Respiratory disease is second only to musculoskeletal problems as a cause of loss of performance in competition horses, according to some of the leading researchers around the world. In fact, N. Edward Robinson, BVet Med, PhD, MRCVS, of Michigan State University, who was recognized for his lifetime achievements in studying the equine respiratory system with a featured lecture at the 1997 American Association of Equine Practitioners meeting, said that airway inflammation coupled with mucus accumulation was found in 54% of horses in a Swiss study. Robinson said a British researcher reported on 300 horses referred to the Edinburgh Veterinary College for suspected respiratory problems. Of those, 55% were said to have chronic obstructive pulmonary disease (COPD), better known as heaves.
There are conflicting research reports as to the cause of COPD. For years, many equine researchers and veterinarians have been of the opinion that COPD can be compared to asthma in humans. They believe that one cause is an allergic reaction on the part of the horse's respiratory system to certain types of dust, mold, or other substances. However, a recent study by a team of researchers at Cornell University on COPD seemed to contridict that theory. The Cornell team, led by Dorothy Ainsworth, DVM, PhD, said that its study strongly indicates that COPD is not an allergic reaction.
"The original thinking," said Ainsworth, "was that heaves or 'allergic airway disease' was similar to human asthma. In that condition, there is a predominance of T-lymphocytes of the Th-2 variety, which produce cytokines, such as Interleukin 4, 5, and 10, that drive the allergic response. Another type of immune response is that classified as the Th-1 response, which has gamma interferon as the predominant cytokine."
In the study, Ainsworth said, the researchers sampled control horses and horses with heaves (or, as they referred to the condition, recurrent airway obstruction--RAO). Both groups of horses, she said, were examined three separate times to see if the peripheral blood lymphocytes and cells in the pulmonary fluids had a Th-1 or Th-2 biasing.
"There was no difference between the two groups, leading us to conclude that this is not an immune-mediated (allergic) disorder."
The team's finding, if it stands the test of additional scientific scrutiny, might not help to solve the problem of heaves in horses, but it could be a step toward better understanding of the condition by eliminating one of the potential causes of COPD.
Ainsworth said that she is of the opinion, in the wake of the research results, that COPD is a non-specific respiratory reaction.
Another study reported on earlier this year seems to contradict Ainsworth's findings. Jean-Pierre Lavoie of the University of Montreal cited several studies from around the world (including Ainsworth's) and came up with the following conclusion:
"Taken together, the studies cited [in this research paper] are suggestive, but not conclusive, that heaves is an allergic condition. The variable results between studies are likely to be due to methodological factors, disease definition, and sampling time. Clearly more studies are needed before the immunological events leading to the pulmonary inflammation in heaves are elucidated."
One thing remains certain, there are a great many horses in the world which suffer from COPD, allergic reaction or not.
Sizing It Up
The possibility of ruling out an allergic response as a cause of COPD still leaves confusion and debate in its wake concerning the primary cause of this disease process.
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. Many in the equine community felt that this was a strong indicator that some horses had an allergic response and others did not, just like some humans with asthma have an allergic reaction to dust while those without the malady might be unaffected.
Does this now mean that heredity is involved? That some horses, while not allergic, 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 (in particular) can bring on an episode of COPD. It also is known that unless the environment and/or feeding programs are changed, the condition can become very difficult to reverse and might compromise the horse's ability to perform.
While there are some therapeutic approaches than can assist the horse with COPD, there is no cure. Nor is there a vaccine that can prevent COPD because it is a non-infectious disease.
Before we can discuss just what COPD is and how it functions in a horse, we must understand how the equine respiratory system works.
How Horses Breathe
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 and passes through the nasal cavity. A horse differs from many mammals in that it is unable to breathe through its mouth. A dog or human in need of additional oxygen, for example, breathes rapidly or pants, with air being taken in through the 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.
After passing through the horse's nasal cavity, air continues its pathway to the lungs by passing over the larynx and pharynx and entering the trachea. Normally, there is a smooth, unimpeded passage of air from nostrils into 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 larynx.
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 are no longer 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 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 that carry 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, similarly, allows the inspired oxygen to cross over and join up with the red blood cells, where it is bound to hemoglobin for transport to the tissues.
How much oxygen is taken in and how much carbon dioxide is expired depend on the amount of exercise the horse is experiencing. When the horse is at rest, its respiration rate is very low, with the animal sometimes taking 10 to 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 at rest, little effort is involved. When it breathes rapidly in an effort to increase the oxygen supply, it is another matter. At that point, 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.
When COPD strikes, the above-described lower airway system is unable to function effectively.
"Airway obstruction or COPD," said Robinson, "is a result of bronco-spasm, mucus accumulation, and probably also inflammatory changes in the airway wall."
Endoscopic examination is an essential part of the evaluation of the horse's airways, Robinson reported. "Inflammation of the lower airway is indicated by varying amounts of secretions and exudates in the airway lumen. Horses with mild disease will have a small amount of mucus that may be white because it contains inflammatory cells. Horses with severe COPD may have copious mucopuruelexudate. In some cases, mucus may be thick and tenacious so that it plugs the peripheral airways."
While the endoscope is highly valuable in pinpointing the presence of COPD and determining its severity, the layman can quickly identify a horse which has a severe case of COPD or heaves. The afflicted horse will use its abdominal muscles to force air from the lungs in what appears to be a bellows effect. In time, the abdominal muscles thicken as a result of heavy use, and a characteristic "heave line" develops.
