Many of you are familiar with the respiratory condition known as “heaves,” also termed recurrent airway obstruction (RAO). Primarily caused by chronic exposure to dusts and molds in hay and bedding, heaves can cripple the function of a horse’s lungs, making it difficult for him to breathe at rest, let alone during physical exertion. In horses with severe heaves, the breathing rate is much higher than normal, the nostrils flare with each breath, there is noticeable chest movement during breathing (inspiration and expiration), and there often is a persistent cough. These signs make a diagnosis of heaves fairly straightforward. Fortunately, a number of drugs and changes in management will bring some relief to horses with this condition.

But heaves is the tip of the iceberg of respiratory ailments that can affect performance. For many of these conditions, the clinical signs are much more subtle, and thus diagnosis is considerably more challenging. In this article, we consider some common respiratory ailments that affect athletic horses and the steps a veterinarian might take to diagnose them.

Form and Function

The respiratory system has two main components–upper and lower. The upper respiratory tract includes the nostrils, nasal passages, pharynx and larynx (throat), and the trachea (windpipe). (See “Anatomy of the Upper Respiratory System” on page 71.) The lower respiratory system is everything beyond the windpipe–the trachea splits into left and right bronchi that supply air to the left and right lungs. Each lung contains a massive branching network of smaller airways, each of which terminates in a small air sac. These air sacs, called alveoli, are in close contact with the bloodstream. This alveoli-blood vessel unit is the site of gas exchange–with each breath, oxygen crosses into the blood while carbon dioxide moves from the blood into the alveoli and is expelled from the body du