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

Nineteen 4-week-old, colostrum-deprived lambs were transtracheally inoculated with respiratory syncytial virus (RSV), Pasteurella haemolytica biotype A serotype 1, or RSV and P haemolytica. Pneumonic lesions were more frequent, more extensive, and more severe in lambs inoculated with RSV and P haemolytica than in lambs inoculated with either agent alone. Lesions were seen in 2 of 4 lambs inoculated with P haemolytica alone, in 3 of 4 lambs inoculated with RSV alone, and in 11 of 11 lambs inoculated with RSV and P haemolytica. Grossly, lambs given P haemolytica alone had fibrinous pleuritis and pneumonic lesions with hemorrhagic and necrotic centers which involved approximately 14% of the lung surface. Lambs inoculated with RSV alone had multifocal areas of consolidation and hemorrhage that involved 5% of the lung surface. Lambs in 2 groups inoculated with RSV and P haemolytica had lesions characteristic of both agents over 15% to 21% of the lung surface. Histologically, P haemolytica alone caused acute fibrinous pneumonia with necrosis of the lung parenchyma and fibrinous pleuritis; RSV alone caused interstitial pneumonitis, bronchiolitis, and hemorrhage. In combination, the agents caused interstitial pneumonitis and severe exudative pneumonia with focal necrosis and hemorrhage. Lesions seen in lambs given RSV and P haemolytica or in lambs given P haemolytica alone were grossly and histologically similar to those seen in naturally occurring cases of acute pneumonic pasteurellosis. Seemingly, the virus caused a lesion that compromised the lungs and thus permitted P haemolytica to become established and to produce a more severe pneumonic lesion than it could produce alone.