Parker County Today December 2017 - Page 54

our expertise: VET COLUMN Exercise Intolerance: Part 2 The Lower Airway Tom Hutchins, DVM, DABVP Dr. Tom Hutchins A 52 s mentioned in the previous installment, second to musculoskeletal disease, respiratory disease is the most common cause of poor performance, interruption in training, and premature retirement among performance horses. Inflammatory airway disease (IAD) and exercise- induced pulmonary hemorrhage (EIPH) are the most common conditions originating from the lower respiratory tract associated with poor race performance. Low-grade chronic obstructive pulmonary disease (COPD) is a common source of exercise intolerance in middle-aged performance horses. The young racehorse population is frequently exposed to viral respiratory pathogens that impair pulmonary defense by damaging mucociliary clearance mechanisms, destroying bronchial-associated lymphoid tissue, and impairing pulmonary macrophage function. Regeneration of the mucociliary apparatus requires approximately three weeks after recovery from viral respiratory disease, and racehorses are rarely permitted sufficient time for convalescence after overt disease. Strenuous exercise and long distance transport impair pulmonary immunity and promote deep inhalation of dust particles. The presence of blood within the lung tissue, as occurs with EIPH, initiates an inflammatory response resulting in bronchiolitis and airway inflammation. Inflamed airways are fragile and are predisposed to further hemorrhage with exercise. Therefore, repeated episodes of EIPH appear to create self-perpetuating lower respiratory tract inflammation and hemorrhage. Impaired pulmonary defense, irritant exposure, and episodes of pulmonary hemorrhage not only predispose racehorses to develop chronic IAD, but promote development of fulminant pneumonia and pleuropneumonia which may result in prolonged recovery, permanent pulmonary damage, premature retirement, or even death. Thoracic auscultation with the rebreathing procedure should be performed in all horses with poor performance or exercise intolerance. The increased depth of repiration caused by the rebreathing bag accentuates abnormal lung sounds, and the rebreathing procedure allows the clinician the subjectively evaluate the time period for induction of and recovery from labored breathing. Listening to the lungs and rebreathing facilitate identification of the abnormal lung sounds characteristic of bronchopneumonia (crackles), pleuropneumonia (silence in the lower lung fields), and COPD (end expiratory wheeze). Horses with IAD and EIPH rarely have abnormalities that can be heard even with rebreathing, but the procedure may induce coughing. Diagnostic investigation of poor performance should routinely include endoscopic examination of the upper and lower respiratory tract. Endoscopic examination allows for direct visualization of inflammatory exudate or blood in the airway, and eliminates upper respiratory tract disorders as the source of poor performance. Bronchoalveolar lavage (BAL) or a transtracheal wash (TTW) is indicated in horses with endoscopic evidence of lower respiratory tract disease to confirm the source of poor exercise performance, characterize the inflammatory process, and/or quantitate the severity of pulmonary hemorrhage. Inflammatory airway disease (IAD) occurs in 22% - 50% of thoroughbreds and Standardbred racehorses and is a common cause of impaired performance and interruption of training. Chronic cough and mucoid to mucopurulent nasal discharge are common clinical findings in racehorses with low-grade airway inflammation. Horses with IAD show poor exercise tolerance at race speeds and perform several seconds slower than previous performances. Proposed causes of lower airway disease in racehorses include recurrent pulmonary stress, deep inhalation of particulate matter, exposure to noxious