There is strong evidence for the efficacy of treatment for both bronchodilators and glucocorticoids in heaves, and we have a reasonably good idea of how and why they work in this disease. This knowledge has largely been extrapolated to the treatment of IAD. For both, the mainstay of treatment has become a combination of environmental remediation, corticosteroid therapy, and bronchodilators. We must acknowledge, however, that our understanding of these drugs in horses with IAD is much less complete.
Corticosteroids remain the cornerstone of successful treatment for IAD, as it is important to counteract the inflammation that characterizes this disease. Bronchodilator drugs will help to relieve coughing due to acute bronchospasm, but only consistent anti-inflammatory therapy, in conjunction with avoidance of environmental triggers, will break the vicious cycle of inflammation, airway hyper-reactivity and bronchoconstriction. Corticosteroids activate glucocorticoid receptors, thus putting into motion a profound inhibition of the arachadonic acid cascade and limiting production of leukotrienes and other inflammatory molecules. Response to steroids can vary considerably from horse to horse.
The choice of whether to use systemic drugs in combination with inhaled drugs or alone may depend on a number of factors, including severity of disease and finances, as aerosolized drugs and their delivery devices are quite expensive; as well as known and putative side effects. It is important to remember that corticosteroids can, among other things, adversely affect tissue growth and protein use, impair the barrier function of the intestinal mucosa, cause immune suppression, and suppress adrenal function, so they must be used with caution.
Systemic Corticosteroids: Multiple studies have demonstrated the positive effects of corticosteroid drugs on horses with heaves, but the evidence for their use in IAD, despite good clinical response anecdotally, is less robust. Prednisolone and dexamethasone are the corticosteroids used most frequently in the treatment of RAO and IAD. Triamcinolone acetonide has also been shown to relieve airway obstruction in heaves. Triamcinolone, however, is anecdotally more closely associated with the development of laminitis in horses than other corticosteroids, and has been shown to cause profound and persistent hyperglycemia and hypertriglycerridemia (3-4 days) in horses after a single injection of triamcinolone, which may explain the anecdotal reports. (French 2000) Thus, its use is discouraged in the treatment of IAD.
Inhaled Corticosteroids: The use of inhaled corticosteroids has truly revolutionized the treatment of IAD. While initial systemic tapered corticosteroid therapy is often necessary with all but very mild IAD, regular inhaled therapy is essential for long-term success in most cases. The most important factor that limits regular use of inhaled corticosteroids is cost, because drugs such as fluticasone and beclomethasone are very expensive. When assessing the effects of corticosteroids on horses with airway disease, it is important to note what delivery device and drug formulation was used because certain devices deliver more drug to the lower airways, and certain drug formulations, such as QVAR, a proprietary formulation of beclomethasone, have been shown to reach the lower airways more reliably, at least in humans. For this reason it is very difficult to make comparisons of drugs across studies that used different delivery devices. Moreover, the FDA has phased out the use of chlorofluorocarbon (CFC) propellants in metered dose inhalers (MDIs) in accordance with the Montreal protocol in order to protect the ozone layer. Thus studies employing CFC inhalers are not directly comparable to those using the currently available hydrofluoroalkane (HFA) inhalers.
Aerosolized corticosteroids are stated to be preferred over systemic in order to decrease potential side effects. This is well-documented in humans, but while this is a rational approach in horses, there is little documentation to support it. It is important to point out for sport horses that may undergo testing that fluticasone propionate or its metabolites can be detected in blood for a minimum of 72 hours after being given by inhalation and urine for approximately 18 hours after inhalation. (Gray 22012)
Fluticasone propionate is thought to be the most potent of the inhaled steroids, has the longest pulmonary residence time, and causes the least adrenal suppression. On the other hand, newer formulations of beclomethasone dipropionate that incorporate HPA as the propellant have more uniform particle size, and are more uniformly mixed. A recent study found that dexamethasone (0.05 mg/kg IM q 24 hours) and inhaled fluticasone (3000 ug q12 hours) were both effective at decreasing airway hyperreactivity in horses with IAD, but neither had a significant effect on clinical signs or the number of inflammatory cells in the bronchoalveolar lavage fluid. Moreover, the treatments had no residual effect 3 weeks after discontinuation. (Leguillette, 2017)
In our clinic, we use both QVAR and Flovent and the deciding factor as to which one is used is often the cost. For reasons that are not well understand, some horses seem to do better on one drug versus the other, and the clinician must maintain a certain flexibility in choosing drugs. In our clinic, we frequently treat with an initial course of parenteral corticosteroids, typically, a four-week, decreasing course of prednisolone, followed by inhaled corticosteroids.