Influence of weaning on fecal shedding of pathogenic bacteria in dairy calves.

The objectives of the current research were to determine the effect of weaning on fecal shedding of Salmonella and Escherichia coli O157:H7 in dairy calves and to examine cultured isolates (to include Enterococcus) for antimicrobial susceptibility. This research was conducted on one large commercial dairy (>3000 head) in the southwestern United States. Two collections were made, during the winter (January 2009) and summer (July 2009) seasons. For the winter collection, two groups of calves were sampled (group 1: n = 18 pens, 69 head, ∼12 weeks of age; group 2: n = 19 pens, 75 head, ∼10 weeks of age). Fecal samples were collected from all calves via rectal palpation 2 days pre- and again 2 days postweaning. For the summer collection, one group of calves housed in 40 pens were utilized and 79 and 76 calves sampled 7 days pre- and 5 days postweaning, respectively. Fecal samples were collected into sterile palpation sleeves, placed on ice, and shipped to our laboratory for bacterial culture of E. coli O157:H7, Salmonella, and Enterococcus. Antimicrobial susceptibility was determined on select isolates. No differences (p > 0.10) in prevalence of Salmonella or E. coli O157:H7 were observed due to weaning in the winter collection. In the summer collection, more (p < 0.01) fecal samples were Salmonella positive preweaning (15.2%) as compared to postweaning (2.6%). No differences were observed for antimicrobial susceptibility in isolates collected pre- as compared to postweaning in either winter or summer collections, with the exception that multidrug-resistant Enterococcus faecium isolates preweaning were resistant to six antibiotics compared to seven or eight antibiotics postweaning (summer collection). Results of the current research indicate that the weaning of dairy calves does not increase the prevalence of pathogenic bacteria or substantially modify antimicrobial susceptibility profiles of these bacteria.