Determining chronicity and frequency of histologic lung lesions in feedyard cattle mortalities

Several etiologic factors contribute to bovine respiratory disease, the primary cause of feedyard mortality. We performed a cross-sectional observational study to determine the frequency and chronicity of lesions in fatal feedyard pneumonia cases. Postmortem examinations were performed (n = 443), and 2 lung samples were collected from each animal. Histologic sections were scored using 10 classifications: 5 types of bronchopneumonia (BP; 3 acute, 2 chronic), 4 types of interstitial pneumonia (IP; 2 chronic, 2 acute), and 1 category for other findings. Cases with chronicity in either section were classified as chronic. Results from both lung sections were combined into a case-level diagnosis of BP, IP, or BP with IP (BIP). After exclusions because of autolysis or missing data, our diagnosis frequencies for 352 cases were: BP (150; 42.6%), IP (86; 24.4%), BIP (71; 20.2%), and Other (45; 12.8%). Of cases with an IP component, chronic lymphoplasmacytic IP (CLIP) was identified in 59 of 157 (37.6%) cases. A generalized linear model was used to evaluate the probability of case-level chronicity and potential associations with diagnosis and days on feed. Model-estimated probability of chronic lesions was higher in cases with IP (51% IP; 60% BIP) compared with BP (38%). Of 71 BIP cases, 27 had acute BP and acute IP; 18 had acute IP and chronic BP; 0 had chronic IP and acute BP; 5 had chronic IP and chronic BP; 14 had CLIP and acute BP; and 7 had CLIP and chronic BP. In our final dataset of 352 feedyard pneumonia cases, IP was found histologically in the lungs of 157 cases, either alone or in conjunction with BP; cases with an IP component were more likely to have chronic lesions.