Occurrence of and risk factors for extended-spectrum cephalosporin-resistant Enterobacteriaceae determined by sampling of all Norwegian broiler flocks during a six month period

: All broiler flocks reared and slaughtered in Norway from May-October 2016 (n = 2110) were screened for the presence of extended-spectrum cephalosporin (ESC) -resistant Enterobacteriaceae. Furthermore, we investigated possible risk factors for occurrence of such bacteria in broiler flocks. The odds of a flock being positive for ESC-resistant Enterobacteriaceae increased if the previous flock in the same house was positive, and if the flock was reared during September-October. However, we cannot exclude seasonal fluctuations in occurrence of ESC-resistant Enterobacteriaceae during the months November to April. The overall occurrence of ESC-resistant Enterobacteriaceae was 10.4%, and primarily linked to the presence of blaCMY (82.6%) in positive isolates. We describe the first findings of Escherichia coli with blaCTX-M-1, Klebsiella pneumoniae with both blaCTX-M-15 and blaSHV-12, and K. pneumoniae with blaCMY isolated from Norwegian broiler production. This study gives us a unique overview and estimate of the true occurrence of ESC-resistant Enterobacteriaceae in Norwegian broilers over a six-month period. To the best of our knowledge, this is the most comprehensive study performed on the occurrence of ESC-resistant Enterobacteriaceae in a broiler population.