RECURRENT AIRWAY OBSTRUCTION

Recurrent airway obstruction (RAO) is a common, performance-limiting, allergic respiratory disease of horses characterized by chronic cough, nasal discharge, and respiratory difficulty.

 Most horses exhibit clinical signs when stabled, bedded on straw, and fed hay, whereas elimination of these inciting factors results in remission or attenuation of clinical signs. 

The pathophysiology involves small-airway inflammation (neutrophilic), mucus production, and bronchoconstriction in response to allergen exposure.

Etiology

The average age at onset is 9 yr. Approximately 12% of mature horses have some degree of allergen-induced lower airway inflammation. 
There is no breed or gender predilection; however, there does appear to be a heritable component to susceptibility.

Clinical Findings

Horses present with flared nostrils, tachypnea, cough, and a heave line.

 The typical breathing pattern is characterized by a prolonged, labored expiratory phase of respiration. 

The abdominal muscles respond by assisting with expiration, and hypertrophy of these muscles produces the classic heave line. 
 Wheezes are generated by airflow through narrowed airways and are most pronounced during expiration. 
Crackles may be present and are associated with excessive mucus production. Mild to moderately affected horses may present with minimal clinical signs at rest, but coughing and exercise intolerance are noted during performance. 

This is referred to as summer pasture–associated obstructive pulmonary disease. The management is similar to that of a horse with heaves, with the addition of pasture avoidance.

Diagnosis

The diagnosis of RAO is determined in most horses on the basis of history and characteristic physical examination findings.
 Hematology and serum chemistry results are unremarkable.
 Radiographic findings in horses with RAO are peribronchial infiltration and overexpanded pulmonary fields (flattening of the diaphragm).

Bronchoalveolar lavage is rarely required for diagnosis of fulminant RAO and is not innocuous in horses that are dyspneic at rest.

Treatment

The single most important treatment is environmental management to reduce allergen exposure. 

The most common culprits are organic dusts present in hay, which need not appear overtly musty to precipitate an episode in a sensitive horse. 

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 Soaking hay with water before feeding may control clinical signs in mildly affected horses but is unacceptable 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 consists of a combination of bronchodilating agents 

Severely affected horses are ideally controlled with aerosolized bronchodilators 
Horses with mild to moderate airway inflammation can be treated with aerosolized corticosteroids and aerosolized or systemic (clenbuterol) bronchodilators.

It is inappropriate to treat RAO with bronchodilators as the sole therapy.
 NSAIDs, antihistamines, and leukotriene-receptor antagonists have not demonstrated therapeutic benefit.

References
Merks veterinary manuals