Changes in the rates of field isolation and antimicrobial susceptibility of bacterial pathogens collected from fall-placed feedlot steers between arrival at the feedlot and 90 to 120 days on feed

The change in bacterial recovery and phenotypic antimicrobial susceptibility of 3 important bovine respiratory disease bacteria (Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni) between arrival and 90 to 120 days was observed in 295 healthy fall-placed feedlot calves in western Canada using deep nasal swabs. At the arrival sampling, 28%, 28%, and 9% of calves were culture positive for Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni, respectively; these rates changed to 23%, 14%, and 16%, respectively, in the post-arrival period. A decrease in antimicrobial susceptibility was observed between the arrival and post-arrival sample periods, with numerically fewer pan-susceptible isolates and little change in multi-drug resistant isolates. Between the 2 sampling periods, statistically significant decreases in susceptibility to tilmicosin and tulathromycin were observed for Mannheimia haemolytica and tildipirosin for Pasteurella multocida isolates. Results of this study suggest that changes to both the proportions of bacteria isolated and the antimicrobial susceptibility of isolates occur between arrival and ~90 days post-arrival.