This study evaluated the nasopharyngeal microbial flora and antimicrobial susceptibility patterns of the one hundred and thirty holstein calves with dairy calf pneumonia from dairy farms of Mashhad Suburb between September 2002 and August 2003 .The most common micro- organisms isolated were Pasteurella multocida 80 (61.54%), Mannheimia haemolytica 41 (31.54%), Bacillus sp. 15 (11.54%), Staphylococcus sp. 3 (2.31%), Streptococcus sp. 4 (3.08%), Pseudomonas sp. 3 (2.31%),Proteus sp. 3 (2.31%) and E coli 5 (3.84%). Antimicrobial susceptibility testing was performed on all M. haemolytica and P. multocida employing the disk diffusion method (Kirby-Bauer). Each strain was tested with 10 antimicrobial agents. With 7 (17.07%), 6 (14.63%), 4 (9. 76%) and 1 (2.44%) of M. haemolytica were resistant to lincomycin, gentamicin, oxytetracycline and chloramphenicol, respectively. However, resistance to penicillin, lincomycin, amoxicillin, gentamicin and oxytetracycline was observed in 10 (12.50%), 6 (7.50%), 6 (7.50%), 5 (6.25%) and 5 (6.25%) of P. multocida isolates, respectively. All M. haemolytica and P. multocida tested were found susceptible to florfenicol and cephalothin. The results show the need for local veterinarians and producers to be more responsible in the use of antibiotics in the treatment of pneumonia in calves, and growing danger of the dissemination of strains of M. haemolytica or P. multocida resistant to most antimicrobials which could complicate in the future the treatment of pneumonia in these animals.

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