Respiratory Disease in Horses

Various sources indicate that respiratory problems rank second in importance to lameness problems in limiting the performance of athletic horses. Surveys have shown that over 25% of equine veterinary calls are due to some sort of respiratory system problem. This results in a significant negative economic impact on the equine industry in the form of veterinary costs, lost time in training and showing, early retirement from performance, and even loss of life. This article on the equine respiratory system will be divided into two parts. Part one will begin with a general overview of equine respiratory system anatomy and physiology. Then the discussion will shift to the veterinary examination of the respiratory system and special diagnostic procedures that are used. In part two of the article, specific diseases of the lower respiratory tract will be covered, along with discussion of treatment and prevention.

Anatomy and Physiology
The respiratory system of the horse can be divided into an upper and a lower portion. The upper respiratory tract consists of the nostrils, nasal cavity, paranasal sinuses, pharynx, and guttural pouches. The lower respiratory tract begins with the trachea, which is the long tube that extends from the pharynx to the lungs. The trachea branches into the right and left lung lobes. The left side has only two lobes, the cranial and caudal. The right side has three lobes, the cranial, intermediate, and caudal. The branch of the lung that enters the right lung is more horizontal, which may predispose this side to more disorders. The bronchi continue to branch into smaller and smaller tubes, eventually reaching the very small branches called bronchioles. These bronchioles eventually lead to tiny air sacs called alveoli, where gas exchange between air and blood occurs. The equine lungs contain over 10 million air sacs.

The major function of the respiratory system is to exchange gases, oxygen from the air into the blood stream and carbon dioxide from the blood into exhaled air. This exchange of gases occurs in the air sacs. Inhaled air is warmed and humidified as it passes through the respiratory tree. The air is also filtered of airborne particles so that it is clean by the time it reaches the air sacs. This filtering action is a product of the mucociliary apparatus. The respiratory tract is lined with cells that produce mucus that trap particles from the inhaled air (mucus also helps protect the respiratory tract lining from dehydration). Tiny hair like projections called cilia also line the respiratory tract. Their wave like motion moves the mucus up and out of the lower respiratory tract at a rate of over a 1 1/2 cm per minute. Within 1 hour, 90% of the material trapped in the mucus is removed in the normal horse – a pretty good air filtration system. Various types of white blood cells are spread out throughout the lining of the respiratory tract. These white blood cells play an important role in removing bacteria and noxious agents and in signaling the immune system when infection threatens.

How much air does a horse breath? Horses are obligate nose breathers, they cannot breathe effectively through the mouth. They are highly adapted with a rigid upper airway to move large quantities of air. At rest a horse breathes 8 to 16 times per minute moving about 20 to 25 gallons of air per minute. In galloping horses, breathing and locomotion are coupled, or performed synchronously. With each stride there is one breath. Exhaling occurs as the horse lands on the front limbs. At a gallop, horses have between 110 and 150 strides per minute (and the same number of breaths per minute) and can move over 400 gallons of air per minute! Think of eight 50-gallon barrels lined up to get a picture of this volume. That’s a lot of air!

Examination of the respiratory system
An important aspect to the veterinary examination of the respiratory system is the history. What problems or symptoms have been observed? When did they begin? Are they getting better, worse, or staying the same? What is the vaccination history? Has there been exposure to other sick horses? General, non-specific symptoms of respiratory disease may include poor performance and exercise intolerance. With infectious diseases, there may be fever, dullness, and poor appetite. Signs more specific to the respiratory system include elevated respiratory rate, increased respiratory effort, nasal discharge, enlarged lymph nodes, and coughing.

An integral instrument in the examination is, of course, the stethoscope. The technical term for listening with a stethoscope is auscultation. Auscultation of the trachea might reveal rattle-like sounds of mucus and exudates fluttering about as air rushes in and out. With diseased lungs, abnormal sounds include wheezes as air moves through constricted airways and crackly sounds of fluttering mucus and exudate. Often it is helpful to the veterinarian to force the horse to take deeper breaths, so that abnormal sounds will be more easily heard. One way to force deeper breaths is to have the horse breath in a plastic bag. Another is to hold the horse’s nostrils closed for 30 seconds or so. Since horses only breath effectively through their nose, this essentially forces them to hold their breath. Once released, they will take several deep breaths, making abnormal sounds more audible. If there is inflammation in the airway, these deep breaths also may elicit a coughing response. Blood samples may be drawn to look for signs of inflammation, such as elevated white blood cell counts. If there is a nasal discharge, nasal or pharyngeal swabs may be used to collect samples for bacterial culture. Serology can be used on blood samples to test for viruses such as rhinopneumonitis (“rhino”).

Special procedures are sometimes indicated, helping to make an accurate diagnosis and to assist in prescribing treatment. Nasal washes are sometimes used to collect samples to perform highly sensitive PCR tests to identify the bacterium that causes strangles. An endoscopic examination is performed in some cases. In this procedure, a flexible endoscope is inserted through the nasal passage. This enables the veterinarian to visually examine the nasal passages, the pharynx, and even the trachea down to the level where it divides, looking for evidence of abnormalities and disease. A “trans tracheal aspirate” is a procedure used to collect samples of fluid from the lower trachea. In this procedure, a trocar is inserted through the skin and into the trachea on the underside of the horse’s neck. A long tube is fed through the trocar, into and down the trachea. Sterile saline is flushed into the tubing and then aspirated back out. The collected fluid is examined for cell types and cultured for bacteria. Isolated bacteria can then be tested in the laboratory to find out which antibiotics are most effective. Another procedure used to collect samples from even deeper in the respiratory tree is the “broncho-alveolar-lavage” (BAL). A very long tube is inserted through the nasal passage, down the trachea and large bronchi, and into the small bronchi. A small balloon at the tip of the tube can then be inflated. In this manner, fluid can be infused and aspirated out, similar to the tracheal wash. But in this case, the fluid collected is from much lower in the respiratory tree. BAL is very useful in diagnosing heaves. Ultrasound is another procedure used to evaluate the lungs of a horse. Placing the ultrasound probe on the skin surface between the ribs, the veterinarian can evaluate the lungs for excess fluid around the lungs, thickening of the lung lining, and lung consolidation. Radiographs, although helpful in evaluation foals, is difficult to perform in adult horses.

In part two of this series, the discussion will shift to diseases of the equine lower respiratory tract, including comment on treatment and prevention.

Special thanks to Boehringer-Ingelheim for illustrations in this article and to Ken West and Dr. Robert Stenbom from technical services.