Editor’s Note: N. Edward Robinson, BVetMed, PhD, is the Matilda Wilson professor in large animal clinical sciences at Michigan State University. He is one of the world’s leading researchers in equine airway disease, and he has lectured on this topic around the world. Following is information from Robinson on the latest in inhalation therapy for equine airway disease.

In horses, inhalation therapy is suitable for treating heaves (recurrent airway obstruction) and inflammatory airway disease (IAD). Heaves is an allergic response to organic dusts in the environment. Usually these dusts originate in poorly cured or dusty hay, but horses can also respond to dusts in pastures or tree pollens. Like people, some horses are more susceptible to these antigens than others. When the heaves-susceptible horse inhales dusts, his airways become inflamed and neutrophils invade the airway lumen, or cavity. Neutrophils are the warriors of the white blood cells. They evolved to respond rapidly to infection and “eat” things the horse’s body considers invaders. The role they play in heaves is not clear.

However, neutrophil products initiate over-proliferation of the mucus cells in heaves-susceptible horses, and that can lead to excessive mucus secretion. At the same time, the airway smooth muscle becomes hyperresponsive to stimuli (such as allergens and dusts) so that if the particulate (dust) load is high enough, bronchospasm develops. Bronchospasm is constriction of the smooth muscles around the small airways that restricts airflow and oxygen exchange; this tightening of the airways means that not enough air gets through. This leads to the respiratory distress that is typical of heaves.

Many heaves-susceptible horses can have significant airway inflammation even without signs of respiratory distress. This inflammation might be enough to maintain excess mucus secretion, but not to cause bronchospasm. So, the absence of obvious distress in a horse with a history of heaves doesn’t mean his disease is under control