The official title for an all-too-common respiratory ailment in horses is chronic obstructive pulmonary disease (COPD). For the average horse owner, however, that rather cumbersome title has been reduced to a single word: heaves. Other people call it broken wind.
In this article, we'll stick with the acronym COPD. While there is some debate about the primary cause, it is the general consensus that COPD is caused by an exposure to dust, molds, and other air pollutants.
The reason for debate about primary cause is the puzzling fact that two horses might share the same environment and feed supply, with one suffering from COPD and the other showing no signs of the malady. Does this mean that heredity is involved? That some horses are predisposed to COPD and others are not? Additional long-term research will be necessary before those questions are answered with certainty.
What is known is that dust and mold, particularly, can bring on an episode of COPD. It is also known that unless the environment and/or feeding program are changed, the condition can become permanent and compromise the horse's ability to perform.
While there are some therapeutic approaches that can assist the horse with COPD, there is no cure. Nor is there a vaccine that can prevent COPD since it is a non-infectious disease.
Before we can discuss just what COPD is and how it functions in a horse, we first must understand how the equine respiratory system works.
The Respiratory System
Basically, the respiratory system for equines and other mammals functions to exchange oxygen and carbon dioxide. Oxygen enters the lungs during inspiration, and carbon dioxide exits with expiration. Oxygen is the fuel to be burned, and carbon dioxide is the exhaust to be removed.
Air enters through the nostrils, then passes through the nasal cavity. A horse differs from many mammals in that it does not normally breathe through its mouth. A dog in need of additional oxygen, for example, breathes rapidly or pants, with air being taken in through its open mouth. Because of this ability, a dog has very small nostrils. The horse, on the other hand, has large nostrils that can be flared even wider when the demand for oxygen increases.
There is another distinct difference between the horse and the dog, as well as some other mammals--the nasal cavity of the equine is quite long. This is of particular advantage in cold weather because it allows the inspired air to be warmed before it reaches the lungs.
After passing through the nasal cavity, air continues its pathway to the lungs by passing over the larynx and pharynx, then entering the trachea. Normally, there is a smooth, unimpeded passage of air from the nostrils into the lungs.
As with all mammals, there are defense mechanisms to keep unwanted matter from entering the lungs. When the horse swallows, for example, the pharynx and soft palate position themselves in such a way that food is directed into the esophagus rather than the trachea. The larynx, which houses the horse's vocal cords, also serves as a barrier that prevents food from moving into the trachea.
The 50-60 cartilaginous rings of the horse's trachea form incomplete hoops opening dorsally. Smooth muscle fibers of the tracheal muscle join the inner surface of the free ends of the cartilaginous rings. Extra cartilaginous plates at the foot of the trachea fill in the gaps between the free ends of the main rings. Irregular plates support the left and right principal bronchi going to the lungs.
At the end of the trachea is the bronchial tree. The conducting airways of the bronchial tree divide into smaller and smaller bronchi. When cartilaginous plates no longer are present in the walls of the smallest bronchi, the airway is termed a bronchiole. The bronchioles, in turn, join with the alveolar ducts that terminate in the functional units of the lungs where gas exchange actually occurs--the alveoli. The alveoli are small outpouchings along the walls of the alveolar sacs and alveolar ducts. It is through the walls of these tiny pouches that gas exchange actually takes place. The exchange occurs between air within alveoli and blood within capillaries in the alveolar walls.
The alveoli have very thin walls between oxygen-laden air in the lungs and the blood vessels that contain red blood cells carrying oxygen throughout the body.
Because of the thin structure of the walls, the carbon dioxide is able to move out of the blood, permeate the thin lining, and be expired. The alveolar structure allows the inspired oxygen to cross over and join with the red blood cells, where it is bound to hemoglobin for transport to the tissues.
When the blood leaves the lungs, it is saturated with oxygen. The oxygen is transported through the arterial system to tissues where it is "burned" in cell metabolism. The use of oxygen produces "exhaust"--carbon dioxide. Now the venous blood supply takes over. It picks up the carbon dioxide and returns it to the lungs, where it is expired.
How much oxygen is taken in and how much carbon dioxide is expired depend on how much the horse is exercising. When the horse is at rest, its respiration rate is very low, with the animal sometimes taking 10-14, or even fewer, breaths per minute.
