Equine recurrent airway obstruction: pathogenesis, diagnosis, and patient management

Equine recurrent airway obstruction (RAO), also called heaves, broken wind, emphysema, and chronic bronchiolitis, is a condition that affects mature horses maintained in confinement housing. The terminology for the disease results from the clinical manifestations of end-stage disease. Recurrent obstruction of the lower airways is characterized by bronchospasm, excessive mucus production, and pathologic changes of the bronchiolar walls leading to terminal airway obstruction [2,9]. Because the total cross-sectional area of the small airways is quite large, clinical signs are observed in the resting horse when obstructive disease involves most of the small airways. There is no breed or gender predisposition that is recognized, although the incidence of disease seems to increase with age [1,3]. Some authors suggest a hereditary basis of disease, because specific lines seem to be predisposed to the development of heaves. Equine RAO causes a reproducible series of clinical signs in affected horses. These changes may be seasonal and more intense when horses are housed indoors and exposed to dust, poor-quality hay, ammonia fumes, fungal spores, and other nonspecific stimuli [1,9]. In contrast, summer-associated obstructive pulmonary disease (SAOPD) is a syndrome similar to heaves; however, the clinical signs are triggered by exposure to late summer pollen in the southeastern United States [4-7]. The primary difference between heaves and summer-associated disease is the local climate, with exposure to specific allergens that induce clinical signs of airway obstruction. The presenting complaint for horses with RAO varies but often includes a chronic cough. Exercise intolerance, respiratory distress, mucopurulent nasal discharge, abnormal pulmonary sounds, and an enlarged field of percussion may also be observed. The diagnosis is more difficult when clinical signs are mild and exhibited by an occasional cough or minimal abdominal effort. Rebreathing is a simple procedure that can enhance pulmonary sounds on auscultation, but in horses with less severe disease, ancillary diagnostics are required [8]. Blood gas analysis at rest and 5 minutes after exercise can give the clinician an indication of gas exchange impairment. This is particularly important if the PaO2 values at rest are low (normal values at sea level are 100 +/- 5 mm Hg or 13.3 +/- 0.7 kPa) [9]. When pulmonary function testing is performed in horses suffering from RAO, increased pulmonary resistance is frequently identified. Standard pulmonary testing is not sensitive enough to detect mild airway obstruction. Horses with RAO have histologic evidence of pulmonary disease during remission, indicating that although clinical signs are recurrent, the disease is permanent. The goal of therapy is to reduce airway inflammation and broncho-constriction while enhancing mucociliary clearance mechanisms. Corticosteroids and bronchodilator therapy are used regularly for treatment of RAO.