Acquisition and epidemiology of antibiotic-resistant Escherichia coli in a cohort of newborn calves.

OBJECTIVES The acquisition of antibiotic-resistant commensal Escherichia coli was examined in a cohort of newborn calves. METHODS Faecal samples were collected weekly from calves over a 4 month period and screened for E. coli resistant to ampicillin, apramycin and nalidixic acid at concentrations of 16, 8 and 8 mg/L, respectively. E. coli viable counts were performed on samples from a subset of calves. RESULTS All calves acquired ampicillin- and nalidixic acid-resistant E. coli, while only 67% acquired apramycin-resistant E. coli during the study. Sixty-seven per cent of samples were resistant to at least one of the three antibiotics. Prevalence of ampicillin and nalidixic acid resistance was high initially and declined significantly with age (P < 0.001). No temporal or age-related pattern was observed in the prevalence of apramycin resistance. Housing the cohort had a significant effect on the prevalence of nalidixic acid resistance (P < 0.001). Total and ampicillin- and nalidixic acid-resistant E. coli counts declined with calf age (P < 0.001), with the rate of decline in ampicillin-resistant counts being greater than that for total counts (P < 0.001). The proportion of total E. coli counts that were resistant to ampicillin or nalidixic acid also declined with age (P < 0.001). CONCLUSIONS Cohort calves rapidly acquired antibiotic-resistant bacteria within days of birth. Carriage of resistant bacteria was associated with both age and housing status of the cohort.