However, when strenuous exercise is involved, the respiration rate increases dramatically as the lungs move into high gear to satisfy the demand for oxygen throughout the body. When the horse breathes while at rest, little effort is involved. That changes when it breathes rapidly in an effort to increase the oxygen supply. With that demand, the intercostal muscles and diaphragm are called on to expand the chest, which in turn allows for an expansion of the lungs. The lung expansion allows a greater quantity of air to flow in.
There are three terms that must be addressed at this point:
Tidal volume--The amount of air that is inspired and expired in a single breath.
Respiration rate--The number of breaths taken per minute.
Minute volume--The amount of air a horse inspires or expires in one minute.
During strenuous exercise, the respiratory system must increase the minute volume significantly. To increase the minute volume, both respiration rate and tidal volume must increase. This means, in essence, that the horse has to breathe more rapidly and deeply in order to increase minute volume.
Minute volume has been measured in liters of air expired per minute. It has been found that the horse at rest might expire as little as 150 liters per minute, while the horse being exercised at a fast gallop might expire as much as 1,500 liters per minute.
Nature has both helped and hindered the horse in its effort to increase the flow of oxygen and remove carbon dioxide. The horse is constructed in such a way that respiration is assisted by limb motion. This is especially true when the animal is traveling at a gallop. At this gait, the respiration rate and stride rate are normally one to one. In other words, at every stride, the horse takes a huge breath. Inspiration occurs when the forefeet are being extended, and expiration occurs when the forefeet strike the ground. This unique feature is of benefit because it reduces the workload of the respiratory muscles, thus reducing the potential for fatigue.
The bad news is that the number of strides taken in a given distance also limits the intake of oxygen. The horse will reach a point at which it simply is running at its maximum stride frequency and, thus, has reached its limit to inspire oxygen.
The normal horse can handle the oxygen demand when the workload produces a heart rate up to 180 beats per minute. When the horse is stressed to the point that the heart rates hits 200 and beyond, the respiratory system might be unable to deliver a sufficient quantity of oxygen for fuel.
At this point, the blood is no longer saturated with oxygen when it leaves the lungs. This lack of oxygen saturation is called arterial hypoxemia. It is a condition that has been recorded in racing Thoroughbreds, Quarter Horses, and Standardbreds.
With that as background, it is time to take an in-depth look at chronic obstructive pulmonary disease.
A group of researchers penned this description of COPD in an abstract printed in the British Veterinary Journal:
"Present evidence suggests that chronic obstructive pulmonary disease of horses is a delayed hypersensitivity response to inhaled antigens, particularly thermophilic (growing best in high temperatures) molds and actinomycetes (a form of fungus) that grow in damp hay.
"Within several hours of exposing COPD-susceptible horses to such hay, neutrophils invade the lung and accumulate in the lumens of airways, particularly bronchioles. The inflammatory response is accompanied by increased levels of histamine in broncho-alveolar lavage fluid, increased plasma levels of the inflammatory mediators thromboxane and 15-hydroxyeicosatetraenoic acid (15-HETE), and decreased production of prostaglandin by the airway mucosa.
"During acute exacerbations of COPD, airways exhibit non-specific hyper-responsiveness and become obstructed as a result of bronchospasm and the accumulation of mucus and exudates.
"Bronchospasm is due largely to activation of smooth muscle muscarinic receptors by acetylcholine (a substance normally present in many parts of the body which has important physiological functions). The diffuse airway obstruction leads to uneven distribution of ventilation, ventilation/perfusion mismatching, and hypoxemia. As a result of the increased respiratory drive caused by hypoxemia and the presence of airway obstruction, horses adopt a characteristic breathing strategy in which very high peak flows at the start of exhalation rapidly diminish as exhalation proceeds."
As stated in the final sentence of the above abstract, horses with COPD have difficulty in clearing their lungs of air before taking another breath. The use of abdominal muscles to assist in pushing air from the lungs results in the characteristic "heavey" look to afflicted horses.
