Potentiation of bovine respiratory syncytial virus infection in calves by bovine viral diarrhea virus

Abstract

The potential synergistic effects of BVDV and BRSV infections were investigated. Single virus infections often produce mild, if any, clinical disease in controlled animal challenge studies. The multi-etiologic nature of bovine respiratory disease is well-documented, and the potential effects of dual infection more closely reproduces natural disease and the significance of pathogenic viruses. This is important in designing vaccination programs for disease control. Fourteen Hereford beef calves weighing approximately 205 kilograms were obtained from a ranch in western Nebraska. The calves were seronegative to IBR, BVD, PI3, and BRSV, and were confirmed to be free of persistent BVD infection through repeated negative virus isolation. Random assignment to form treatment groups allotted four calves to a BVD challenge alone, four calves to a BRSV challenge alone, four calves to dual BVD, BRSV challenge, and two calves to a negative control group. Evaluation methods included pre- mortem clinical scoring, hematology, and virus isolation. Euthanatized calves were evaluated by virus isolation, gross pathologic, and histopathologic evaluation. Severity of clinical signs (depression, serous to mucopurulent nasal discharge, and dyspnea) and extent of lung injury was greater in calves inoculated with dual BRSV/ BVDV than in calves inoculated with either virus alone. Potentiation of BVDV infection of the digestive tract by BRSV was indicated by augmentation of clinical diarrhea in calves dually-infected with BRSV and BVDV compared to calves infected only with BVDV. Gross and histopathologic lesions correlated to pre-mortem clinical observations. BVDV and BRSV potentiate each other in dual virus infection, causing more severe clinical signs and lesions than in infection with either virus alone. This more closely mimics natural infections, and better establishes the clinical significance of these pathogens than single virus controlled challenges studies.