Heaves (Recurrent Airway Obstruction)

By | 2012-11-11

Practical Management of Acute Episodes and Prevention of Exacerbations

Heaves, also known as recurrent airway obstruction (RAO) and chronic obstructive pulmonary disease (COPD), is an inflammatory condition in horses that results from the inhalation of dust in moldy hay and bedding. The condition affects primarily the small airways of horses and causes bronchospasm, bronchial hyperresponsiveness, mucus plugs, and pathologic changes of the bronchiolar walls, leading to obstruction of terminal airways. The mechanisms by which dust inhalation causes lower airway inflammation remains ill-defined, although evidence exists that a hypersensitivity reaction to specific antigens present in hay may be implicated. However, a wide range of particles is present in the horse’s environment that also could be implicated in the development of heaves.

The treatment of heaves aims at (1) preventing further inhalation of offending dust in hay, (2) decreasing inflammation of the lower airways, and (3) providing symptomatic relief of airway obstruction. Although environmental dust control is pivotal to prevent the exacerbation of heaves, medications often are required for immediate improvement of airway function.

It is currently unknown whether a mechanistic relationship exists between heaves and inflammatory airway disease (inflammatory airway disease) in young performing horses and therefore findings regarding the treatment of heaves may not necessarily be appropriate for inflammatory airway disease.

Acute Episodes

The primary goal of therapy during acute exacerbation of heaves is to relieve airway obstruction primarily by the administration of antiinflammatory agents and bronchodilators.

Table Medications Recommended for the Treatment of Heaves

Medication Dosage*
Corticosteroids
dexamethasone 20-50 mg** IV, IM, or PO q24h
dexamethasone 21-isonicotinate 0.04 mg/kg IM q3d
prednisolone 2.2 mg/kg PO q24h
isoflupredone acetate 10-14 mg** IM q24h
triamcinolone acetonide 20-40 mg** IM
beclomethasone dipropionate 3500 μg/horse q12h in MDI (Equine AeroMask)
1320 μg/horse q12h in MDI (3M Equine Aerosol Delivery System)
fluticasone propionate 2000 μg/horse q12h in MDI (Equine AeroMask)
Bronchodilators
clenbuterol 0.8-3.2 μg/kg orally twice daily
0.8 μg/kg IV
aminophylline 5-10 mg/kg orally or IV twice daily
fenoterol 1-2 mg/horse in MDI (Equine AeroMask)
albuterol 0.8-2 μg/kg in MDI
ipratropium bromide 2-3 μg/kg q6h with mechanical nebulizer
90-180 μg/horse q6h in MDI (Equine AeroMask)
1200 μg/horse q6h with DPI
salmetreol 63-210 μg q8h (Equine AeroMask)
Cromones
sodium cromoglycate 80 mg/horse q24h for 4 days with a mechanical nebulizer
200 mg/horse q12h in MDI (Equine AeroMask)
nedocromil sodium 10-20 mg q8h in MDI (Equine AeroMask)

IV, Intravenous; IM, intramuscular; MDI, metered-dose inhaler; q12h, every 12 hours; DPI, dry powder inhaler.

* Suggestive dosages are indicative only.

** The usual dose for a horse that weighs 450 to 500 kg.

Corticosteroids Recommended for the Treatment of Heaves

Bronchodilators Recommended for the Treatment of Heaves

Expectorant, Mucolytic, and Mucokinetic Agents

Expectorants are drugs that increase pulmonary secretion, whereas mucolytic agents loosen secretions. The term mucokinetic agent may be preferred because it indicates that the therapy is aimed at increasing the clearance of the respiratory tract secretions. Although the administration of mucokinetic agents may help loosen the secretions in the large airways, evidence of their efficacy in improving the clinical signs of heaves is sparse. Clenbuterol, because of its bronchodilator and mucokinetic properties, may be preferred to clear mucus from the airways. Dembrexine (Sputolysin) and potassium iodide also improve clearance of bronchial secretions. Potassium iodide should be administered with caution to heavey horses because it is irritating for the respiratory tract and can induce or worsen bronchospasm. Nebulization with N-acetylcysteine (1 g/horse q12h via mechanical nebulizer) depolymerizes mucus by breaking disulfide bridges between macromolecules and has been advocated in the treatment of horses.

