Between 1994 and 2002, a total of 390 (46.3%) Mannheimia haemolytica, 292 (34.7%) Pasteurella multocida, and 160 (19.0%) Histophilus somni were isolated at the Oklahoma Animal Disease Diagnostic Laboratory from lungs from 6-18-month-old beef cattle with pneumonia. The ratio of M. haemolytica isolations to P. multocida isolations decreased from 3.1 in 1994 to 0.8 in 2000 while increasing to 1.5 in 2002. Mannheimia haemolytica isolations significantly (P < 0.05) decreased from 62.5% in 1994 to between 30.6% and 50.4% in 1998--2002. Pasteurella multocida isolations significantly (P < 0.05) increased from 20.0% in 1994 to between 28.6% and 47.4% in 1998--2002. Histophilus somni isolations were <19% except in 1998 (40.8%) and 1999 (23%). Antimicrobial susceptibilities for M. haemolytica significantly declined for erythromycin (P = 0.0001), florfenicol (P = 0.0004), spectinomycin (P = 0.0001), and tilmicosin (P = 0.03). For P. multocida, antimicrobial susceptibilities significantly declined for erythromycin (P = 0.0001), florfenicol (P = 0.004), spectinomycin (P = 0.03), sulfachloropyridizine (P = 0.028), tetracycline (P = 0.017), tilmicosin (P = 0.0001), and trimethoprim/sulfamethoxazole (P = 0.0003). Antimicrobial susceptibilities for H. somni were variable for spectinomycin and sulfachloropyridizine, whereas susceptibilities to other antibiotics remained consistently high.
