Recurrent airway obstruction is a common, allergic respiratory disease of horses that is similar to asthma in people. Exposure to the allergen causes inflammation and constriction of the small airways, leading to chronic cough, nasal discharge, and breathing difficulty. Episodes of airway obstruction tend to occur when susceptible horses are stabled, bedded on straw, or fed hay, whereas elimination of these factors results in remission or lessening of signs. The average age at onset is 9 years. Approximately 12% of mature horses have some degree of lower airway inflammation induced by allergens. The disease affects horses of all breeds. There does appear to be an inherited component to susceptibility.

Horses with recurrent airway obstruction typically have signs such as flared nostrils, rapid breathing with labored exhalation, and cough. The cough may be productive and often occurs during feeding or exercise. The abdominal muscles must work hard to assist with exhaling, and overdevelopment of these muscles produces a line along both sides of the abdomen known as a “heave line.” Wheezing may be caused by airflow through narrowed airways, and is most obvious when the horse is exhaling. Mild to moderately affected horses may have few signs at rest, but coughing and exercise intolerance are seen during athletic performance.

The diagnosis of recurrent airway obstruction is made on the basis of the horse's history and physical examination. Chest x-rays may not be necessary in horses with typical signs, unless there is no response to standard treatment after several weeks of therapy. However, x-rays are sometimes used to distinguish between recurrent airway obstruction and other diseases with similar signs, such as interstitial pneumonia, pulmonary fibrosis, or bacterial pneumonia.

The single most important treatment is environmental management to reduce the horse's exposure to allergens. Medication will improve the signs of disease, but respiratory disease will return after medication is discontinued if the horse remains in an environment where allergens are present. The most common culprits are organic dusts present in hay.

Horses should be maintained at pasture with fresh grass as the source of roughage, supplemented with pelleted feed. Round bale hay is particularly allergenic and is a common cause of treatment failure for horses on pasture. Horses that remain stalled should be maintained in a clean, controlled environment. Complete commercial feeds eliminate the need for roughage. Hay cubes and hay silage are acceptable low-allergen alternative sources of roughage and may be preferred by horses over the complete feeds. Soaking hay with water prior to feeding may control signs in mildly affected horses but is not effective for highly sensitive horses. Horses maintained in a stall should not be housed in the same building as an indoor arena, hay should not be stored overhead, and straw bedding should be avoided.

Medical treatment typically consists of a combination of bronchodilating agents to open up the air passages and provide rapid relief of airway obstruction and corticosteroids to reduce lung inflammation. Some of these medications are given in an aerosol form.

Summer Pasture-associated Obstructive Lung Disease

Horses on late-summer pasture in the southeastern United States may have signs similar to those of horses with heaves. This likely reflects sensitivity to molds or grass pollens and is referred to as summer pasture-associated obstructive lung disease. The management is similar to that of a horse with heaves, with the addition of pasture avoidance. These horses should be maintained in a dust-free stable.

Last full review/revision July 2011 by Bonnie R. Rush, DVM, MS, DACVIM; Neil W. Dyer, DVM, MS, DACVP; Joe Hauptman, DVM, MS, DACVS; Ned F. Kuehn, DVM, MS, DACVIM; Stuart M. Taylor, PhD, BVMS, MRCVS, DECVP; Wendy E. Vaala, VMD, DACVIM; Maureen H. Milne, BVMS, MVM, DCHP, MRCVS