Overhydration by the massive administration of isotonic saline solution combined with bronchodilators or mucokinetic agents has been used to treat airway obstruction of horses with heaves. Although in a controlled laboratory setting this author failed to find an improvement in the pulmonary mechanics of heaves-affected horses with overhydration alone, it occasionally was associated with improved airway function of some clinical cases particularly when heaves-affected horses were refractory to other modes of therapy including potent corticosteroids. The proposed beneficial effects of this treatment are improved mucus transport and removal of mucus plugs related to the liquefaction of excessively viscous mucus. This treatment should be administered with caution as a number of side effects, including dyspnea and colic, have been observed with its use.

Antitussive agents are rarely indicated in the treatment of equine heaves because cough is a mechanism essential for the clearance of respiratory secretions.

Prevention Of Exacerbation

Environmental Changes

Clinical exacerbation of heaves occurs when susceptible horses are exposed to environmental dust particles. Drugs administered to heaves-affected horses will have only transitory effects if concurrent strict dust control measures are not applied. A wide diversity of particles may be found in a barn, including molds, noxious gases, endotoxins, and other irritants. The greatest exposure to particles small enough to be inhaled deep into the lungs of horses occurs when they are eating hay. For this reason, long-term management of heaves depends primarily on the replacement of hay in the diet by non-dusty hay alternatives. The airways of heaves-affected horses are hyperreactive, and therefore any inhaled irritants also potentially could contribute to the airway obstruction in susceptible horses.

The reversal of clinical signs of heaves with strict environmental changes may take up to 3 to 4 weeks. The remission time correlates with age and the duration and severity of illness. Horses kept permanently outdoors and fed grass or other hay substitutes usually remain free of clinical signs. Horses do well when kept outdoors even in very cold conditions, as long as they have access to enough food, fresh water (heated water tub), and shelter. The replacement of hay by less dusty feed can induce clinical remission in stabled horses. Pelleted hay, hay silage, and hydroponic hay are well tolerated and free of dust. Hay soaked in water for 2 to 4 hours before feeding may control heaves in some horses, whereas in others only partial improvement often is noted. Wood shavings, shredded paper, peanut kernels, and peat moss are good substitutes for straw, although a recent study failed to find differences in airway function in heaves-susceptible horses fed silage that were bedded on good quality straw or shavings. Other commonly made recommendations include removing the horse from the stable when cleaning the box stalls and watering the aisles before sweeping to decrease the amount of dust particles suspended in air. Proper ventilation is also important, although identifying the proper ventilation system, which would minimize dust, is problematic.

Aerosol Medications

Aerosol medications, in particular steroids such as BDP and FDP, are quite effective to prevent relapses, if given long term (see “Aerosolized Drug Delivery Devices” and “Use of Aerosolized Bronchodilators and Corticosteroids”). These drugs prevent the cascade of inflammation that is the hallmark of the allergic process and may reduce the previous remodeling of the airway (airway wall thickening via epithelial hyperplasia and goblet cell metaplasia). Although little information exists in the literature, the use of 10 puffs of BDP (84 meg/puff) or FDP (220 μg/puff) given daily or every other day has been reported to be an effective means to prevent exacerbations during periods of susceptibility but does not replace the need for environmental changes.

Alternatively, the prophylactic administration of sodium cromoglycate (Intal, 80 mg q24h for 4 days) by inhalation in heaves-susceptible horses in clinical remission prevented the appearance of clinical signs for up to 3 weeks after they were introduced to a dusty environment. The administration of sodium cromoglycate using a dose metered inhaler and a treatment mask facilitated drug administration and therefore decreased treatment failure resulting from inadequate drug administration. A similar mast cell blocker is nedocromil sodium (Tilade) that is given at a dose of 10 to 20 puffs (1 mg/puff) three times per day. These two mast cell blockers may be effective in preventing exacerbations in horses that do not respond to inhaled steroids, or as supplements to reduce the need for steroids. The problem with mast cell blockers is the need for large and frequent dosing.