A ten-year (2000-2009) study of antimicrobial susceptibility of bacteria that cause bovine respiratory disease complex-Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni-in the United States and Canada

Bovine isolates of Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni, collected from 2000 to 2009, were tested for in vitro susceptibility to ceftiofur, penicillin, danofloxacin, enrofloxacin, florfenicol, tetracycline, tilmicosin, and tulathromycin. Ceftiofur remained very active against all isolates. Penicillin retained good activity against P. multocida and H. somni isolates with no appreciable changes in susceptibility or minimal inhibitory concentration (MIC) distributions with time. While there was no obvious trend, the percent of M. haemolytica that were susceptible to penicillin ranged from 40.9% to 66.7%. Danofloxacin MIC50 and MIC90 values for M. haemolytica and P. multocida did not change beyond a single dilution over the 6 years it was included in the testing panel. The MIC90 for H. somni increased beyond 1 dilution. Enrofloxacin MIC50 values for the 3 pathogens also did not change over time, unlike the MIC90 values, which increased by at least 4-doubling dilutions. Ninety percent or more of M. haemolytica and H. somni isolates were susceptible to florfenicol, while susceptibility among P. multocida was 79% or greater. Less than 50% of the isolates tested as susceptible to tetracycline in many of the years. All 3 organisms showed declines in tilmicosin and tulathromycin MIC50 and MIC90 values over the years in which they were tested.