Occurrence of major and minor pathogens in calves diagnosed with bovine respiratory disease

•H. somni and Bovine coronavirus were positively associated with pneumonia in calves.•Higher numbers of H. somni and M. haemolytica were associated with BRD.•The genetic background of BRD-associated Pasteurellaceae has no value in diagnostics. Bovine respiratory disease (BRD) is caused by a mixture of viruses and opportunistic bacteria belonging to Pasteurellaceae and Mycoplasma bovis. However, these organisms are also commonly isolated from healthy calves. This study aimed to determine whether the organisms are present in higher numbers in calves sick with acute BRD than in clinically healthy calves, and further to genetically characterize bacteria of the family Pasteurellaceae to understand whether particular types are associated with disease. Forty-six clinically healthy and 46 calves with BRD were sampled by broncheoalveolar lavage (BAL) method in 11 herds geographically spread over Denmark to determine presence and quantity of microorganisms by culture and quantitative real time qPCR. Isolates of Pasteurellaceae were tested for antibiotic resistance and were whole genome sequenced to determine genotypes. Histophilus somni was in particular positively associated with BRD, suggesting particular importance of this organism as likely aetiology of BRD. In addition, quantification of bacteria revealed that higher counts of H. somni as well as of M. haemolytica was also a good indicator of the disease. Pasteurellaceae isolates were susceptible to the commonly used antibiotics in treatment of BRD, and genotypes were shared between isolates from clinically healthy and sick calves.