Managing Airway Disease
Researchers have proven that anomalies of the equine upper respiratory tract, such as left laryngeal hemiplegia (roaring) and dorsal displacement of the soft palate, negatively impact performance. But alterations in normal airway structure and function can occur at any point along the respiratory tract, even beyond the larynx, within the depths of the lung, or hidden beyond the probing tip of the veterinarian's endoscope. While heaves (recurrent airway obstruction, formerly known as chronic obstructive pulmonary disease), is a very familiar and severe condition of the lower respiratory tract in older horses, inflammatory airway disease (IAD)--a low-grade inflammatory syndrome of the airways in younger horses--is less commonly recognized.
Despite its apparently taciturn nature, IAD is thought to be a ubiquitous and important performance-limiting condition of the lower respiratory tract in the young and otherwise healthy athletic horse.
"While the exact prevalence of IAD in athletic horses is unknown, it is suspected that IAD is an important contributor to exercise intolerance and poor performance in horses," explains Susan Holcombe, VMD, PhD, Dipl. ACVS, ACVECC (emergency and critical care), an associate professor at Michigan State University's (MSU) College of Veterinary Medicine.
"Some studies have reported that up to 50% of racing Thoroughbreds and Standardbreds have IAD," she adds.
In this article we will look at why respiratory conditions such as IAD are important, how they are thought to affect performance, and diagnostic and management strategies for both noncompetitive and competitive horses.
Structure and Function of the Respiratory System
The equine respiratory system traditionally is divided into two segments. The upper respiratory tract extends from the nostrils to the top of the trachea, just past the larynx (voicebox), while the lower respiratory tract extends from the larynx, located at the back of the throat, down the neck and into the thorax. Within the thorax the trachea divides into two tubes, the chief bronchi, each leading to one of the two lungs. Within each lung, the chief bronchi further divide and subdivide, becoming narrower and narrower. At this point the airways are referred to as bronchi and bronchioles. The airways continue to branch within the lungs and ultimately end at the alveoli--microscopic air sacs located at the end of the bronchioles where gas exchange (respiration) occurs.
The oxygen in the inspired air diffuses into the bloodstream in the alveoli, and the carbon dioxide diffuses out of the bloodstream into the alveoli, where it is expired. The oxygenated blood in the lungs then circulates throughout the body.
How IAD Affects Performance
Ed Robinson, BVetMed, MRCVS, PhD, is the Matilda R. Wilson Chair in Large Animal Clinical Sciences at MSU and the director of the Equine Pulmonary Research Laboratory. Robinson presented TheHorse.com's free Webinar, "Managing Noninfectious Respiratory Problems," sponsored by Boehringer Ingelheim Vetmedica Inc., in June 2009.
According to Robinson, "An unobstructed flow of air to and from the alveoli where gas exchange occurs is imperative in athletic horses. To deliver enough oxygen to their working muscles, exercising horses have a respiratory rate as high as 120 breaths per minute and the flow of air through the nose can be as high as 90 liters per second. Therefore, any factor that impedes the delivery of oxygen to the alveoli can negatively impact performance."
In horses with IAD airways are inflamed (thickened), narrower than normal due to smooth muscle spasm, and lined with a thicker layer of mucus than a normal airway. These changes are thought to occur secondary to a number of inciting causes, such as allergy, inhalation of an excess number of small particles, or inhalation of endotoxin (a toxin produced by Gram-negative bacteria such as Escherichia coli). Together, these changes result in a decrease in movement of air to and from the alveoli.
The "Average" Horse
In some older horses (8-10 years or older), airway disease can easily be diagnosed based on history and physical examination findings alone. A classic finding in horses with severe airway disease (heaves) is evidence of airway obstruction even while at rest. Veterinarians can examine older horses with heaves via endoscopy and perform either a transtracheal wash or bronchoalveolar wash (BAL); however, in older nonathletic horses, these tests are usually not necessary.
Once a horse is diagnosed with airway disease, treatment goals are threefold: 1) improve the environment; 2) reduce airway inflammation; and 3) dilate the airways to reduce airway obstruction.
"Of these three goals, improving the horse's environment is the most important factor," explains Robinson.
Don't be deceived by the apparent simplicity of this recommendation. According to Robinson, controlling the horse's environment is easier said than done.
"It is critical for horse owners to focus on the horse's breathing zone--a two-foot sphere around the horse's nose from where he draws his breath," Robinson says.
"A variety of changes can greatly improve a horse's breathing zone," he advises. "Ideally, the horse should spend the majority or, preferably, all of its time outside the barn (ensuring the horse is protected from the elements). Also, rather than providing a round bale, scatter the hay on the ground. Not only does this reduce inhalation of particles and mold spores, but (it) also encourages the drainage of mucus from the airways during feeding."
Studies have shown these small changes can result in a remarkable improvement in the lung function of horses with IAD. Nonetheless, once a horse has airway inflammation, he will always be susceptible to future attacks. Therefore, acute flare-ups of IAD should be managed medically to further improve lung function.
