Little is known about the drivers of critically important antibacterial resistance in species with zoonotic potential present on farms (e.g. CTX-M β-lactamase-positive Escherichia coli). There is also debate about the influence of farms on the circulation of resistance in local human populations. This was a two-year surveillance study on 53 dairy farms. E. coli positive for blaCTX-M were detected in 224/4145 (5.4%) of all samples from faecally-contaminated sites. E. coli positive for blaCTX-M were more prevalent (98/631; 15.5%) in calf samples and less prevalent (12/630; 1.9%) in samples collected from pastureland, including publicly accessible sites. Multilevel, multivariable logistic regression showed antibiotic dry cow therapeutic choice to be associated with risk of blaCTX-M positivity, including use of cefquinome or framycetin; 74% of blaCTX-M-positive E. coli were framycetin-resistant. Low temperature was associated with low risk of blaCTX-M positivity. This was additional to the effect of temperature on total E. coli density, a finding with profound implications for surveillance. There was no evidence that study farms had a significant impact on circulating blaCTX-M plasmids in the local human population: across 296 fully sequenced E. coli isolates, two cattle isolates shared blaCTX-M plasmids with eight urinary isolates collected in parallel.

Evidence for reduced CTX-M carriage in cattle-associated Escherichia coli at low temperatures and on publicly accessible farmland: implications for surveillance and potential for farm-to-human transmission