POTENTIAL MORPHOLOGIC AND PHYSIOLOGICAL FACTORS THAT MAY PREDISPOSE THE BOVINE LUNG TO RESPIRATORY-DISEASE

The morbidity associated with bovine pneumonic pasteurellosis in the U nited States cattle industry is a challenging and persistent veterinar y issue. Causes for the apparent susceptibility of cattle to this dise ase complex are only partially appreciated. Genetic selection of domes ticated cattle has resulted in animals with small lungs relative to bo dy size and metabolic demands. Bovine lungs have about 25% of the lung volume per unit of body weight as compared with the mammalian mean. F urthermore, bovine alveolar surface area expressed as a proportion of body weight or per unit of basal oxygen consumption is less than half the mammalian mean. The resting tidal volume of cattle is approximatel y twice that of the mammalian mean while the resting rate of pulmonary airflow is about three times that of the mammalian mean. Reduced bovi ne respiratory capacity can contribute to decreased heat tolerance and decreased resistance to aerogenic and vascular exposure to infectious , toxic, noxious, or bioactive substances, particularly during periods of great excitement or exertion. Immunity is a dynamic physiologic st ate that is mediated by lymphocytes and involves an inducible ability to react to an antigen. Humoral immunity denotes an immune state resul ting from production of antibodies by B-cell lymphocytes. This article discusses the various aspects of bovine lung morphology that predispo se this species to respiratory disease in the context of various immun ologic factors. Specific anatomy of the bovine respiratory system, wit h a focus on the upper and lower respiratory tracts, the pleural space , pulmonary circulation, and the interstitium, is reviewed.