A report on COPD from equine veterinarians at the University of Illinois in Urbana used layman's terms to describe the malady. First, they compared the disease to emphysema in humans. Then, the informative article, penned by Kimberly Meenen of the Veterinary Extension Department, had this to say:
"The causes of COPD in horses have not all been worked out, although allergies and chronic respiratory infections have been strongly implicated. Most likely, the disease begins with irritants such as dust and mold causing a hypersensitivity type of allergic reaction in the small airways of the lungs. Immune cells infiltrate the area, damaging the lining of the small airways of the lungs, and stimulating mucus production and bronchoconstriction. These constricted small airways of the lungs cause the horse to have to work harder in order to breathe, especially on expiration.
"Horses that have had this disease for a long time develop thickening of the abdominal muscles used on expiration, producing the characteristic 'heave line' on the sides of the horse's belly near the flank. Other signs seen in long-term cases include a white nasal discharge and a chronic cough not associated with sickness that is worse in the morning, while eating, or with exercise. Horses that are just developing COPD may only exhibit mild exercise intolerance or an intermittent cough.
"Since it is more commonly associated with stabled horses, it is believed that the disease has something to do with molds growing in bedding, feed, and on barn walls. COPD is traditionally seen in the late summer or fall due to high pollen counts. Also, horses may be worse when they are brought into tight, closed-up barns for the winter and fed hay rather than allowed to continue pasture-grazing. Other horses are more allergic to certain pollens present during spring and may be more severely affected at that time. Therefore, COPD is likely a year-round disease that becomes clinical when the particular allergen (or allergens) to which the horse is sensitive is around.
"Horses diagnosed with COPD don't recover from the disease or ever become truly cured, but they can be successfully managed."
That final sentence is ominous and places the burden of prevention on the horse owner.
Perhaps key in the prevention aspect is quality of hay. The horse is an herbivore, so hay is a main ingredient in its diet. About 65% of the horse's digestive capacity is in the lower gut, where large microbial populations aid in the digestion of fibrous food. For proper digestive tract function, it has been estimated, a horse must consume on a daily basis 1% of its body weight in long-stem dry matter. During winter months or in a stable environment, this means hay.
This is not to say that pelleted and cubed feeds are not available and, in some cases, appropriate. However, feeding hay or allowing the horse to graze freely on grass is much more what nature intended. In addition to providing the roughage a horse needs, hay also serves as something of a pacifier. Some veterinarians have observed that some horses are more apt to have wood chewing and stall weaving problems, to mention only two, if fed a non-hay diet.
That being said, it must also be pointed out that all hays aren't equal. Hay varies in nutrient values and quality. When one considers COPD, quality becomes of utmost importance, no matter what type of hay is being fed--legume or grass.
Many horse owners have found that alfalfa can be a high-quality forage for horses. However, as with grass hay, not all alfalfa is created equal. In order for it to be a nutritious, problem-free forage for horses, it must be properly cured.
This can be a problem in hot and humid states where it is difficult to cure the hay properly in the field before baling. If the hay is not properly cured, mold and dust are culprits that can be implicated in COPD.
The problem in hot and humid states is that the moisture content of the hay remains too high. It is ironic that in Kentucky, for example, it is difficult to get the moisture content low enough for baling, and in the mountain states, it is often difficult to have enough moisture on the hay at baling time to prevent the cured leaves from crumbling and falling off the stems. In those areas, balers often head into the fields in the middle of the night or early in the morning after a dew has fallen.
While dryness can be a problem in maintaining quality, the reverse problem--too much moisture--is of major concern when one is taking steps to prevent COPD. The basic rule of thumb is that hay should not be placed in bales when the moisture content is above 20%. However, hay can be baled with higher moisture content--up to 35%--providing one treats it with a preservative.
If the moisture content exceeds 20% and no preservatives are added, the hay will heat when packed tightly into a bale. The heating will reduce the quality of the hay and can result in moldy, dusty forage that carries with it a COPD threat. In a worst case scenario, baled moist hay sometimes can produce enough heat to stimulate spontaneous combustion and a disastrous barn fire.
If dusty and moldy hay is fed in a tightly enclosed manger, there is no way the horse can avoid inhaling the dust and spores of mold. Feeding hay on the ground lessens the inhalation problem, but does not eliminate it.
In many areas, hay is put into large round bales. These are very handy, especially if hay is the only ingredient in the diet and is fed free-choice. The challenge with round bales, even if the hay is put up properly, is to prevent mold from developing during storage. If the round bales are placed on the ground, there usually is enough moisture on the bottom part of the bale to stimulate the development of mold. If round bales are to be fed to horses, they should either be stored on a dry floor under a roof or should be wrapped with a protected covering.
