*2.1.2 Viral agents*

The most common viral agents associated with BRD include bovine herpesvirus type 1 (BHV-1), parainfluenza-3 virus (PI3), bovine viral diarrhea virus (BVDV) and bovine respiratory syncytial virus (BRSV) [13]. These viral pathogens can induce primary infection with mild clinical signs of BRD and predispose cattle to bacterial infection [18, 21]. Viral infection can impair the mucosal barrier and respiratory pathogen clearance, damage the lung parenchyma and suppress immune responses in cattle. Combined, the effects of viral infection facilitate the proliferation of opportunistic bacterial pathogens in the upper respiratory tract and translocation of these pathogens into the lung and cause infections to the compromised lung [21]. A recent human study suggested that respiratory viruses can also affect the structure and composition of nasal microbiota, which may be another way through which virus weakens host resistance to bacterial pathogens [22].
