Lower Airway Disease
Coughing, one of the most common signs of respiratory disease, can occur in response to irritation from viruses, bacteria, or inhaled environmental dusts and allergens. However, most veterinarians and researchers will agree that irritation from the dusts in feed and bedding is the most common cause of coughing. In the worst case, this irritation can lead to development of "heaves," formerly known as chronic obstructive pulmonary disease (COPD). This is one of the most common respiratory disorders of horses, particularly in temperate climates like those of North America and northern Europe.
In June 2000, an international group of leaders in equine lower airway disorders met to discuss the current understanding of equine chronic airway disease (including heaves). They set the stage for new research in this area over the coming decade. This article is an update on these important problems, with particular reference to the information presented at this international workshop (see Robinson 2001).
The term COPD is used in human medicine to describe a severe and progressive inflammatory lung disease that is particularly prevalent in smokers. Because there are some similarities between COPD in humans and heaves in horses, COPD was also used to describe the equine condition. However, research over 10-15 years has shown that the human condition and the equine condition are quite different; in fact, heaves in horses more closely resembles human asthma than human COPD.
Like human asthma, heaves is characterized by recurrent bouts of reversible narrowing (bronchospasm) and inflammation of the airways. A key feature is the reversible airway narrowing. With improvements to the horse's environment and use of bronchodilator drugs, the airway obstruction wanes, but it can recur when the horse is exposed to a dusty environment. Logically then, the preferred proper name for true heaves is recurrent airway obstruction or RAO.
Researchers and clinicians have recognized that there are other types of chronic airway diseases in horses for which a diagnosis of heaves (RAO) does not fit. Young athletic horses, for example, are commonly afflicted by lower airway inflammation that results in excess mucus within the lungs and poor exercise performance. It is possible that, over a period of years, this condition progresses to full-blown heaves (and therefore shares a common cause with heaves). However, although some researchers suspect a link between these two conditions, currently there is no evidence to support this contention. For the moment, this chronic respiratory disorder in young horses is termed inflammatory airway disease (IAD). The term small airway inflammatory disease (SAID) has also been applied in this same context.
The Clinical Picture
The clinical signs of RAO (and IAD/SAID) can vary greatly from horse to horse. As well, clinical signs in an individual horse can wax and wane. In general, signs usually worsen when horses are moved to an indoor (barn) environment, but improve if they are turned out to pasture. Climate also seems to play a big role. Affected horses tend to be worse in the spring and summer, particularly when it's hot. In some southern states, the term summer pasture-associated obstructive pulmonary disease is used to describe a heaves-like condition observed in horses when they are turned out to pasture. This is probably associated with exposure to respiratory allergens that occur in pastures.
The early signs of heaves can be quite subtle, including a mild nasal discharge (which can have a white, creamy appearance or be more yellow in color), a cough that is most notable during and after exercise, and perhaps a lack of vigor during exercise. The horse might cough repeatedly, particularly during feeding or other activities that can increase the amount of respiratory irritants in the horse's immediate environment (e.g., stall mucking and re-bedding). A horse might cough in a series of spasms, as if unable to "clear his throat." The rate and depth of breathing might be increased (the normal resting respiratory rate in adult horses is 12-20 breaths per minute).
Typically, there is a progressive worsening of clinical signs over a period of months to years. Importantly, however, appropriate treatment and prevention measures can slow or even halt the progression of illness.
In severely "heavey" horses, characteristic signs include frequent coughing, exercise intolerance, and labored breathing--almost like a horse recovering from a hard bout of exercise. Rather than the gentle chest motion associated with restful breathing, there is a marked increase in chest movement, nostril flaring, and a noticeable abdominal push during expiration. To compensate for the airway obstruction, the horse uses his abdominal muscles in an attempt to generate greater airflow. These abdominal muscles enlarge in response to the extra work, resulting in development of a characteristic "heave line." This extra effort can also take its toll on the horse's energy reserves--some horses with severe heaves will lose weight.
