THE MERCK VETERINARY MANUAL
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Recurrent Airway Obstruction in Horses

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Recurrent airway obstruction (RAO) is a common, performance-limiting, allergic respiratory disease of horses characterized by chronic cough, nasal discharge, and respiratory difficulty. Episodes of airway obstruction are seen when susceptible horses are exposed to common allergens. 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.

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.

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. Cough may be productive and often occurs during feeding or exercise. The abdominal muscles respond by assisting with expiration, and hypertrophy of these muscles produces the classic heave line. Characteristic auscultatory findings include a prolonged expiratory phase of respiration, wheezes, tracheal rattle, and overexpanded lung fields. 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. Horses with RAO are not typically febrile unless secondary bacterial pneumonia has developed.

Horses from the southeastern USA may demonstrate clinical signs on late-summer pasture, which likely reflects sensitivity to molds or grass pollens. 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.

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). Thoracic radiographs are of little benefit in confirming the diagnosis of RAO and may not be necessary in horses with characteristic clinical signs, unless there is no response to standard treatment after 14 days of therapy. However, they may help identify the most important differential diagnoses, including interstitial pneumonia, pulmonary fibrosis, or bacterial pneumonia.

Bronchoalveolar lavage is rarely required for diagnosis of fulminant RAO and is not innocuous in horses that are dyspneic at rest. It is indicated in horses with mild to moderate disease with poor performance and coughing during exercise. Neutrophilic inflammation (20%–90% of total cell count) confirms the presence of lower airway inflammation and differentiates horses with eosinophilic pneumonitis, fungal pneumonia, or lungworm infestation from horses with heaves. Curschmann's spirals may be seen on cytologic evaluation and represent inspissated mucus/cellular casts from obstructed small airways.

The single most important treatment is environmental management to reduce allergen exposure. Medication will alleviate clinical signs of disease; however, respiratory disease will return after medication is discontinued if the horse remains in the allergen-challenged environment. 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 should be maintained at pasture, with fresh grass as the source of roughage, supplemented with pelleted feed. Round bale hay is particularly allergenic and 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 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. Horses with summer pasture–associated obstructive pulmonary disease should be maintained in a dust-free, stable environment.

Medical treatment consists of a combination of bronchodilating agents (to provide relief of airway obstruction) and corticosteroid preparations (to reduce pulmonary inflammation). Bronchodilator therapy (β agonists and parasympatholytic agents) will provide immediate relief of airway obstruction until clinical signs of disease are controlled by corticosteroids. Severely affected horses are ideally controlled with aerosolized bronchodilators (eg, albuterol, ipratropium) and systemic corticosteroids (eg, dexamethasone 0.1 mg/kg/day, IV). 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.

Last full review/revision January 2014 by Bonnie R. Rush, DVM, MS, DACVIM

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