Respiratory Diseases Affecting Equine Athletes

There are a variety of respiratory diseases and abnormalities that affect the performance of equine athletes. These diseases and abnormalities fall into two sections depending on what region of the respiratory system they affect, whether it is the upper or lower respiratory tract.

Within the lower respiratory tract, the two diseases that affect this area are exercise-induced pulmonary hemorrhages (EIPH) and equine asthma. Exercise-induced pulmonary hemorrhage is defined as the presence of blood in the tracheobronchial airways during and after exercise in a horse. The bleeding occurs from the bronchial arterial circulation, extensive bronchiolitis in the dorsal regions of the caudal lobe within the lungs, or from interstitial fibrosis and sequestration of macrophages containing hemosiderin. There is no scientifically proven cause to why EIPH occurs, however it is suspected that factors such as lungs with poor collateral ventilation and lung segments with diseased small airways that fail to inflate regularly are suspected reasons for this condition. During the repair response of these diseased segments, there is bronchial arterial neovascularization that occurs and as a result, these blood vessels bleed. Exercise-induced pulmonary hemorrhage can be diagnosed by an endoscopic observation of the lungs, paying particular attention to see if there is blood visible in the airways following exercise. If there are hemorrhages observed in the upper airway system, such as a guttural pouch mycosis or ethmoidal hematoma, then these do not qualify as EIPH. There are a few prevention and control measures that can be put in place to prevent this condition from developing or progressing in severity. There can be improvements in ventilation to minimize strain on the lungs, altered training and fitness conditions, or respective convalescence after any viral or bacterial respiratory infection. The most common treatment drug administered for EIPH is furosemide. This drug is used along with estrogens, vitamin K, and vitamin C to enhance its effectiveness. The image below illustrates a horse with EIPH.

The second lower respiratory tract is equine asthma. Equine asthma is characterized by a persistent cough that becomes apparent during exercise or feeding. This can directly impact their exercise ability, as well as make them appear constantly out of breath. There is no direct way to diagnose this condition, there are indicators that can be observed. If a horse produces strained breathing sounds when respirating, such as wheezing and a rattle, then it can be deduced that a horse has equine asthma. There is no medication or drug that can be used to treat asthma, rather ways to mitigate and decrease the frequency of coughing episodes. One of these methods is the simultaneous application of bronchodilators and corticosteroids. The bronchodilators will open the lung airways, allowing for easier respiration. In the meantime, the corticosteroids eliminate any inflammation caused by irritants entering the airway. Both these medications can be given in an oral, inhalator, or injectable composition. In addition to treatment, there are multiple preventative measures that can be implemented to reduce equine asthma. Depending on the horse and the trigger of their asthma, soaking hay, limiting stall time, proper stable ventilation and use of shavings are actions that will prevent the irritation of the airway system in terms of dust, limiting inflammation of the lungs.

The three most common upper respiratory diseases in equine athletes include , laryngeal paralysis, dorsal displacement of the soft palate (DDSP), and nasopharyngeal collapse. Laryngeal collapse occurs when one or more of the supporting cartilages of the trachea partially collapse. This forms a “roaring” or wheezing sound when a horse respirates, which becomes more extreme when the horse is exercised. Similar to EIPH, this condition can be diagnosed through an endoscopy procedure conducted by a veterinarian. In addition, an ultrasound of the larynx can be performed to observe if the muscle fibers in this area are unbalanced and/or uneven. Unlike most respiratory diseases, there are multiple treatment options that prove to be quite effective. These include:

1. Prosthetic Laryngoplasty - The paralyzed cartilage is “tied back” into an open position through an incision in the throat latch area.
2. Ventriculectomy/cordectomy - The ventricle and the vocal cord  is removed to widen the airway.
3. Arytenoidectomy - The removal of the paralyzed arytenoid cartilage which acts to enlarge the opening to the trachea.
4. Neuromuscular pedicle graft - Surgery that re-innervates the muscles that controls abduction of the arytenoid cartilage.

All four of these options are surgical and have a longer recovery time post procedure. Despite this, these treatments can lessen or eliminate the laryngeal paralysis that has occurred, allowing an equine athlete to return to exercise and performance with minimal issues. The image below compares a normal larynx to a horse with laryngeal hemiplegia.

The second upper respiratory disease is DDSP. Horses are mammals that breathe through their nose, and to accommodate this, their mouth anatomy is different from that of most mammals. As a result, the larynx rests upon the dorsal palate, allowing for easier respiration through the nose. Horses that experience DDSP appear to have atypical muscle contractions within the muscles surrounding the soft palate, which creates respiratory resistance. This leads to more strenuous and labored breathing, creating issues and a decrease in athletic performance. There are two surgical procedures that can resolve DDSP, such as sternothyrohyoid myectomy (alters the anatomy of the upper respiratory tract) and Staphylectomy (resection of the posterior soft palate to remove tissue and open up the airway). In addition, a horse can be treated with anti-inflammatory drugs and a rest from exercise to decrease the severity of DDSP.

Lastly, a smaller portion of equine athletes suffer from epiglottic entrapment. This disease involves the epiglottic becoming lodged in tissue surrounding the larynx, which in turn creates respiratory resistance as the laryngeal opening is narrowed. The best treatment is surgery to fix the anatomical placement of the epiglottis, and has been proven to be quite effective.

References

1. Equine Asthma. Western College of Veterinary Medicine. University of Saskatchewan.
2. EIPH – Bleeding in Performance Horses. Equine Medicals.  
3. Laryngeal hemiplegia in Horses. American College of Veterinary Surgeons.
4. Larson, Erica. EIPH in Horses: What You Need to Know. The Horse.
5. Mazan, Melissa. Equine Exercise Physiology-Challenges to the Respiratory System. Animal Frontiers.  U.S. National Library of Medicine.
6. Rush, Bonnie R. Dorsal Displacement of the Soft Palate in Horses - Respiratory System. Merck Veterinary Manual.

Photo credits

1. EIPH – Bleeding in Performance Horses. Equine Medicals.
2. Rush, Bonnie R. Dorsal Displacement of the Soft Palate in Horses - Respiratory System. Merck Veterinary Manual.