Experimental respiratory syncytial virus pneumonia in young calves: microbiologic and immunofluorescent findings.

Young calves were inoculated with respiratory syncytial virus (RSV) intranasally or by a combined intranasal and intratracheal route and were killed between postinoculation (initial) days (PID) 1 and 14. Viral antigens were detected by immunofluorescence in nasopharyngeal cells from calves killed between PID 2 and 10. Evidence of infection of the trachea and lungs with RSV was obtained by immunofluorescence and virus isolation in calves inoculated by the combined route, but not in calves inoculated intranasally. Within the lungs, RSV antigens were observed in epithelial cells of bronchioli and alveoli. The only virus detected in inoculated calves was RSV. With the exception of 1 calf, bacteria or mycoplasmas were not isolated from the lower respiratory tracts of inoculated calves. Antibody to RSV was not detected in calves killed up to PID 5, but 4 of 5 colostrum-deprived calves killed between PID 10 and 13 had antibodies to RSV. Preexisting, maternally derived antibody to RSV did not protect the calves from infection. Seemingly, the clinical signs of pneumonia and pathologic lesions observed in inoculated calves were caused by RSV infection.