Airway Obstruction, Recurrent
Basic Information 
Definition
• A respiratory disease more frequently seen in middle-aged to older adult horses that is characterized clinically by recurrent episodes of airway obstruction when horses are exposed to organic dust or other particulate matter.
• Horses exhibit severe coughing and exaggerated respiratory effort that is more evident on expiration than inspiration. These attacks are initiated by an influx of neutrophils into the airways and by bronchoconstriction associated with excessive mucus and airway smooth muscle. At least in the initial stages, attacks of airway obstruction can be reversed by environmental remediation.
Clinical Presentation
History, Chief Complaint
• Typically presents initially with a seasonal cough that worsens over a period of years.
• As the disease progresses, the cough is accompanied by tachypnea and evident respiratory distress.
• During an exacerbation of more severe disease, the horse will have flared nostrils and evident abdominal press seen more in expiration but also evident on inspiration.
• Hypertrophy of the expiratory muscles may result in a “heaves line.”
• A marked nasal discharge is usually seen, ranging from serous to mucopurulent.
• Horses in advance stages of disease may be cachectic because their work of breathing exceeds their ability to eat.
Physical Exam Findings
• Physical examination findings usually are concordant with the above history. A moderate tachycardia may also be auscultated.
• Thoracic auscultation may reveal either loud bronchovesicular sounds or widely dispersed crackles and wheezes.
• In very severe cases, the lungs may be strangely silent because of the very small amount of airflow that the animal is able to achieve. In these cases, auscultation of the trachea is compatible with the presence of mucus.
• Lung fields are often enlarged because of air trapping, so percussion will reveal larger than normal lung fields.
Etiology and Pathophysiology
• RAO is akin to environmentally induced asthma in humans in that it is caused by chronic exposure to organic and inorganic dusts.
• Organic dusts are replete with endotoxin, β-glucans from mold, and various allergens. This exposure results in an outpouring of neutrophils into the horse’s airways.
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