Age-related decline in carriage of ampicillin-resistant Escherichia coli in young calves.

The presence of ampicillin-resistant Escherichia coli (Ampr E. coli) in the fecal flora of calves was monitored on a monthly basis in seven cohorts of calves. Calves were rapidly colonized by Ampr E. coli, with peak prevalence in cohort calves observed in the 4 months after the calves were born. The prevalence of calves yielding Ampr E. coli in cohorts consistently declined to low levels with increasing age of the calves (P < 0.001).