Treatment history and antimicrobial susceptibility results forMannheimia haemolytica Pasteurella multocida andHistophilus somni isolates from bovine respiratory diseasecases submitted to the Iowa State University Veterinary Diagnostic Laboratoryfrom 2013 to 2015

摘要

Bovine respiratory disease is the most costly disease facing the cattle industry. Increasing resistance to antimicrobial treatment has been presented as a significant contributing factor, often through summarized susceptibility testing data. We assessed the relationship between previous antimicrobial treatment and antimicrobial susceptibility results from isolates of Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni cultured from bovine respiratory cases submitted to the Iowa State University Veterinary Diagnostic Laboratory from 2013 to 2015. Antimicrobial susceptibility data from 1,251 bacterial isolates were included for analysis. More bacterial isolates from cattle that received antimicrobial treatment showed resistance compared to isolates from untreated cattle, and the percentage of resistant isolates increased as the number of antimicrobial treatments increased. Resistance to enrofloxacin, spectinomycin, tilmicosin, and tulathromycin was present in >75% of M. haemolytica isolates from cattle that had received 3 or more antimicrobial treatments; resistance to each of those 4 antimicrobials was present in ≤10% of M. haemolytica isolates from untreatedcattle. Similar but less dramatic trends were apparent for isolates ofP. multocida and H. somni. The percentageof multi-drug resistant bacterial isolates also increased with the number oftreatments. Results of our study suggest that previous antimicrobial treatmentmay have a profound effect on antimicrobial susceptibility testing. Summarizedsusceptibility results from diagnostic laboratories should not be used to makegeneralized statements regarding trends in antimicrobial resistance withoutproviding context regarding antimicrobial treatment history.