Transmission dynamics of extended-spectrum β-lactamase and AmpC β-lactamase-producing Escherichia coli in a broiler flock without antibiotic use

•Transmission rates and routes of ESBL/AmpC-EC were quantified in an organic broiler flock.•ESBL/AmpC-EC prevalence peaks at day 3, with almost 100% of broiler samples positive.•A reproduction ratio of 1.7 indicates that ESBL/AmpC-EC transmit and persist in the flock.•Previous infectiousness of a broiler reduces the probability to become infectious again.•Total ESBL/AmpC-EC persistence seems to depend on shifts in phylogenetic groups. Extended-spectrum and AmpC β-lactamase-producing Escherichia coli (ESBL/AmpC-EC) are found throughout the broiler production pyramid. Transmission of resistance between broilers and humans could occur at any point, representing a potential public health issue. Insight in farm transmission dynamics could provide a basis for control, leading to fewer contaminated broilers. The aim was quantifying transmission rates and routes of ESBL/AmpC-EC, and specific phylogenetic groups, in an organic broiler flock without antibiotic use. In each of two consecutive production rounds, 80 randomly chosen broilers were followed individually. Cloacal swabs from these, 20 other randomly chosen broilers, and 11 environmental samples were taken at several moments from arrival till slaughter. ESBL/AmpC-EC were isolated by selective pre-enrichment, and ESBL/AmpC-genes and E. coli phylogenetic groups were determined. Transmission parameters (β) were estimated using a Generalised Linear Model with a susceptible-infectious-susceptible model. Effect of direct broiler contact as compared to contact through the environment and previous carriage c.q. infectious status and their interaction were included as explanatory variables. Multiplying β by the length of the infectious period gives the reproduction ratio (R). On day 1, prevalence was 28.8% (95%CI 19.2–40.0%) and 0.0% (95%CI 0.0–4.5%) among individually followed broilers, in round 1 and 2 respectively. In round 2, the environment was positive before arrival of day-old chicks. After 3 days, almost 100% of broilers and environmental samples were positive in both rounds. Most samples were positive for CTX-M-1 group genes, and A1 and B1 were predominant phylogenetic groups. From day 3 there was a shift towards more phylogenetic groups. R was 1.70 (95%CI 0.55–5.25) for total ESBL/AmpC-EC. Risk for broilers to become infectious was lower if previously infectious (βpreviously infectious=0.02 vs. βnot previously infectious=3.41; P