Chronic Airway Disease in the Horse

History. A 10-year-old horse is presented with a 2-year history of coughing and a progressive loss of exercise toler­ance. Recently, the horses problem has become so severe that it has difficulty breathing while resting in its stall.

Clinical Examination. Inspection reveals a thin horse with flared nostrils and an anxious expression. The respira­tory rate is elevated, and respiratory movements are accentuated. During inhalation the intercostal spaces are pulled in between the ribs. The initial part of exhalation is characterized by a rapid relaxation of the rib cage. This is followed by a prolonged contraction of the abdominal muscles, which is terminated immediately before the next inhalation. During the prolonged contraction of the abdominal muscles, wheezes can be heard when you place your ear close to the nostrils.

The horse has an elevated pulse rate. The mucous mem­branes of the gums have a bluish tinge. Auscultation of the thorax reveals increased breath sounds over all the lung fields and musical wheezes audible at end exhalation. Excessive mucus pooled in the airway is viewed through an endoscope advanced into the trachea.

Because the horse is being examined at a teaching hospital, there are facilities for measurement of lung function. The change in pleural pressure (ΔPpl) during each breath is 25 cm H₂O (normal, 5 cm H₂O), and airway resistance is 3 cm H₂O∕L∕sec (normal, 1 cm H₂O∕L∕sec). Administration of atro­pine intravenously decreases ΔPpI to 7 cm H₂O and airway resistance to 1.5 cm H₂O∕L∕sec.

Comment. The respiratory distress, cough, and lack of exercise tolerance indicate a respiratory problem. The increased effort ofbreathing documented by the elevated ΔPpl could be caused by airway obstruction, a decrease in lung compliance, or increased breathing resulting from increased metabolic rate. The latter cause is eliminated because the horse is resting in the clinic. The mucus in the airway and elevated airway resistance confirm airway obstruction. Musical wheezes at the end of exhalation typify airway disease and result from increased air turbulence or vibration of mucus and the airway walls. Airway obstruction is caused in part by bronchospasm result­ing from parasympathetic activity, because it is reversed by atropine, a parasympathetic antagonist. Atropine does not return resistance to normal, so there is also considerable obstruction by mucus and swelling of the airway wall.

The flared nostrils are an effort to reduce the work ofbreath­ing by dilating the upper airway. The blue-tinged mucous membranes indicate desaturation of hemoglobin because of impaired oxygen uptake in the diseased lungs.

Retraction of the intercostal muscles during inhalation indicates a major decrease in pleural pressure as the respira­tory muscles work to inflate the lung and pull air through the obstructed airways. The prolonged contraction of abdominal muscles, or heaving, represents an effort by the horse to force air out through obstructed airways. Weight loss is probably a result of the increased work ofbreathing. Coughing is an effort by the horse to expel the excessive mucus.

Treatment. This horse has heaves (also known as recurrent airway obstruction [RAO∣), a problem exacerbated by stabling in a dusty barn and eating poorly cured, moldy hay. Heaves is the result of an allergic response to particles, anti­gens, and endotoxin in the hay and barn dust. The best treatment for the horse is to keep it out at pasture and supplement its diet with pelleted feed rather than hay. In many cases, including this horse, additional treatments are needed when the horse endures a crisis. Treatment is aimed at dilating the airways (bronchodilator such as clenbuterol), oxygen therapy when warranted, and corticosteroids (inhala­tion or systemic) when severely affected. With good manage­ment control, some horses do not need constant treatment. In advanced stages and when treating some performance horses, however, treatment such as bronchodilators and inhaled steroids may be necessary.