Introduction - At present, Mannheimia haemolytica (M.h.) is recognized to be the major bacterial pathogen involved in Bovine Respiratory Disease (BRD) outbreaks. Aim - The study is aimed at providing information on the prevalence of Mannheimia haemolytica, alone or in association with other pathogens, during Bovine Respiratory Disease (BRD) outbreaks involving dairy calves. Mannheimia haemolytica isolates were submitted to assessment of sensitivity to antibiotics widely used in buiatrics. Materials and methods - During 2012-2016, 185 acute respiratory disease outbreaks involving dairy cattle operations located in the Po Valley (Italy) were examined. From calves aged 10-180 days, 715 nasal swabs from animals showing acute respiratory signs and 66 respiratory tracts from died calves belonging to 39 BRD outbreaks were collected and submitted to laboratory investigations at detecting viral and bacterial respiratory pathogens. M.h. isolates were tested for their antimicrobial susceptibility by Kirby-Bauer method. Results and discussion - Results confirm that BRSV is the main viral pathogen involved in BRD. Laboratory investigations on nasal swabs and lung specimens collected from calves with acute clinical signs of disease allowed the isolation of 136 Mannheimia haemolytica strains with a prevalence of 17% and 24% for nasal swabs and lung specimens, respectively. A level of susceptibility >75% to amoxicillin+clavulanic acid, ampicillin, ceftiofur, florfenicol, trimethoprim+sulfonamides was demonstrated. Conversely, a level of resistance >50% to sulfadiazine and tylosin was detected. Our results are in accordance with other surveys carried out in Northern Italy. Conversely, disagreement with European reports regarding antibiotic sensitivity of M.h. was observed. European data regarding amoxillin-clavulanic acid, thirdgeneration cephalosporins, florfenicol, and fluoroquinolones showed lower resistance than those emerging from our findings. This phenomenon is caused by the extensive use in our cattle operations of antibiotics in general and of the abovementioned antibiotic classes in particular. Conclusions - To limit the extensive use of antibiotics in BRD, preventive strategies should be carried out by using vaccines. For the control of M.h. - LKT pathogenic effect, monovalent and/or combo vaccines have long been available.
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