Molecular and Pathomorphological Studies of Lung Lesions in Bovine Respiratory Disease Associated with Macrolide-Resistant Bacterial Pathogens

This study aimed to evaluate factors influencing the severity of histopathological pneumonia in bovine respiratory disease (BRD) cases, with reference to the complexity of bacterial pathogens and molecular profiles of macrolide resistance genes. A total of 80 formalin-fixed paraffin-embedded (FFPE) lung tissue samples from calves (n=40) and adult cattle (n=40) with fibrinous pneumonia were examined. Histopathological severity was assessed using a modified semi-quantitative scoring system based on inflammation extent, necrotic areas, and vascular damage. The presence of five major bacterial pathogens (Pasteurella multocida, Mannheimia haemolytica, Histophilus somni, Mycoplasma bovis, and Trueperella pyogenes) and three macrolide resistance genes [erm(42), msr(E) and mph(E)] were analysed using multiplex real-time PCR. Histopathological evaluations revealed significantly higher severity scores for fibrinonecrotic pneumonia compared to other pneumonia types. Molecular analyses showed multiple bacterial pathogens in 68.8% of cases, significantly correlating with increased histopathological scores. Regarding macrolide resistance genes, at least one resistance gene was detected in 76-95% of positive cases, with the msr(E) + mph(E) combination being the most frequent. The combined effect of pathogen number and resistance gene load on pneumonia severity was confirmed by ROC analysis for its ability to predict severe pneumonia. In conclusion, our findings highlight the importance of molecular-based pneumonia severity scoring in guiding rational antimicrobial use and herd management strategies in BRD.