What is Inflammatory Airway Disease [IAD]?
IAD is a disease of the airways which disturbs normal lung function (most often causing exercise intolerance) in addition to acting as a source of irritation (causing cough). Clinical signs are associated with swelling or obstruction and thus narrowing of the lower airways, which prevents air from reaching the maximum number of air sacs (alveoli) for gas exchange. In horses suffering from IAD, the airways are narrower because of inflammatory secretions, excessive mucus and fibrous tissue. In order to overcome the resulting increase in airway resistance, the horse has to work harder to achieve each breath and ultimately becomes exercise intolerant.
Horses with IAD most commonly present with a history of cough (which may be triggered by exercise), decreased performance, higher breathing rate or nasal discharge (usually clear to mucoid). The clinical signs are usually subtle and can affect horses of any age. In the worst case, long standing airway inflammation may lead to development of “heaves” (also known as recurrent airway obstruction or RAO), which tends to affect mature to older horses. Horses with heaves may even present to the veterinarian in an emergency setting, as affected horses exhibit significant lower airway inflammation and obstruction resulting in markedly increased work of breathing. When horses have an exacerbation of RAO, their whole bodies can be seen to rock with the effort they expend in breathing. Guidelines for the definition, diagnosis and treatment of IAD and RAO were published in 2007 by the American College of Veterinary Internal Medicine, under the co-authorship of Dr. Andrew Hoffman at Tufts University.
Horses can develop inflamed airways in response to particulates in their environments, such as plant pollens, and the molds and spores found in even the best of hay. Subsequently, the lung mounts an inflammatory response with its full complement of cells and mucus. Eventually, the tissues thicken, and permanent obstruction may occur. Airway hyperreactivity may also be triggered by a viral disease, such as equine influenza, for months after the primary disease has resolved. You may note this as a persistent cough, especially during exercise, because inflamed airways are more reactive, or “twitchier” than those of normal horses. This means that the airways, which are already narrowed, constrict even more in response to a number of stimuli. These stimuli may be the same particulates that initiated the original airway inflammation (such as molds and pollens), cold air or gases, such as ammonia.
The diagnosis of IAD is best based on lung function testing and bronchoalveolar lavage (BAL), to direct therapy and monitor treatment success. Although IAD is rarely a curable disease, clinical signs and airway obstruction may be alleviated by administration of inhaled or systemic corticosteroids, bronchodilators and mast cell stabilizers or changing the environment. In this context, pulmonary function testing has been utilized to better monitor and characterize the disease severity, evaluate the treatment response and objectively guide the long term management of horses with airway inflammation, in order to preserve athletic performance and health.