA

Logistic regression was used to model associations between antimicrobi al treatment and resistance among fecal Escherichia coli of finisher p igs at the farm level. Four sets of potential risk factors representin g different levels of refinement of antimicrobial use on farms were mo delled on resistance to antimicrobials. Final models for each antimicr obial were constructed from treatment and management variables signifi cant on initial screening, and corrections for overdispersion were mad e. In general, in-feed antimicrobial treatment of pigs was more consis tently associated with an increased risk of resistance than individual -animal treatment. Antimicrobial treatment in starter rations was sign ificant in final models of resistance to ampicillin, carbadox, nitrofu rantoin, sulfisoxizole, and tetracycline. Treatment in grower-finisher rations was significantly associated with resistance to ampicillin, s pectinomycin, sulfisoxizole, and tetracycline, There was little eviden ce that in-feed antimicrobials increased the risk of resistance to gen tamicin, which is a drug used only for individual-pig treatment in thi s study population. These results suggest that antimicrobial medicatio n of rations of post-weaning pigs selects for and maintains antimicrob ial resistance among E. coli of finisher pigs. Although resistance was common on farms that did not medicate rations of post-weaning pigs, t he results indicate that antimicrobial use does increase the risk of r esistance to the antimicrobials studied. (C) 1998 Elsevier Science B.V .