To decrease inflammation in the â€¨airways, corticosteroid drugs are indicated for treating IAD rather than non-â€¨steroidal anti-inflammatory drugs (NSAIDs). Evidence suggests the use of NSAIDs in people can make airway disease worse. Corticosteroids, on the other hand, reduce inflammation, resolve bronchospasm, prevent permanent thickening of the airways, and clear up mucus. The corticosteroids of choice are oral prednisolone (not prednisone), dexamethasone, or the inhaled corticosteroid fluticasone.
In addition to their direct benefits, corticosteroids also improve the efficacy of bronchodilators, which make up the second recommended class of pharmaceutical agents for IAD. Of the available bronchodilators, the two most commonly administered drugs are albuterol and clenbuterol. Both dilate the airways by causing smooth muscle relaxation. These bronchodilators can be administered to horses 30 to 60 minutes prior to exercise to minimize cough and improve lung function.
Owners should be aware that while antihistamines are commonly included in products marketed for horses with IAD, there is no evidence that antihistamines improve airway function.
In athletic horses, the history and physical findings remain important tools in diagnosing IAD, but the advanced diagnostic testing--most commonly endoscopy and a BAL--are often indicated.
Holcombe explains, "The changes in the lungs of IAD-affected athletic horses can be subtle, resulting in few outward signs of disease; however, even these mild changes can have a big impact on performance."
In terms of diagnostic endoscopy, one goal is to rule out other causes of respiratory tract disease that can hamper performance in young, fit, athletic horses. Examples include dorsal displacement of the soft palate, epiglottic entrapment, and roaring (left recurrent laryngeal neuropathy), which are described on page 28, and also lymphoid hyperplasia (rapid growth of normal cells that resemble lymph tissue). A second goal is to identify and quantify the amount of mucus present in the lower airways, as mucus accumulation is associated with reduced performance.
"In a study published in the July 2006 edition of the Equine Veterinary Journal, our research group reported that moderate to severe tracheal mucus--Grade 2 to 4 out of 4--visible on endoscopy was a risk factor for poor racing performance," relays Holcombe. "Thus, endoscopy is a valuable tool for identifying poor performance attributable to a health issue in the respiratory tract in racehorses, and likely other sport horses, as well."
In addition to endoscopy, a veterinarian can use either a transtracheal wash or BAL to help identify horses with IAD. While the transtracheal wash is easier to perform than BAL, results of the transtracheal wash can be equivocal. This is because the isolated cells might not necessarily reflect what is happening in the lower airways, as the collected cells could be from either the lower or upper respiratory tract. To abrogate this concern, the veterinarian often performs a BAL instead. The BAL collects cells from the small airways deep in the lungs only (assuming the technique is performed appropriately using correct sterile technique). These cells are subsequently examined under a microscope.
"In horses with IAD, we see a high number of polymorphic mononuclear cells (PMNs), primarily the white blood cells called neutrophils, rather than macrophages," explains Robinson.
The number of neutrophils shows the severity of inflammation. The more neutrophils that are present in the BAL, the worse the airway disease is.
Treating IAD in athletic horses Management guidelines for IAD in athletic horses are similar to the recommendations described above: improve the environment, reduce airway inflammation, and dilate the airways to reduce airway obstruction.
The major hurdle for elite athletic horses is that they are frequently shipped to other barns for competitions, and controlling a horse's environment is not typically an option for most owners or trainers.
"For these horses, decrease dust and particle exposure as much as possible," says Robinson. "Use shavings for bedding, feed the horse from the floor to encourage mucus drainage, and supplement the diet with a complete, naturally low-dust pelleted feed to reduce the amount of hay required."
Likely one of the most challenging aspects of maintaining athletic horses with IAD is medication. Unlike owners of pleasure horses, owners of performance horses are required to adhere to strict drug withdrawal times for their animals. For example, the withdrawal times for albuterol and Ventipulmin (clenbuterol HCl) can be up to 72 hours prior to performance. Thus, Robinson's recommendation to administer these products approximately 30 minutes prior to exercising is simply not an option for many equine athletes.
"In these cases, you have to manage the horse to resolve the airway inflammation so that it doesn't need emergency medications when it is required to compete," says Robinson.
Holcombe agrees, adding, "There simply are no other therapies currently available."
IAD is a challenging disease to control in any horse, but it is more so in athletic horses. In that particular equine population, even very small changes in lung function can have a huge impact on performance. This poses an additional level of complexity that keeps veterinarians like Robinson and Holcombe on their toes.
Robinson concludes, "Understanding IAD is going to require large-scale epidemiological investigations so that we can more accurately understand its causes. It is likely that the horses we presently lump together as having IAD may have multiple causes of their problem that we don't understand at present. It is becoming clear from research being conducted in mice and humans that these types of respiratory problems involve a complex interaction of genetic and environmental factors. The long-term goal is to identify risk factors and genetic or other markers of susceptibility to IAD and to manage horses accordingly to prevent the development and impact of airway disease."
About the Author
Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.