Detection of Pasteurella multocida and macrolide resistance genes in calf pneumonia: an immunohistochemical and molecular approach.

This study aimed to detect Pasteurella multocida by immunohistochemical (IHC) methods in formalin-fixed paraffin-embedded (FFPE) lung tissues obtained from pneumonic calves and to investigate the presence of macrolide resistance genes (erm(42), msr(E), mph(E)) in positive cases using real-time PCR. Among lung tissues collected from 100 necropsied calves, histopathological analysis revealed fibrinous bronchopneumonia as the most common diagnosis (56/100). P. multocida was detected immunohistochemically in 55 out of 100 cases (55%) using a specifically produced polyclonal antibody. The highest prevalence of P. multocida was observed in cases of fibrinous bronchopneumonia (82%). At least one of the targeted macrolide resistance genes was detected in 39 (71%) of the 55 IHC-positive cases. The most frequently encountered resistance profiles were the msr(E) + mph(E) combination (n = 12; 22%) and the triple gene combination erm(42) + msr(E) + mph(E) (n = 12; 22%). In conclusion, this study demonstrated that P. multocida plays a key role in fibrinous pneumonia in calves and that these strains harbor high levels and diverse types of macrolide resistance genes, which may contribute to treatment failures. Therefore, routine molecular resistance screening and the development of rational therapeutic strategies are recommended for veterinary practice.