IAD: Cough, Poor Performance, Mucus in the Airways--What Is So Important About That?

The lower respiratory tract responds to a variety of inhaled irritants in a stereotypical way by coughing, increasing mucus secretion, and bronchospasm. The purpose of such response is to limit further inhalation of irritants and to accelerate their removal from the respiratory tract. Additional defense mechanisms involve alveolar macrophages, airway-associated lymphoid tissue, and systemic immunity. Substances irritant to the airways may be infectious agents (viruses, bacteria, mycoplasma, fungi), parasites, endotoxins, molds, dust, noxious gases, or pollutants. Airway inflammation may result if exposure to the irritant is of sufficient magnitude. Lower airway inflammatory diseases include infectious and parasitic diseases, interstitial pneumonia, recurrent airway obstruction (RAO) or heaves, and inflammatory airway disease (IAD). Inflammatory airway disease is an incompletely characterized pulmonary disorder that is most commonly observed in young, athletic horses. Contrary to other lower airway inflammatory diseases, clinical signs of IAD are subtle and include chronic cough, excess mucus in the trachea, and poor performance; otherwise, horses have a normal attitude and appetite, are not febrile, and do not exhibit increased respiratory effort at rest. Recently, a panel of international experts recommended using the term IAD instead of small-airway inflammatory disease, small-airway disease, and bronchitis/bronchiolitis, which have been used in the past to describe the disease. In addition to terminology issues, confusion may result from horses with IAD being included in broader disease categories such as undifferentiated pulmonary disorders, chronic obstructive pulmonary disease, and lower airway disease. The purpose of the following discussion is to clarify our current understanding of IAD focusing on clinical characteristics.