Answered by, Elizabeth Weber, DVM, Ocala, FL
Courtesy of AAEP
Question: What, in your opinion, is the leading cause of poor performance due to respiratory disease?
Answer:Unfortunately, this is a question with no straightforward answer! While some respiratory problems are inherently more severe than others, effect on performance will depend on the type of performance in question, the population of horses looked at, and the degree of severity of the abnormality. In addition, some individuals tolerate certain respiratory issues better than others, in the same way that some can be more or less stoic about musculoskeletal issues.
In many horse populations, recurrent airway obstruction (RAO, also often referred to as COPD or heaves), is a common performance-limiting respiratory issue, affecting approximately 12% of mature horses to varying degrees. RAO is an allergic respiratory disease in which common environmental allergens cause an inflammatory response in the lungs, resulting in coughing, nasal discharge, respiratory difficulty, and exercise intolerance. Treatment involves environmental management to reduce exposure to these allergens and medical therapy to treat the symptoms, but there is no cure for it. Due to both its widespread occurrence and the degree of effect on performance (due to coughing during exercise and exercise intolerance), in my opinion RAO is a leading cause of poor performance in show and pleasure horses. However, the average age of onset is 9 years, so while it is an important cause of poor performance in mature horses, it is of less concern in the younger horse population.
In other populations of performance horses, laryngeal hemiplegia is a significant cause of poor performance, especially when it has progressed to complete paralysis. In a normal horse, the two arytenoid cartilages in the larynx are pulled upward and outward as the horse breathes in, opening the airway to allow air to flow through normally. With laryngeal hemiplegia, the nerve controlling one of these cartilages (nearly always the left side) does not function normally, and so the cartilage does not move as it should. The degree of severity can range from a mildly delayed opening to complete paralysis of the cartilage. When complete paralysis of one cartilage occurs, air has significantly less room to pass through the larynx, causing an increase in airway resistance and an increase in respiratory effort. In horses doing strenuous work, such as racehorses or high-level sport horses, this results in an abnormal respiratory noise and exercise intolerance, leading to poor performance in many cases. This condition can be addressed with surgery (prosthetic laryngoplasty, often referred to as a “tie-back”) and performance typically improves post-operatively, but these horses still do not generally reach their full performance potential.
There are certainly other causes of poor performance related to the respiratory tract (exercise-induced pulmonary hemorrhage, dorsal displacement of the soft palate, epiglottic entrapment, and shipping fever, just to name a few), and severity of all issues can be variable. However, in my opinion, the above are two of the most significant due to the combination of their prevalence and their impact on normal respiration, leading to decreased performance.