EFFECT OF CONCURRENT EXPERIMENTALLY-INDUCED BOVINE RESPIRATORY SYNCYTIAL VIRUS AND BOVINE VIRAL DIARRHEA VIRUS-INFECTION ON RESPIRATORY-TRACT AND ENTERIC DISEASES IN CALVES

Objective-To compare experimentally induced concurrent bovine viral di arrhea virus (BVDV) and bovine respiratory syncytial virus (BRSV) infe ction with single virus infection. Animals-9- to 12-month-old calves. Procedure-Calves were allotted to 4 groups: 1, mock-infected control ( n = 3); 2, BRSV infected (5); 3, BVDV infected (5); and 4, concurrent BRSV and BVDV infected (5). Total and differential WBC counting was do ne. Concentration and duration of BVDV in nasal secretions and serum, and duration of BRSV in nasal secretions were determined. Concentratio n of BVDV in various tissues was determined, and isolation of BRSV fro m lung tissue was attempted. Histologic examination and immunohistoche mical analysis were done to detect lesions and distribution of viral a ntigens, respectively. Results-Calves with concurrent infection develo ped more severe clinical signs of disease (fever and diarrhea), leukop enia, and more severe lesions. They also shed virus from nasal secreti ons in greater concentration and for longer duration, and BRSV was iso lated from their lungs. Calves with concurrent infection also had more extensive lung lesions. Alimentary epithelial necrosis and severe lym phoid depletion were associated with BVDV infection in calves with or without concurrent BRSV infection. BVDV antigen in lymphatic tissue wa s detected in stromal cells only. Conclusions-Concurrent infection wit h BRSV and BVDV resulted in more severe respiratory tract and enteric disease than did infection with either virus alone, possibly indicatin g synergistic effect between the viruses. BVDV's role in causing respi ratory tract disease is attributable, indirectly, to effects on the ho st's immune system, not to infection of the lungs.