A 4-year survey of antimicrobial susceptibility trends for isolates from cattle with bovine respiratory disease in North America.

The antimicrobial susceptibility trends of bovine respiratory disease (BRD) pathogens isolated from 1988 to 1992 were determined. A total of 880 isolates representing Pasteurella haemolytica, Pasteurella multocida, and Haemophilus somnus were used in the study. Overall, resistance to ampicillin, tetracycline, erythromycin, and sulfamethazine was frequently encountered among strains of P. haemolytica and P. multocida. Ceftiofur, an extended-spectrum cephalosporin originally marketed in 1988 for the treatment of BRD, was very active against the BRD pathogens tested; the MIC of ceftiofur for 90% of isolates tested was < or = 0.06 microgram/ml. Resistance to spectinomycin varied on the basis of the breakpoint used. Substantial variation in the year-to-year susceptibility of BRD pathogens to tilmicosin, a new macrolide antimicrobial agent, was observed. The proportion of susceptible P. haemolytica isolates ranged from 84.7% in the second year to 7.1% in the third year and 78.2% in the fourth year. Similar fluctuations were observed with strains of P. multocida.