Cross-sectional study of calves from Norwegian dairy herds with enzootic pneumonia: pathogen occurrence, antimicrobial resistance, culture result interpretation, and sampling site agreement

Background Enzootic pneumonia in calves is associated with pathogens such as Pasteurellaceae bacteria and Mycoplasmopsis bovis . Some of these bacteria are commensals in healthy calves, complicating culture interpretation, discouraging laboratory diagnostic use, and limiting knowledge of their impact. This study aimed to investigate the occurrence and antimicrobial resistance pattern of respiratory pathogens in calves from dairy herds with enzootic pneumonia. Culture results between upper and lower airway sites in healthy and sick calves were also compared to evaluate their diagnostic value. A cross-sectional study was conducted on 131 calves (72 healthy and 59 sick) from nine Norwegian dairy herds. Nasal swabs (NS), nasopharyngeal swabs (NPS), and serum were obtained from each calf, and bronchoalveolar lavage (BAL) from 113 calves. Results At the calf level, Pasteurella multocida was present in 60%, Mannheimia haemolytica in 55%, and Histophilus somni in 3%. At the sampling site level, from NS, NPS, and BAL, P. multocida was detected in 47%, 45%, and 27%, and M. haemolytica in 46%, 37%, and 8%, respectively. H. somni was detected in ≤ 2% per site. P. multocida appeared as pure culture in 73% (22/30) of positive BAL cultures. Serum antibodies to M. bovis were not detected. Disc diffusion showed that most Pasteurellaceae isolates were susceptible to penicillin and other common pneumonia antimicrobials. The exceptions were penicillin-resistant M. haemolytica isolates in two calves. Logistic regression identified dominant, abundant cultures of P. multocida from BAL as the only bacterial risk factor for pneumonia. Agreement for P. multocida detection between NS and NPS cultures and BAL was fair (kappa). Positive predictive values of P. multocida culture results at different abundance levels in NS and NPS, relative to BAL, were at most 49%. Conclusion P. multocida was the predominant bacterium in lungs of calves from problem herds, and its abundance in the lungs may differentiate infection from colonization in sick calves. M. haemolytica and H. somni appeared to have less clinical impact. Antimicrobial resistance appeared not to be a problem. Upper airway cultures were inaccurate predictors of bacterial presence and abundance in the lungs, and must be interpreted carefully alongside other diagnostic tools.