Interstitial pneumonia in feedlot cattle: concurrent lesions and lack of immunohistochemical evidence for bovine respiratory syncytial virus infection

The objectives of this study were to describe the nature and distribution o f microscopic lung lesions in feedlot cattle with interstitial pneumonia an d to determine whether bovine respiratory syncytial virus (BRSV) antigen wa s present in affected lungs. Lungs with macroscopic lesions compatible with interstitial pneumonia were collected from cattle from 5 west-central Sask atchewan feedlots that had been on feed for greater than 60 days at the tim e of death. Interstitial pneumonia was most consistently present in dorsal portions of caudal lung lobes and in 21/28 cases (75%) had a multifocal to coalescing distribution. All 28 lungs exhibited hyaline membrane formation and some degree of type II alveolar epithelial cell hyperplasia, consistent with an acute to subacute duration. Twenty-one of 28 cases (75%) had concu rrent bronchopneumonia in at least 1 lung lobe; bronchopneumonia was grossl y evident in 9/28 cases (32%). Chronic bronchitis or bronchiolitis was pres ent in at least 1 section in 12/28 (43%) of the lungs, and 25/28 (89%) had at least 1 focus of bronchiolitis fibrosa obliterans. Bronchopneumonia and bronchiolitis fibrosa obliterans were markedly less common in 10 sets of bo vine lungs obtained from an abattoir. Bovine respiratory syncytial virus an tigen was demonstrated using immunohistochemistry in 2/28 cases and was ass ociated with bronchiolar epithelial necrosis that was more severe than the bronchiolar lesions in the BRSV antigen-negative cases. Interstitial pneumo nia in feedlot cattle in this study was more frequently associated with sup purative bronchopneumonia and bronchiolitis fibrosa obliterans than with BR SV infection.