Phenotypic antimicrobial resistance (AMR) of avian pathogenic Escherichia coli (APEC) from broiler breeder flocks between 2009 and 2018

Colibacillosis is one of the most important diseases in poultry production. The use of antimicrobials remains a therapeutic cornerstone for avian pathogenic E. coli (APEC), thereby contributing to the development of antimicrobial resistance (AMR). The aim of this study was to characterize AMR in broiler breeder flocks reared under commercial conditions. Data covering 10 years, from 2009 to 2018, were used to evaluate the phenotypic AMR of 264 APEC isolates obtained from 158 broiler breeder flocks of a large company in Portugal. The APEC isolates were tested against eleven antimicrobials by the Kirby-Bauer disc diffusion test. The annual proportion of AMR was calculated by dividing the number of APEC isolates with phenotypic resistance by the total number of APEC isolated that year. Similarly, the overall AMR of the whole period was calculated. The relationship of antimicrobial resistance with time (years) was investigated with a generalized linear model using logistic regression. The overall AMR of the 10-year period was: amoxicillin 78%, ampicillin 73.5%, tetracycline 63.3%, doxycycline 56.4%, apramycin 34.5%, neomycin 68.2%, enrofloxacin 32.6%, flumequine 39.4%, co-trimoxazole 47.7%, florfenicol 46.6% and lincospectin 66.3%. Over time, a significant decrease in AMR was observed for amoxicillin and ampicillin, neomycin, flumequine, co-trimoxazole, florfenicol and lincospectin. Multidrug resistance (MDR) decreased from 100% in 2009 to 48% in 2018. Only seven (2.7%) APEC strains were fully susceptible to all tested antimicrobials. The decrease over time of AMR in APEC likely reflects the efficacy of manifold improvements in broiler breeder production systems. A further reduction in AMR is still desirable.