Relationship between level of antibiotic use and resistance among Escherichia coli isolates from integrated multi-site cohorts of humans and swine.
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The objective of this longitudinal ecological study was to examine the relationship between the prevalence of antibiotic-resistant (AR) commensal Escherichia coli isolates from both monthly human wastewater and composite swine fecal samples and the concurrent aggregated monthly antibiotic use recorded within each host species in multi-site vertically integrated swine and human populations. In addition, human vocation (swine worker versus non-swine worker), swine production group, and season were examined as potential confounding variables. Human and swine E. coli isolates (n=2469 human and 2310 swine, respectively) were tested for antimicrobial susceptibility using a commercial broth microdilution system. In the human population, among swine workers the relative odds of tetracycline resistance were increased significantly for tetracycline (class) drug use at the third quartile and above of mean monthly dosage (MMD) (OR=1.8) as compared to the referent category (non-use). The relative odds of ciprofloxacin resistance were significantly increased for ciprofloxacin use in non-swine workers (OR=5.5) as compared to the referent (non-use). The relative odds of tetracycline resistance were increased significantly for chlortetracycline use in medicated feed for the upper tertile of MMD category (OR=2.9) as compared to the referent category (no use) across all swine production groups. While high variability among seasonal samples over the 3-year period was observed, no common seasonal trends relating to antibiotic use and prevalence of resistance over the 3-year period were apparent. The overall effects of concurrent human and swine antibiotic use on AR E. coli levels were inconsistent and modest in this study.