Bronchopneumonia with interstitial pneumonia (BIP) is a fatal lung disease of feedlot cattle that has been recognized by veterinarians for 40 years but it is not well characterized or understood and there are no specific preventions. At postmortem examination, BIP resembles the combination of bacterial pneumonia and “acute interstitial pneumonia” (AIP), a disease thought to be caused by a toxicant in the feed such as 3-methylindole. BIP was the second most common reason for death (or euthanasia) from respiratory disease in the 9909 feedlot deaths that we studied. The observed frequency was twice what was expected from random concurrence of BP and AIP. BIP affected steers more than heifers whereas AIP had a strong heifer predominance, and the seasonality and number of days on feed differed in BIP vs AIP. These findings define BIP as a frequent and unique cause of death in feedlot cattle, different from simple concurrence of AIP and BIP. We next compared the lung of 18 BIP cases in feedlot beef cattle to that of 24 cases with only bacterial pneumonia and 13 cases of AIP cases. Microscopic findings and infectious agents were comparable in BIP vs BP cases and in BIP vs AIP cases, with differences reflecting a more chronic disease involving less-virulent bacteria in BIP vs BP. Mycoplasma bovis infection was similarly frequent among groups, and a viral cause of BIP was not identified. We also studied blood cytokines and lung surfactant function in these animals. Overall, these studies established BIP as a distinct condition in which chronic bacterial pneumonia precedes acute interstitial lung disease. The findings support a role of chronic inflammation in heightened sensitivity to 3-methylindole or another lung toxicant. This advances our understanding of this disease, and lays the groundwork to investigate the specific cause, in the anticipation of developing methods to prevent this frequent cause of death in feedlot cattle.
Beef Cattle Research Council
Natural Sciences and Engineering Research Council of Canada