In vitro activity of twelve veterinary antibacterial drugs against Mannheimia haemolytica and Pasteurella multocida isolated from Dutch calves

Results of susceptibility tests of clinical isolates of animal pathogens ar e periodically summarized and reported by the Animal Health Service. Howeve r, these results are based upon qualitative test methods. In the present pa per results of quantitative susceptibility tests of twelve antibacterial ag ents against Mannheimia haemolytica (MHA) and Pasteurella multocida (PMU) i solated from Dutch calves in 1996 and 1997 are presented. Minimum inhibitor y concentrations of amoxicillin, ceftiofur, tetracycline trimethoprim-sulph amethoxazol, tilmicosin, neomycin, gentamicin, spectinomycin,flumequine, en rofloxacin, chloramphenicol and florfenicol were determined. No resistance was detected for, ceftiofur and florfenicol. Three strains had an intermedi ate susceptibility to tilmicosin. The resistance percentages of MHA and PMU for neomycin, gentamicin, spectinomycin, flumequine, enrofloxacin, and chl oramphenicol varied from 2 % to 16%. Higher resistance percentages (16% - 5 3%) were observed for amoxicillin, tetracycline, and trimethoprim-sulphamet hoxazole. The MIC breakpoints used to determine whether a strain is suscept ible, intermediate, or resistant are arbitrary and discussed in this paper.