Dynamics of faecal shedding of Extended Spectrum beta-Lactamase or AmpC-producing Escherichia coli on dairy farms

Objectives: To explore the dynamics of faecal ESBL/AmpC shedding in dairy cattle and farmers, a study was conducted to examine changes in shedding by individual animals as well as enviromental exposure, and to study the association beween antimicrobial use (AMU) and ESBL/AmpC shedding. Methods: The study comprised a cross-sectional survey on 20 farms and a 1 year follow-up on 10 farms. Faecal samples were cultured by both direct inoculation on MacConkey agar + 1 mg/L cefotaxime (MC+), and enrichment in LB-broth + 1 mg/L cefotaxime with subsequent inoculation on MC+. Dust samples were collected using Electrostatic Dust fall Collectors (EDC). Human faecal samples were collected by the farmers. Presence of ESBL/AmpC genes was screened by PCR and sequencing. Using mixed effects logistic regression, odds ratios were determined and population attributable fractions (PAF) calculated subsequently. Results: In phase 1, 8/20 farms were positive for ESBL/AmpC, and, with 2 negative farms, were selected for phase 2. Transient shedding of dominant allele variants was observed in the animals. EDCs and human faecal samples did not reflect what was observed in the animals. AMU was related to shedding of ESBLs in the next sampling moment (OR 14.6 (3.0-80.0)) and PAF of AMU was 0.36 (0.08-0.77). Calves fed with colostrum from cows on dry-off therapy was not a risk factor (OR 1.7 [0.7-4.9, p=0.28]).Conclusions: The presence ESBL/AmpC could only be partly explained by AMU. No link was shown in shedding between cattle and humans or the environment. Interventions should focus on prevention of introduction.