Association between tetracycline consumption and tetracycline resistance in Escherichia coli from healthy Danish slaughter pigs.

It has been recognized that exposure to antimicrobial agents can exert a selective pressure for the emergence of antimicrobial resistance. The objective of this study was to investigate an association between the probability of isolating a tetracycline-resistant Escherichia coli isolate from the intestinal tract of healthy pigs and patterns of tetracycline consumption in the herds of origin, together with other risk factors. Data on antimicrobial resistance, antimicrobial consumption, and pig herd demographics were obtained from different Danish surveillance programs. Descriptive statistics were performed for the risk factors in relation to the susceptibility status. Logistic regression analysis was performed to identify risk factors with significant effect on the log odds of tetracycline resistance of E. coli isolates. The model showed that an increase in the interval between last prescription and sampling date would decrease the probability of isolating a resistant E. coli isolate (p-value = 0.01). Also, a direct association between treatment incidence rate in a herd and probability of resistance was detected (p-value = 0.03). Other risk factors found to have a significant effect in the isolate susceptibility status were number of produced animals in the year and year of sampling. Other antimicrobial consumption risk factors, such as number of prescriptions and amount prescribed, although not included in the final model, presented indirect impact in the tetracycline resistance probability. From this study, we can infer that tetracycline usage, the time span between last treatment and sampling date, together with herd size and the proportion of animals being treated in a herd, increase the probability of obtaining a resistant isolate.