An Update on Feedlot AIP

Abstract

Acute interstitial pneumonia (AIP) has been recognized in feedlot cattle for decades, but the cause is still unknown. AIP is most common in cattle on feed greater than 45 days; heifers may be disproportionately affected. Clinical signs suggest severe respiratory distress, but a definitive diagnosis can only be made at postmortem. At postmortem, the lungs fail to collapse, interstitial emphysema and edema is evident, and a “checkerboard” pattern of light and dark colored, independently movable lobules is commonly seen. Histologically, hyaline membranes, type II pneumocyte hyperplasia, and later, cellulate infiltrates are found. No specific treatment is reliably effective. Known causes of AIP outside feedlots include 3-methylindole, perilla mint, and moldy sweet potato toxicity. Bovine respiratory syncytial virus can cause lesions similar to AIP. Bronchopneumonia and histologic evidence of chronic bronchiolar injury have been described in cattle with feedlot AIP, suggesting that chronic airway inflammation may predispose cattle to the disease. Recent research suggests that 3-methylindole, hormonal influences (including the feeding of melengestrol acetate), and bacterial lung infection may influence the development of feedlot AIP; however, no evidence is currently strong enough to support any of these as a single cause. Recent research does not support acute BRSV infection as a cause. Other insults speculated to cause feedlot AIP, such as dust exposure, heat, and allergic reactions, have not been investigated. Currently it seems most likely that feedlot AIP has a multifactorial etiology, and the relative importance of various factors may differ among feedyards. More information is needed before informed recommendations regarding prevention of feedlot AIP can be made.