The horse’s respiratory tract is a long tube which terminates in alveoli. The alveoli are microscopic air sacs where oxygen enters the blood stream and exchanged for carbon dioxide. Oxygen-rich air enters the nares (nostrils) and travels through the nasal passages, paired paranasal sinuses, and nasopharynx (the region extending from the nasal passages to the trachea). The nasopharynx is located above the soft palate, which is an extension of the roof of the mouth called the hard palate.
The horse’s soft palate is long, extending from the end of the hard palate to the base of the epiglottis. The epiglottis lies on top of the soft palate, and helps hold the soft palate in place. Horses are obligate nasal breathers because of the anatomical structure of the epiglottis and soft palate. Obligate nasal breathers’ means that a normal horse is not capable of breathing from his mouth.
The larynx (voice box) demarcates the junction between the upper and lower airways and is located at the entrance to the trachea.
The epiglottis is one of several cartilaginous structures that make up the larynx. Other important larynx structures include the aryepiglottic folds, vocal cords, and glottic cleft (entrance to the larynx). Only when the epiglottis is lying flat on the soft palate can the air pass through the glottic cleft of the larynx to enter the trachea.
The air then passes down the trachea into the thorax (chest). Within the thorax, the trachea divides into two tubes called bronchi, each of which leads to one of the lungs. The bronchi then subdivide into progressively narrowing tubes called bronchi and bronchioles. Lying at the end of the bronchioles are the alveoli—microscopic air sacs where the actual respiration process (the exchange of oxygen for carbon dioxide) occurs.
Pulmonary defense mechanisms
The respiratory system protects against infection via clearance mechanisms, the mucosal lining provides a barrier, mucosal secretions trap debris, and the specialized cells which have small cilia or “hairs”. The mucus and cilia extends from the pharynx to the respiratory bronchioles. The mucous layer is constantly propelled upward by the ciliated respiratory epithelium; this action continually moves inhaled particles and debris out of the respiratory tract. Smoke inhalation and viruses’ damage and destroy the ciliary hair cells. If your horse becomes infected with influenza or herpesvirus, these two viruses destroy the cilia and regrowth of these cells takes approximately 21 days. Therefore it is imperative that your horse rest at least three weeks after a respiratory infection.
Some of the frequently diagnosed respiratory tract problems in horses include:
- Infections (e.g., equine herpesvirus, equine influenza, strangles, pneumonia, Rhodococcus equi infection in foals);
- Dorsal displacement of the soft palate (DDSP, when the soft palate moves abnormally in an upward direction so the end of the soft palate rests above, instead of below, the epiglottis);
- Epiglottic entrapment by the aryepiglottic fold in the larynx;
- Laryngeal hemiplegia (roaring, left laryngeal hemiplegia), caused by paralysis of the left arytenoid cartilage and vocal fold, resulting in a failure to achieve full abduction of these structures during respiration;
- Guttural pouch the guttural pouches are paired extensions of the Eustachian tubes that connect the pharynx to the middle ear. The guttural pouches can be infected with Strep. Equi and develop chondroids (hardened pus). Guttural pouch tympany (guttural pouch filled with air) and guttural pouch mycosis (fungal infections.
- Temporohyoid osteoarthropathy this is a progressive disease of the middle ear and components of the temporohyoid joint. The cause is thought to be an inner/middle ear infection that originates from the blood.
- Exercise-induced pulmonary hemorrhage (EIPH, due to small vessels rupturing in the lungs);
- Recurrent airway obstruction (RAO, heaves) and inflammatory airway disease (IAD).
- Rhinitis and sinusitis nasal airways can be infected with a variety of viral, bacterial, fungal and parasitic agents.
- Lymphoid pharyngeal hyperplasia this is a common condition involving the upper respiratory tract of 2 and 3 year old racehorses. Mild cases respond to rest, if the hyperplasia is due to influenza, herpes or S equi subsp. equi the horse will require a longer rest period.
- Arytenoid chondritis this condition usually begins as an infection and becomes a chronic inflammatory syndrome
- Lower respiratory tract infections include bacterial pneumonia, pulmonary abscesses, and pleuropneumonia (bronchopneumonia in which the infection has spread to involve the pleura and pleural space).
Diagnosis of respiratory ailments
One of the most important diagnostic tools for assessing the respiratory tract is the endoscope. Arizona Equine Medical and Surgical Centre veterinarians have both a one and three meter video endoscope. Our veterinarians pass the flexible scope through the nasal passages to the nasopharynx. We can then directly evaluate the soft palate, larynx, guttural pouches, and the trachea where many of the abnormalities are found (i.e., DDSP, epiglottic entrapment, roaring). Some horses require additional testing on a high-speed treadmill, video endoscopy, or an “overground endoscope” that remains in place while the horse trains. Endoscopy is also a primary diagnostic tool for EIPH, as it helps the veterinarian identify blood in the trachea that is coming from the lower airway.
Ultrasound, radiographs (x rays) are all important useful tools to diagnose respiratory tract disorders.
Respiratory conditions can affect any horse at any age. Vaccination for herpes and influenza are important for competition horses and horse that intermingle with other horses. For some conditions, such as roaring and EIPH, there are currently no known ways to avoid the problem. For other diseases, particularly RAO, owners are encouraged to consider the horse’s breathing zone—a 2-foot sphere around the horse’s nose from where he draws his breath—and minimize the amount of dust and debris in this zone. Housing environment is critical as well. Horses with IAD or RAO need as much pasture time and fresh air as possible. Also, remove the horse from the stable during clean out, use low-dust bedding, and use feeds with little dust.
Images used under creative commons license – commercial use (5/6/2016) Pete Markham (Flickr)