Respiratory Viruses and Bacteria in Cattle

Abstract

Bovine respiratory disease (BRD) is the principal source of economic loss for the North American beef industry and a significant health problem in the dairy industry as well. The pathogenesis typically involves some combination of predisposing stress which compromises respiratory defense mechanisms and coincidental primary infection with one or more respiratory viruses. Viral infection and the host’s response to it further compromise defense and facilitate colonization of deeper pulmonary tissues by bacteria normally carried in the nasopharynx, especially members of the family Pasteurellaceae. The viruses most frequently associated with BRD include infectious bovine rhinotracheitis virus, a type 1 bovine herpesvirus (BHV1), parainfluenza virus type 3 (PI3), bovine respiratory syncytial virus (BRSV), and bovine viral diarrhea virus (BVDV). Other viruses which may be involved and could be underestimated are bovine adenovirus and bovine coronavirus (BCV). Secondary bacterial pneumonia is typically attributed to members of the family Pasteurellaceae, including Mannheimia haemolytica (formerly Pasteurella haemolytica), Pasteurella multocida, and Haemophilus somnus. Other bacteria that have been isolated with some frequency are mycoplasmas, especially Ureaplasma diversum, Mycoplasma dispar, Mycoplasma bovis, and Mycoplasma bovirhinis. Chlamydia spp. have been recovered from pneumonic lungs of cattle with BRD, coincidentally with Pasteurellaceae and mycoplasmas. In conclusion, there is ample evidence to support the synergistic effect of combined viral and bacterial infections in BRD.