Recurrent airway obstruction (RAO) is the most recent name given to a common, chronic equine respiratory condition. In RAO the small airways in the lung become hypersensitive to allergens, spasm and constrict, and become blocked by mucus. 

RAO is poorly understood, but is allergic in nature. Molds and dusts primarily found in hay cause a chronic inflammatory response deep in the lungs. Over time, inflammatory cells and scarring surround and infiltrate the delicate respiratory surfaces and thicken the small airways. Airways constrict, and mucus plugs and obstructs the airways. Scarring reduces the normal compliance of the lung tissue, preventing normal air movement and inflation. Severe episodes brought on by extra exposure to allergens may cause life-threatening respiratory distress. 

This chronic process often results in asthma-like signs: rapid or difficult breathing, coughing and exercise intolerance. Severely affected horses become thin from the increased effort required to breathe.

Some become depressed and may have a poor appetite. The classic heave line seen in horses with RAO is the overgrowth of the abdominal muscle, which is caused by excessive abdominal exertion needed to breathe. 

RAO occurs worldwide in horses fed hay. In the U.S. it most commonly occurs in the Northeast and Midwest.

Diagnosis in moderate and severe cases is often evident on veterinary physical examination. In more subtle cases, the use of a re-breathing bag may be helpful for a vet to hear the characteristic respiratory noises.  Additional diagnostic tests rely on sampling and analysis of the respiratory secretions from within the small airways, BAL and Trans-tracheal aspirate. 

The most important aspect of treatment is management change. Airborne dusts and allergens must be reduced in the environment. This may require pelleted feeds or soaking of hay. Steroids, inhaled, injected and oral, may have value in treatment but will not work if the environment is not changed to reduce inhaled allergens.