Levels and trends of antimicrobial resistance in Campylobacter spp. from chicken in the UK

Campylobacter spp. are the most common bacterial cause of foodborne illness in the UK, with chicken considered to be the most important vehicle of transmission for this organism. It is estimated there are 500,000 cases of campylobacteriosis in the UK annually, with Campylobacter jejuni (C. jejuni) and Campylobacter coli (C. coli) accounting for approximately 91% and 8 % of infections, respectively. Although severe infection in humans is uncommon, treatment is seldom needed for human infection but usually involves the administration of a macrolide (e.g., azithromycin) or a fluoroquinolone (e.g., ciprofloxacin). An increased rate of resistance in Campylobacter in chicken to such antimicrobials could limit effective treatment options for human infections and it is therefore important to monitor changes in rates of resistance over time. In this report we analysed trends in antimicrobial resistance (AMR) in C. jejuni and C. coli isolated from chicken in the UK. The chicken samples were from chicken reared for meat (ie. broiler chicken as opposed to layer chicken (ie. egg-laying chicken)) and included chicken sampled at slaughterhouses as well as from retail stores in the UK. Datasets included AMR results from retail surveys of Campylobacter spp. on chicken sampled in the UK from various projects in the time period from 2001 to 2020. In the retail surveys, samples were obtained from stores including major and minor retail stores throughout the UK (in proportion to the population size of each nation) and Campylobacter spp. testing was performed using standard methods with the majority of isolates obtained from direct culture on standard media (mCCDA). Data from national scale surveys of broiler chicken, sampling caecal contents and carcase neckskins at slaughterhouses, undertaken by APHA in 2007/2008, and between 2012 and 2018 were also included in the study. In the APHA-led surveys, Campylobacter were isolated using standard culture methods (culture onto mCCDA) and antimicrobial susceptibility testing was performed by a standard microbroth dilution method to determine the minimum inhibitory concentration (MIC) of isolates. Care was taken when comparing data from different studies as there had been changes to the threshold used to determine if an isolate was susceptible or resistant to an antimicrobial in a small number of scenarios. Harmonised thresholds (using epidemiological cut-off (ECOFF) values) were employed to assess AMR with appropriate adjustments made where required to allow meaningful comparisons of resistance prevalence over time. Data from additional isolates where resistance to antimicrobials were predicted from genome sequence data were also considered.