The common link between heaves and IAD is airway inflammation. In horses with heaves, the inflammation that we see on a lung wash, otherwise known as a bronchoalveolar lavage or BAL is due to cells called neutrophils, whereas in horses with IAD, the culprits may be neutrophils, mast cells, or a combination of the two. Horses rarely have eosinophils as the inciting inflammatory cells, unlike cats and humans with asthma, who commonly have high levels of eosinophils in their airways. We know far more about what happens on a microscopic basis in the lower airways of horses with heaves than we do about horses with IAD. Horses with heaves develop increases in airway smooth muscle, fibrous tissue, epithelial tissue and mucus, all of which contribute to the marked bronchoconstriction, or narrowing of the airways, that result in abnormal breathing and air hunger. Although we can reverse bronchospasm due to abnormal smooth muscle contraction with drugs such as albuterol and we can reduce mucus production with use of corticosteroids (see below), the fibrosis and epithelial thickening are difficult to reverse, and abnormal lung function in horses with overt heaves can still be detected even when the horses are in remission.
When horses have chronic, long-standing airway dysfunction, as with heaves, they often develop abnormal breathing patterns at rest. The time that they spend breathing out (expiration) is usually prolonged, and they often develop hypertrophied external abdominal oblique muscles that are used for pushing air out of the lungs. Just as a body builder develops an exaggerated look from over-use of specific muscles, so a horse with heaves develops a heave line from muscular overuse during breathing. Some horses become quite thin when they have chronic heaves. We know from studies in our laboratory that these horses burn the same amount of calories that they would if they were trotting all day and all night. It would be impossible for them to eat enough to satisfy their caloric needs.
We know much less about the pathology of IAD. We do know that they produce excessive amounts of mucus, which we see both on endoscopy
Moderate amount of mucus in airway
and BAL cytology. Because most horses with IAD have airway hyperreactivity, we surmise that they have either overly active airway smooth muscle or excessive amounts of smooth muscle in their airways. A recent study showed that airways of a large majority of actively racing horses have neutrophilic inflammation of the small airways as well as smooth muscle hyperplasia – and the distribution suggests that the inciting agent is inhaled. This correlates well with the clinical data showing that airway inflammation is associated with high levels of particulates, including endotoxin, beta glucans, ammonia, and other airway irritants.