If unprotected round bales are fed, it is wise to peel off the outer layer so that only green, leafy hay remains. A horse's eating habits also can cause potential COPD problems when round bales are used. Some horses seem literally to burrow into a round bale, burying their noses in the hay in an effort to get at the tastiest morsels. In so doing, they inevitably inhale particles of dust and debris that can bring on or, at least exacerbate, COPD. Even the cleanest of hay can harbor some mold spores.
Diagnosis And More Prevention
The ultimate approach for diagnosing COPD is an endoscopic examination, but there are other telltale signs that anyone can observe.
In the early stages of COPD, it is likely that the animal will cough during the beginning of the ride or during exercise. As the exercise continues, the cough diminishes.
In the early stages of the disease, the horse owner has a fighting chance to prevent it from becoming permanent. The first step, of course, is to remove the hay or other material that brought on the allergic reaction in the first place and replace it with material that is mold and dust-free. In many cases, turning a stabled horse out on green grass will help.
If one must feed hay that is suspect, water can be the best ally. However, simply sprinkling dusty hay with water won't do the trick. The hay should be completely immersed in water so that the mold is washed off and there is no dust.
The information from the University of Illinois adds this advice:
"For horses that become worse when stabled, leaving them outside year-round and providing them with a three-sided shelter for protection from the wind, rain, and cold seem to be the best therapy. If you must bring the horse indoors, a clean, dry barn is essential.
"Stalls must be cleaned regularly to keep ammonia levels down, because ammonia is toxic to the cells lining the respiratory tract. Peat moss, wood shavings, shredded paper, or synthetic bedding material may be more appropriate than straw, which can be full of mold. Also, establish an area away from hay and bedding. Hay stalls, lofts directly over horses, and even storage areas that share the same ventilation as the horse barn are all discouraged.
"Disinfecting walls and floors can decrease mold numbers. Raking, rather than sweeping, dirt floors as well as lightly spraying them down afterwards can decrease dust, another contributor to development and progression of the disease. Good ventilation should not be overlooked.
"Fans and windows in strategic locations can be inexpensive to install and are best in the long run for horses that are exhibiting signs of heaves, and for those that aren't. Consult an environmental engineer on how best to renovate an existing barn or how to build a new barn for maximal ventilation.
"Most horses will respond to these environmental and nutritional changes, but there are a few medications that can be prescribed by your veterinarian for those horses that do not respond."
The drugs available are not curative, but there are some that will provide at least temporary relief for the COPD-afflicted horse. Some of the drugs available are bronchodilators, which, true to their name, dilate the bronchial passages so that more air can flow into and out of the lungs. There also are mucolytic drugs (those that dissolve mucus) that can be helpful.
Corticosteroids, such as prednisone and dexamethasone, can be used to provide at least temporary relief.
An abstract in the British Veterinary Journal, printed in September 1997, reported on the use of ipratropium bromide, a bronchodilator, in treating horses with COPD. Involved in the research were five healthy horses and six which were suffering from COPD. The bronchodilator was administered in powder form with an adapted face mask. Pulmonary function tests were recorded before inhalation and 15 and 60 minutes after inhalation.
To induce airway obstruction in the six COPD horses, the animals were bedded down on straw and fed hay before the tests.
In the healthy horses, administration of the bronchodilator produced no effects on either respiration rate or tidal volume. However, there were changes in the afflicted horses.
According to the abstract, the horses were administered three different dosages of the medication. One dosage was 600 grams, another 1,200, and a third 2,400. The researchers reported that when the medication was administered at the 1,200-gram level, the COPD-afflicted horses showed positive responses in breathing at both the 15-minute and one-hour testing levels. Doubling the dosage produced additional positive effects, they reported, but they were not significant.
The research points out that there are effective medications on the market to facilitate temporary relief of COPD. However, it must be re-emphasized that there are no medications that can cure this disease.
In the most severe, chronic cases, there can be permanent structural changes in the alveolar wall and interstitial tissues.
COPD is definitely a case where an ounce of prevention is worth a pound of cure. Keeping the horses in a clean environment and feeding only green, dust- and mold-free hay can help prevent the disease from occurring.
About the Author
Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.