A longitudinal field trial assesing the impact of feeding waste milk containing antibiotic residues on the prevalence of ESBL-producing Escherichia coli in calves.

A longitudinal field trial was carried out on a farm known to harbour cefotaximase (CTX-M)-positive Escherichia coli, in order to assess the impact of feeding waste milk containing antibiotic residues (WM+AR) on the prevalence of these bacteria in the faeces of calves. Fifty calves were alternately assigned to one of two groups at birth and fed either milk replacer (control group) or WM+AR (treatment group). Faecal samples were collected from all calves daily for the first week after enrolment, twice weekly until weaning, then weekly for a further six weeks. Environmental samples from the calf housing were collected weekly. WM+AR and powdered milk samples were examined for antibiotic residues and CTX-M-positive E. coli. Total E. coli and CTX-M-positive E. coli in faecal samples were enumerated using selective media. Regression analyses were performed on the bacterial count data using a population-averaged approach based on generalised estimating equations (GEE) to account for repeated measurements on individual calves over time. Cefquinome, a fourth generation cephalosporin, was detected in 87% of WM+AR samples at a mean concentration of 0.746 mg/l. All environmental sampling locations yielded CTX-M-positive E. coli. Significantly more pen floor samples were positive in the treatment group. Calves in the treatment group shed greater numbers of CTX-M-positive E. coli than calves in the control group throughout the study, and shedding decreased at a slower rate in the treatment group. CTX-M-positive E. coli persisted in a larger number of calves fed WM+AR compared with calves fed milk replacer where the prevalence in the treatment group declined significantly slower over time. There was no difference between calves fed WM+AR or calves fed milk replacer in the proportion of E. coli isolates that were CTX-M-positive. These findings indicate that feeding WM+AR increased the amount of resistant bacteria shed in the faeces. Shedding of CTX-M-positive E. coli persisted for longer in calves fed WM+AR, and persisted after weaning. Crown