Etiology of Acute Interstitial Pneumonia in Feedlot Cattle: Noninfectious Causes

Acute interstitial pneumonia (AIP) of cattle is a cause of severe dyspnea of acute onset that often ends in death. In feedlots, AIP can be an important cause of morbidity and mortality; however, the cause of feedlot AIP is unknown. In nonfeedlot situations, feed-associated pneumotoxins, particularly 3-methylindole (3-MI, a product of ruminai tryptophan metabolism), are well characterized to cause bovine AIP. A metabolite of 3-MI, 3-methyleneindolenine, has been found to be increased in the plasma and lung tissue of animals dying of feedlot AIP in some studies, suggesting that 3-MI contributes to the pathogenesis. In some lots, heifers die of AIP in greater numbers than steers; evidence suggests that melengestrol acetate may contribute to the development of feedlot AIP in these heifers. Although hypersensitivity is often speculated to cause feedlot AIP, the pathology of AIP is not typical of most lung diseases known to be due to hypersensitivity. No research has been conducted to address the role of hypersensitivity in feedlot AIP. Many anecdotal reports associate airborne dust with feedlot AIP, but no research has been carried out to test the hypothesis that dust exposure induces AIP. The toxic gases nitrogen dioxide and hydrogen sulfide have been incriminated in the etiology of AIP, but there is no evidence to suggest that feedlot cattle are likely to be exposed to sufficient amounts of these gases to induce AIP.