Horses in training face a number of performance limiting conditions. The most common reason is lameness, however respiratory conditions are the second most important reason for poor performance and are often overlooked. They may be categorized as follows: structural, functional, inflammatory andinfectious. Important conditions in each group are discussed below.
Upper airway issues are common performance limiting conditions in many equine performance disciplines, and Thoroughbreds can also be affected. As the upper airway is responsible for the majority of airway resistance, any narrowing due to soft tissue collapse (e.g. the nasopharyngeal walls) has profound effects on overall airflow. The airway may also become narrowed due to the soft palate, which can sometimes leave its usual position beneath the epiglottis and become dorsally displaced, leading to a reduction in airflow. Tissue vibration and turbulent airflow are responsible for noise generation, although not all horses that generate noise have performance limitations. Noise present only on inspiration implies airway collapse, however abnormal sounds during both phases of respiration suggest a fixed obstruction (e.g. nasal mass) should be suspected.Epiglottic entrapment is common in racing Standardbreds and Thoroughbreds but uncommon in other breeds. It may be present at rest and an incidental finding with no effects, or it may create noise during exercise. Arytenoid chondritis varies from mild thickening to cartilage ulceration and deformation, with prognosis for performance poor if the contralateral cartilage is abnormal. This inflammatory condition is of unknown origin.
Laryngeal hemiplegia, a functional disorder, is a frequent cause of exercise intolerance and prevalence increases with age. Paralysis of one side of the larynx, most often the left side, causes the arytenoid cartilage and vocal cords to bulge into the airway, obstructing airflow and compromising the ability of the horse to breathe sufficiently to meet the body’s oxygen requirements under exercise. In addition to the exercise intolerance that results, profound upper respiratory tract noise may be heard.
Lower airway inflammation and infection may affect upper airway function (precipitating collapse or diminishing laryngeal function as explained above) or diminish exercise tolerance in their own right. Inflammatory airway disease (IAD) and exercise induced pulmonary hemorrhage (EIPH) are the most common performance limiting lower respiratory tract diseases in the training horse. Inflammatory changes may be precipitated by both infectious and non-infectious agents, including bacteria, viruses, allergens and particulate irritants . Diminished performance, cough and nasal discharge are common. Racehorses (and other breeds/disciplines that require short bursts of maximal exercise) experience EIPH. This may be related to the occurrence of IAD in some situations, although there is considerable debate as to the cause or causes of EIPH.
By Dr. Peter Morresey, BVSc, MVM, MACVSc, Dipl. ACT, ACVIM, CVA, Rood and Riddle Equine Hospital