Vertical transmission of Escherichia coli carrying plasmid-mediated AmpC (pAmpC) through the broiler production pyramid

In Sweden the prevalence of Enterobacteriaceae with transferable resistance to extended-spectrum cephalosporins (ESCs) is low. However, in broilers ESC-resistant Escherichia coli is common, with a clear dominance of bla(CMY-2). Antimicrobials are rarely used in broiler production in Sweden and cephalosporins are never used. Introduction through imported breeding stock and subsequent vertical transmission of the bacteria through the production pyramid could be one explanation for this high prevalence. To test this hypothesis, paper linings from imported flocks of grandparent animals were screened for the presence of ESC-resistant E. coli and a positive flock, together with its progeny, was followed longitudinally through the production pyramid using boot swabs. The relationship of isolated ESC-resistant E. coli was investigated using multiple-locus variable number tandem repeat analysis (MLVA). ESC-resistant E. coli carrying bla(CMY-2) was isolated from six out of eight imported flocks of grandparent animals. One clone of E. coli carrying bla(CMY-2) occurred in all levels of the production pyramid and in flocks of imported grandparent animals. E. coli carrying bla(CMY-2) is frequently present among grandparent animals imported to Sweden for breeding purposes. The occurrence of one clone in all levels of the production pyramid indicates that its introduction through imported breeding stock and vertical transmission through the production pyramid could be one explanation for the high proportion of Swedish broilers colonized with ESC-resistant E. coli.