Cross-sectional study of calves from Norwegian fattening herds with enzootic pneumonia: pathogen occurrence, clinical relevance, antimicrobial resistance, and agreement between respiratory tract sampling sites.

Bovine respiratory disease (BRD) is a leading cause of morbidity and antimicrobial use in calves. Laboratory diagnostics are limited by difficulty obtaining lower airway samples from live animals and the common presence of many BRD-associated bacteria in healthy calves, complicating culture interpretation. This study aimed to investigate the occurrence, clinical relevance, and antimicrobial resistance patterns of BRD-associated bacteria in calves from Norwegian fattening herds with enzootic pneumonia, and to evaluate agreement between upper and lower airway sampling sites. In a cross-sectional study, 88 calves from seven fattening herds were clinically scored and classified as healthy or diseased. Nasal swabs, nasopharyngeal swabs, bronchoalveolar lavage (BAL) fluid, and serum samples were collected for bacterial culture, antimicrobial susceptibility testing by disc diffusion, and serology. Pasteurella multocida and Mannheimia haemolytica were the most frequently detected bacteria across sampling sites, while Histophilus somni was less common. In BAL samples, P. multocida was the predominant bacterium identified and was significantly associated with clinical disease. Agreement between upper airway samples and BAL for Pasteurellaceae detection was slight to fair at the individual level. However, group-level isolation rates for P. multocida were similar across sampling sites in diseased calves. Most isolates were susceptible to common BRD antimicrobials, although penicillin-resistant M. haemolytica was detected in 18% of M. haemolytica-positive calves. Antibodies against Mycoplasmopsis bovis were not detected. In conclusion, P. multocida appears to be the bacterial pathogen most strongly associated with clinical disease in calves from Norwegian fattening herds with enzootic pneumonia. Upper airway sampling had limited value for individual diagnosis but may be useful for herd-level assessment when diseased calves are sampled. Antimicrobial resistance levels were generally low, although resistance was detected.