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Submitted to: United States Animal Health Association Proceedings
Publication Type: Proceedings Publication Acceptance Date: 9/30/2014 Publication Date: 10/16/2014 Citation: Meinersmann, R.J. 2014. Antimicrobial resistance profiles in pathogens isolated from chickens. 57th American Association of Veterinary Laboratory Diagnosticians/118th United States Animal Health Association Proceedings. October 16-22, 2014. Kansas City, Missouri. Interpretive Summary: Technical Abstract: Antimicrobial resistance profiles are frequently studied from the perspective of epidemiology and not so often from the perspective of population genetics. The population geneticist assumes that gene flow, vertically (generation to generation), horizontally (individual to individual) or migratory (habitat to new habitat) is driven by Darwinian selection. Thus we analyze changes in trait prevalence to see how they correlate with possible selective factors. Antimicrobial usage is certainly a strong selective pressure, but the selective cost of maintaining resistance genes without the presence of the antimicrobials is less clear. With this in mind, we analyzed data from the National Antimicrobial Resistance Monitoring System (NARMS) for isolates collected from chickens at slaughter from 1997 to 2013. In this period we have collected 3,283 isolates of Campylobacter coli, 6,597 isolates of C. jejuni, 16,943 isolates of Escherichia coli and 16,608 isolates of Salmonella enterica. Since all these isolates came from commercially raised chickens throughout the US, the antimicrobial pressures should have been the same for all the bacterial species. Of the several antimicrobial resistance traits that were monitored it was noted that tetracycline resistance has gradually decreased in Campylobacter and E. coli and streptomycin resistance has decreased in E. coli (not monitored in Campylobacter), but in Salmonella the prevalence of tetracycline and streptomycin resistance dramatically increased in that period. A closer look at Salmonella shows that in the observation period the prevalence of serovar Kentucky increased from 25.2% up to 45.7%. The prevalence of serovar Heidelberg decreased from 23.8% down to 5.6% and serovar Enteritidis rose from less than 1% to a peak of 27.3% in 2011 and since has fallen to 15.2%. Such changes were not seen for other serovars. Fascinatingly, when considering the percent of all chicken Salmonella isolates in that time period, streptomycin or tetracycline resistant serovar Enteritidis remained less than 1%, resistant serovar Heidelberg fell from less than 10% to less than 1% for those antimicrobials, but resistant serovar Kentucky rose from 4.2% up to 32.1% for tetracycline and from 2.8% up to 36.9% for streptomycin. Analysis of Salmonella enterica serovar Kentucky by cluster analysis of pulsed-field gel electrophoresis (PFGE) suggests development of two lineages with new antimicrobial resistance; one lineage gaining just streptomycin resistance and another lineage that is simultaneously gaining tetracycline and streptomycin resistance. Thus, the prevalence of Salmonella enterica serovar Kentucky has been dramatically increasing in chickens at slaughter carrying increased resistance to streptomycin and tetracycline but the expansion is probably for reasons other than antimicrobial resistance. |
