Dynamic Changes in Antimicrobial Resistance in Fecal from Neonatal Dairy Calves: An Individual Follow-Up Study.

The prevalence of antimicrobial-resistant (AMR) is typically higher in the feces of young dairy calves than in the feces of older cattle; however, the underlying factors contributing to this difference are poorly understood. In this study, AMR fecal from neonatal calves were characterized both at phenotypic and genotypic levels by individual follow-up sampling. Antimicrobial resistance profiles of isolates from the maternal colostrum were also determined. Most of the fecal AMR emerged in the calves at 2-3 days of age. The was the most prevalent resistance gene detected among AMR fecal from <7-day-old calves, and was also detected in two isolates from the maternal colostrum. Weekly sampling revealed changes in the phenotype of AMR fecal as the calves aged. More than half of the fecal isolates acquired additional resistance to beta-lactams by 21-28 days of age, and minimum inhibitory concentrations were higher in ceftiofur-exposed calves than in unexposed calves. Our findings reveal the dynamic changes in AMR fecal from neonatal calves, and suggest that the feeding of colostrum and ceftiofur administration contribute to the higher prevalence of AMR in young dairy calves.