Outbreak of Respiratory Disease Due to Bovine Respiratory Syncytial Virus with Concomitant Infections by Histophilus somni and Pasteurella multocida in Adult Dairy Cows and Calves from Southern Brazil

Although bovine respiratory syncytial virus (BRSV) is a key contributor to bovine respiratory disease (BRD) worldwide, there are few detailed reports of BRSV-related outbreaks in Brazil. This study describes the clinical, pathological, immunohistochemical (IHC), and molecular findings from a BRD outbreak in adult dairy cows from Southern Brazil. The affected cattle had dyspnea, nasal discharge, and coughing. One cow died, and samples were collected for diagnosis. Histopathology revealed interstitial pneumonia with multinucleated giant cells. IHC identified BRSV antigens in pulmonary tissue. A multiplex real-time PCR identified BRSV, <i>Histophilus somni</i>, and <i>Pasteurella multocida</i> in nasal and oral swabs, while only BRSV and <i>H. somni</i> were detected in the tissues of the cow that died. All animals had co-infections involving BRSV. The average cycle threshold (<i>Cq</i>) values for BRSV were 27.43 (nasal) and 32.68 (oral), with significant differences (<i>p</i> = 0.016), indicating higher nasal shedding. This qPCR assay was effective for detecting BRD pathogens, the quantification of viral and bacterial loads in animals with BRD and can be used for the rapid detection of respiratory pathogens. The elevated BRSV detection in oral samples suggests that this route may be an alternative for the collection of samples in cattle with profuse nasal discharge.