Dynamics of extended-spectrum cephalosporin-resistant Escherichia coli in pig farms: A longitudinal study.

Point prevalence estimates of extended-spectrum cephalosporin-resistant Escherichia coli (ESC-R-Ec) are important surveillance measures but may not uncover the ESC-R-Ec dynamics within pig farms. A longitudinal study was therefore performed by sampling individual pigs, pig farmers and the environment. On average, 30 (range 10-46) piglets of 31 Swiss farms were sampled during the suckling, weaning and fattening stages (n= 2437 samples). In addition, stool from pig farmers and environmental samples were obtained and metadata collected by questionnaires. ESC-R-Ec was identified by routine culture, and clonal relationships and resistance genes were derived from whole genome sequencing data. Working on pig farms was not associated with an increased prevalence of ESC-R-Ec in humans. ESC-R-Ec prevalence significantly decreased from 6.2% to 3.9% and 1.8% for the suckling, weaned and fattening pigs, respectively (P < 0.001). Within the 57 ESC-R-positive suckling piglets, persisting carriage was detected in 25 animals at two consecutive time points and one animal at three consecutive time points. Clonal spread (n=7 farms, 22.6%) and horizontal gene transfer (n=1 farm, 3%) within pigs but not between humans and animals was detected. Liquid manure (n=10 samples, 16.7%) was identified as the major environmental reservoir of ESC-R-Ec in the pig farm environment. Pig farming practices like all-in-all-out systems, but not antimicrobial usage, were associated with reduced risk of ESC-R-Ec at the farm level. As carriage duration is normally short within the individual pigs, the risk of recolonisation and clonal spread of ESC-R-Ec might be reduced by applying appropriate decontamination strategies