Critically Important and of Highest Priority: The Evolving Story of Cephalosporin Use and Antimicrobial Resistance in U.S. Dairy Cattle

Under current United States (U.S.) regulations, a dairy cow is eligible for slaughter 13 days following the last of 2 doses of ceftiofur crystalline-free acid (CCFA) for the treatment of metritis. A matched-pair longitudinal study was employed to monitor levels of antimicrobial resistance among fecal Escherichia coli and Salmonella across 3 dairy farms in the U.S. High Plains to evaluate microbial safety pertaining to antimicrobial resistance on the first-eligible slaughter date. Environmental samples were collected from multiple areas throughout the farm prior to beginning the animal trial. Cows diagnosed with post-parturient metritis via veterinary protocol were pair-matched based on lactation number and calving date. A baseline fecal sample (day 0) was taken prior to the first administered dose of CCFA with a second CCFA dose administered 72 hours later. Additional fecal samples were taken on study days 6, 16, 28, and 56. Samples were processed for E. coli and Salmonella for both the general and third-generation cephalosporin (3GC) resistant populations. Isolates from 3GC-selective plates underwent phenotypic antimicrobial susceptibility testing and whole genome sequencing (WGS). Full-factorial multi-level mixed linear regression showed a significant difference (P ≤ 0.05) in quantitative resistance levels among E. coli populations when comparing treated (metritis) and untreated (control) cattle on days 6 and 16. These resistance levels became similar on days 28 and 56. Overall, levels of Salmonella shedding were higher in both groups on day 0 decreasing further in treated cows on days 6 and 16 and with resistance being infrequent. Resistance differences were observed primarily by dairy farm based on phenotypic antimicrobial susceptibility testing. Resistance genes and Salmonella serotypes were identified from WGS. Based on the results of this study, it is recommended to extend voluntary slaughter withholding period to 28 days following the first administration of CCFA. This is to allow populations of resistant E. coli to decrease to levels equivalent to that of their untreated counterparts. Such an extended slaughter withholding will allow for a reduction of the risk of slaughter fecal contamination by resistant enteric bacterial populations