Contraction of bronchial and bronchiolar smooth muscle contributes to airway obstruction in horses with COPD. As a consequence of airway geometry, smooth muscle contraction causes a greater reduction in airway caliber when the wall is thickened by inflammation, as occurs in COPD. Airway smooth muscle contraction is caused by the complex actions of inflammatory mediators on smooth muscle and on the afferent and efferent neural arcs.

COPD has many similarities to naturally occurring and experimental endotoxin mediated lung diseases in other species. Reversible airway obstruction, airway neutrophilia, bronchial hyperresponsiveness and increased mucus production are common as in emphysema. Again, toxins contribute to pulmonary inflammation in equines.