Conditions or ailments that are the cause of a problem that you see - your observation.

Your vet may diagnose

Recurrent Airway Obstruction, RAO

Synonyms: Chronic Obstructive Pulmonary Disease, COPD, Heaves, Chronic Bronchitis, Chronic Airway Reactivity, Small Airway Disease, Broken Wind, Equine Asthma


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.

my vet's role



Other conditions or ailments that might also need to be ruled out by a vet.

Very Common
Less Common
more diagnoses


The prognosis in mild cases is fair, but management may require long term medication and major management changes. Severely affected horses have a poor prognosis.

RAO tends to recur, and it will recur if horses are fed dusty hay or stabled in environments that trigger this condition.

my role


I might observe

You might make these observations when a horse has this condition.

Very Common
Less Common
more observations

Questions To Ask Your Vet:
  • What can I do to decrease the severity of this condition?
  • Is there a genetic basis for this condition?
  • What management changes do I need to make?
  • Are inhaled medications a good treatment option?
  • What changes should I expect in my horse's ability to perform?

Feed high quality hay with as little dust as possible. Soaking hay may help decrease dust inhalation but may not adequately reduce dust. In severe cases, hay may have to be replaced with pelleted feed.

Be on the lookout for respiratory allergy in pastured horses too. Horses affected by the pasture-associated form of this disease may need to be kept off pasture, especially at times of the year when pollen counts are high.

For stabled horses always maintain excellent ventilation in stalls and maximize turnout. Use low-dust shavings in stalls to minimize dust and particulate in the air. In some cases, paper shavings may be helpful. Do not stable horses where hay is stored. Do not sweep or clean stalls, raising dust, near affected horses.

Related References:

Cha ML, Lais RR, Costa MV: Inhalation Therapy in Horses: In Veterinary Clinics of North America.

Higgins AJ, Snyder JR eds. The Equine Manual. 2nd Ed. Edinburgh: Elsevier Saunders 2006.

Author: Doug Thal DVM Dipl. ABVP