LESIONS IN LAMBS EXPERIMENTALLY INFECTED WITH OVINE ADENOVIRUS SEROTYPE-6 AND PASTEURELLA-HAEMOLYTICA

Twenty-five colostrum-deprived lambs reared in isolation were inoculat ed with a US variant of ovine adenovirus serotype 6 (OAV-6) strain RTS -151, Pasteurella haemolytica, or a combination of the 2 agents. Altho ugh severe pulmonary lesions were caused by each agent, the lesions we re more severe and lasted longer with the combined infection. Lesions induced by OAV-6 alone developed 6-9 days after inoculation and lasted for 15 days, the length of the experiment. The lesions were character ized by suppurative inflammation at the junction of the terminal bronc hioles and alveoli. Air spaces were filled with neutrophils and slough ed epithelial cells, which often contained large intranuclear inclusio ns. Lesions induced by P. haemolytica alone developed within 1 day and persisted for no more than 10 days and were characterized by severe p ulmonary edema with variable amounts of fibrin. Lesions induced by the combined infection had aspects of each infection alone and resulted i n severe disease in 4 of 8 lambs that were permitted to live more than 1 day after inoculation with bacteria. Early pulmonary lesions includ ed edema, limited fibrin deposition, and slight purulent bronchiolitis and alveolitis. Later lesions included necrosis and more fibrin. For lambs inoculated with both pathogens, resolution was incomplete 15 day s after inoculation of virus (10 days after inoculation of P. haemolyt ica). The results presented here corroborate previous findings indicat ing that the RTS-151 variant of OAV-6 is common in lambs and acts in c oncert with P. haemolytica to cause severe and often fatal pneumonia.