Pathogenesis of experimental bovine respiratory syncytial virus infection in sheep

Conventionally reared lambs, seronegative to bovine respiratory syncytial virus (RSV), were inoculated with bovine RSV by an aerosol route. A mild clinical response, characterized by transient fever at postinoculation day (PID) 4, occurred and was accompanied by serous oculonasal secretions in less than 50% of the infected lambs. A decrease in total WBC was observed on PID 6 and 11, with a reduction in the total numbers of lymphocytes. Bovine RSV was recovered from nasal secretions of inoculated lambs from PID 2 to 6. The lambs were euthanatized, and at necropsy, the virus was only isolated from lambs killed between PID 2 and 4. Serum neutralizing antibodies were noticed as early as PID 6 and peaked at PID 13 to 15. Neutralizing antibodies were also detected at low concentration in pulmonary washings. Viral antigen was mainly located in alveolar walls and occasionally in bronchiolar epithelium and alveolar macrophages in lambs killed between PID 2 to 4. Later, specific fluorescence was only present in alveolar walls and was not observed in tissue from lambs killed after PID 22. Budding and free virus-like particles were observed by electron microscopy in type I and type II alveolar epithelial cells. Outstanding macroscopic lesions were not seen. Histologically, mild bronchiolitis and alveolitis were observed.