Prevalence and antimicrobial resistance patterns of Salmonella enterica in preweaned calves from dairies and calf ranches.

OBJECTIVE: To evaluate serovar and antimicrobial resistance patterns of Salmonella enterica isolated from preweaned calves and identify management risk factors associated with fecal shedding of S enterica. SAMPLE POPULATION: Cohorts of 10 to 15 preweaned calves (1 to 84 days of age) on 26 dairies and 7 calf ranches and cross-sectional samples of preweaned calves on smaller farms. PROCEDURES: Calves were evaluated every 2 weeks during a 6-week period. Salmonella isolates obtained from rectal fecal swabs underwent antimicrobial susceptibility testing against 12 antimicrobials. Cluster analysis enabled description of antimicrobial susceptibility patterns. Calf, cohort, and farm risk factors associated with both the prevalence of S enterica and multiple-antimicrobial-resistant S enterica in preweaned calves were identified with repeated-measure logistic regression models.
RESULTS: Salmonella enterica was detected on > 50% of farms and in 7.5% of 3,686 fecal samples. Many isolates (33%) were resistant to multiple antimicrobials. Shedding of Salmonella spp was negatively associated with increasing calf age, herds being closed to incoming cattle, and antimicrobial supplementation of milk replacer; prophylactic antimicrobial treatment in day-old calves increased shedding. No association between farm management and presence of multiple-antimicrobial-resistant S enterica or between calving management and presence of S enterica in calves < or = 1 week old was detected. CONCLUSIONS AND CLINICAL RELEVANCE: In preweaned calves, the most important factors associated with decreased likelihood of fecal shedding of S enterica were the use of antimicrobial-supplemented milk replacer and maintenance of a closed herd. Infection with multiple-antimicrobial-resistant S enterica was not associated with antimicrobial administration.