Bronchopneumonia with interstitial pneumonia (BIP) has been considered a variant of acute interstitial pneumonia (AIP) rather than a distinct disease. This study compared 18 BIP, 24 bronchopneumonia (BP), and 13 AIP cases in feedlot beef cattle. Grossly, BIP cases typically had cranioventral lung lesions of similar morphology and extent as BP cases, but the caudodorsal lung appeared overinflated, bulged on section, and had interlobular edema and emphysema. Gross diagnosis of BIP had 83% sensitivity and 73% specificity relative to histopathology. Histologic lesions of BIP in cranioventral areas were of chronic BP, while caudodorsal lesions included alveolar and bronchiolar damage and inflammation, interstitial hypercellularity, and multifocal hemorrhages. In BIP cases, cranioventral lung lesions were more chronic than caudodorsal lesions. Histologic scores and microbiology data were comparable in cranioventral lung of BIP versus BP cases and caudodorsal lung of BIP versus AIP cases, with differences reflecting a more chronic disease involving less virulent bacteria in BIP versus BP. Mycoplasma bovis infection was similarly frequent among groups, and a viral cause of BIP was not identified. Lesion morphology and similar blood cytokine concentrations among groups argued against sepsis as a cause of lung injury. Surfactant dysfunction was identified in BIP and BP, and was only partially the result of protein exudation. These and other findings establish BIP as a distinct condition in which chronic cranioventral BP precedes acute caudodorsal interstitial lung disease, supporting a role of chronic inflammation in heightened sensitivity to 3-methylindole or another lung toxicant.

Bronchopneumonia with interstitial pneumonia (BIP) of feedlot cattle is characterized by gross and histologic lesions of cranioventral bronchopneumonia (BP) and caudodorsal interstitial pneumonia. This study described the characteristics and frequency of BIP in western Canadian feedlot cattle and identified epidemiologic differences between BIP and either BP or acute interstitial pneumonia (AIP). The study of 9909 deaths on 4 western Canadian feedlots included 1105 BIP, 1729 BP, and 878 AIP cases. A population of 55 cases with gross, histopathology, and microbiology data was used to validate the primary data set. BIP was the second most common reason for death (or euthanasia) from respiratory disease (1105/9909 cases), and the observed frequency was twice what was expected from random concurrence of BP and AIP. Based on logistic regression models, epidemiologic characteristics of BIP were comparable to those of BP, although BIP cases were more chronic with more instances of clinical illness prior to death. BIP was epidemiologically distinct from AIP. Specifically, BIP more frequently affected steers than heifers, deaths occurred earlier in the feeding period at lower body weights and lower daily weight gains, and BIP cases had longer durations from the first clinical illness to death and more separate instances of clinical illness prior to death. Furthermore, death from BIP mainly occurred in winter and fall, while death from AIP was most frequent in summer. These findings define BIP as a unique condition of feedlot cattle and suggest that chronic BP may promote the development of fatal interstitial lung disease in at-risk cattle.