When asked to perform, such horses often will cough in an effort to clear the airways, and their breathing will become extremely labored.
While many horses with airway disease will present these same symptoms, Robinson said, some do not.
"Respiratory disease distress in the resting animal is a sign of severe airway obstruction. Under these conditions, the horse adopts a breathing strategy that allows it to exhale most of its tidal volume early in exhalation. As the airways become obstructed later in exhalation, the horse uses an abdominal push to force a small volume of air through the obstructed air passages. When judging the severity of respiratory distress, it is this change in breathing strategy that is noted by the clinician.
"Although in general the magnitude of airway obstruction is associated with the severity of clinical signs, some horses with quite severe airway obstructions will not adopt the characteristic breathing strategy. Consequently, there is not a particularly tight correlation between the magnitude of airway resistance and the clinician's ability to detect signs of airway distress."
Researchers at Tufts University have developed a method that is effective in detecting small airway disease in the early stages.
A mask is used to measure air flow into and out of the lungs, and to determine the amount of resistance that occurs when the horse breaths an aerosol of histamine. The approach is particularly valuable when a horse shows signs of diminishing performance, but no other outward manifestations that would indicate COPD.
While early detection is important and drug therapy can be helpful, prevention is perhaps the most important aspect in dealing with COPD. This means placing the horse in a dust-free environment.
"The dusts to which horses are exposed most commonly are those found in stables," said Robinson. "Even though horses may inhale dusts generated on the racetrack or in arenas, animals are exposed to these dusts for a matter of minutes per day, whereas they inhale stable dusts for many hours per day.
"Agricultural dusts contain a variety of materials that can be inhaled, including bacteria and bacterial endotoxins; animal-derived components, such as dander, hair, urine, and feces; parts of feed grains and plants; pollens; insect parts and feces; and fungal parts.
"The actual constituents of dust in a stable depend on what is being fed and what is used for bedding, when and how it was grown and harvested, and the conditions under which it was stored."
Good stable ventilation can not be over-emphasized.
"Overall dust levels in stables can be reduced by a low-dust management scheme," said Robinson. "Bedding horses on shredded paper or shavings will reduce dust exposure. Preferably, hay should not be stored in the same barn as the horses, and certainly not above the horse from where it can be dropped down onto the stalls to create the maximum dust levels.
"In large stables, it is wise to keep horses that are dust sensitive together so that one is not feeding hay next door to a horse that is on a pelleted diet," added Robinson. "Dampening aisles during the busy times and before sweeping also helps reduce dust levels."
Another key in the prevention of COPD is the quality of hay fed. 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 that a horse must consume 1% of its body weight in long-stem dry matter on a daily basis. 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; especially if the horse already has developed COPD.
Owners should remember 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 properly cure the hay in the field before baling. If the hay is not properly cured, mold and dust that grow in it are culprits that can be implicated in COPD.
The problem in the northeastern and north central states and even 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 western mountain states, it often is 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.
If dusty, moldy hay is fed in a tightly enclosed manger, there is no way the horse can avoid inhaling the dust and mold spores. Feeding problem hay on the ground and dampening it lessens the inhalation problem, but does not eliminate it.
The very way a horse consumes hay increases the dust problem, Robinson said. "When horses eat hay, they toss the material and release dusts. Under these conditions, the dust concentration in the breathing zone can be 30 to 40 times higher than a few feet away in the stall. When the point dust source is eliminated by feeding a pelleted diet, dust concentrations in the breathing zone decrease to 3% of those recorded when hay is fed."
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 problem 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 either should be stored on a dry floor under a roof, or should be wrapped with a protective 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 can also cause potential COPD problems when round bales are fed. Some horses seem to literally 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.
In the early stages of the disease, the horse owner has a fighting chance to prevent COPD from becoming permanent. The first step, of course, is to remove the hay or other material that brought on the inflammatory 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 have a positive effect.
In some parts of the country, particularly the south, grazing horses sometimes inhale mold spores in the grass that can set off an inflammatory reaction in the lower airway system.
There are drugs available that can aid a horse with COPD, but they are not curative. Some will provide at least temporary relief for the COPD-afflicted horse. Included are bronchodilators, which, true to their name, dilate the bronchial passages so that more air can flow in and out of the lungs.
A major assist in this department came in May of 1998, when the FDA approved the use of the clenbuterol-based Ventipulmin Syrup. The drug had been used successfully in some European countries and Canada, but wasn't approved for usage here until 1998. Ventipulmin can relax smooth muscles and relieve the spasms that constrict bronchial tubes.
There are mucolytic drugs (those that dissolve mucus) that can be helpful, but evidence to support their effectiveness is lacking. Corticosteroids such as prednisone and dexamethasone, can be used to provide temporary relief of inflammation that is at the root of the airway obstruction. However, they will not cure the disease and will need to be used each time the animal has a worsening of its respiratory problem.
However, in all cases, the word "temporary" is key. There is no cure.
COPD is definitely a case where an ounce of prevention is worth a pound of cure. Keeping horses in a clean environment and feeding only green, dust- and mold-free hay generally will prevent the disease from occurring, even in horses which might be predisposed to COPD.
Keen observation is key in preventing COPD. Noting even slight changes in performance levels or rate of breathing at rest and during exercise can provide a clue that a problem is developing, and immediate changes in diet or environment can have a positive result.
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.
POLL: Visits from the Vet