Experimental infection of lambs with bovine respiratory syncytial virus and Pasteurella haemolytica: clinical and microbiologic studies.

Four-week-old lambs were inoculated transtracheally with respiratory syncytial virus (RSV), Pasteurella haemolytica, or RSV and P haemolytica. When given in combination, RSV administration preceded P haemolytica by 3 or 5 days. Lambs inoculated with P haemolytica or RSV developed a mild respiratory tract disease accompanied by a transient pyrexia in a few lambs. By 24 hours after inoculation of bacteria, all lambs inoculated with RSV and P haemolytica were listless, reluctant to move, and exhibited hyperpnea and dyspnea. Most lambs had pyrexia and a few coughed and had serous nasal discharge. These clinical signs persisted for 3 to 4 days and were more pronounced in those inoculated with P haemolytica 5 days after RSV than in those inoculated with P haemolytica 3 days after RSV. Respiratory syncytial virus was isolated from 8 of 15 inoculated lambs and P haemolytica was isolated from 12 of 15 inoculated lambs. All lambs responded serologically to RSV, but none responded to P haemolytica.