The signs of IAD/SAID are much more subtle and most often recognized in horses (e.g., racehorses, eventers, endurance horses) required to undertake strenuous athletic activities, where even minor changes in the efficiency of lung function can limit "top-end" performance. Some reduction in athletic performance is the first clue. Affected racehorses will finish races poorly; riders might complain that the horses are lazy or lethargic during work. Other riders might report that horses cough or toss their heads during "collection," perhaps because there is further obstruction to airflow with neck flexion. In general, the negative effects of IAD/SAID on performance are particularly noticeable in hot weather.
What Causes Heaves and IAD/SAID?
The underlying problem in heaves and IAD/SAID is airway inflammation caused by exposure to various irritants. The main offenders are the organic dusts present in feed and bedding--these dusts can contain molds, insect parts, bacteria, and endotoxins (a component of certain types of bacteria) among other substances, all of which can irritate the sensitive lining of the respiratory tract and trigger an inflammatory reaction. Ammonia (which can accumulate in poorly cleaned or badly ventilated barns), other gaseous pollutants, and pollens also can cause problems in some horses. It has been suggested that a viral infection can trigger susceptibility to heaves, but this hypothesis is untested.
What is more perplexing is why some horses develop problems, while other horses kept in the same environment do not. It is possible that genetic factors play a role in determining susceptibility to heaves, but more research is needed to clarify this issue.
Most of our horses live in a very dusty environment (e.g., box stalls, dry lots, dirt yards), meaning that they can be constantly challenged by these respiratory irritants. The dust concentration in the airspace around the horse's nose, called the breathing zone, is the critical factor. From there, the tiny irritants are inhaled into the deeper reaches of the lungs. It stands to reason that the greater the dust concentration of the breathing zone, the greater the contamination of the lower airways.
Research studies have shown that hay is the primary source of organic dusts (Woods et al. 1993). Horses fed hay will spend large periods of each day with their nose in the hay source, whether it be a hay net in a box stall or a large round bale in a paddock. That scenario provides ample opportunity for contamination of the lower airways with nasty irritants. The situation is much worse if the hay has heavy mold contamination. All of this means that removal of dry hay from the diet of horses with heaves and other chronic lower airway problems is of paramount importance.
The airways that make up the lung are not rigid structures. Their diameter is controlled by a layer of muscle. Inflammation leads to a constriction or tightening of this muscle, the result being a restriction of air flow through the lung. The inflammation also results in thickening of the airway walls and an increase in mucus production. All of these factors obstruct airflow and lead to the clinical signs of respiratory disease and poor exercise performance.
Making the Diagnosis
Step number one in the management of these chronic lung problems is confirming the diagnosis. In horses with severe heaves, the diagnosis is often based on clinical features (e.g., chronic cough that worsens when the horse is housed indoors) and lack of evidence of a bacterial or viral respiratory disease (e.g., no history of fever). On the other hand, the diagnosis of IAD/SAID and mild heaves requires some specialized tests, including endoscopic examination, bronchoalveolar lavage (BAL), and, in some cases, tests of lung function.
Visualization of the windpipe (trachea) with an endoscope is helpful for detection of excess mucus in the airways. It is sometimes helpful to perform this examination about 30 minutes after a bout of exercise, which tends to increase the amount of mucus in the trachea.
BAL involves the flushing of a sterile saline solution (usually 300-500 mL) into the lung, either through an endoscope or a specialized rubber tube, and recovering the fluid and any debris by suction. Currently, this test is the best method for assessment of inflammation in the lower airways. The sample is examined for the number and type of inflammatory cells. In horses with heaves, there is usually a large increase in the proportion of neutrophils (a type of white blood cell). Horses with IAD/SAID can have increased neutrophils or an increase in the number of mast cells (another immune cell that plays a key role in inflammatory responses).
Tests of lung function that measure the degree of airway obstruction or alterations in gas exchange can potentially provide the most sensitive measure of airway obstruction and disease severity, and future technological advancements in this area should improve the ability to detect the early stages of these diseases. Currently, however, lung function testing is restricted to a few veterinary referral centers. More widespread use awaits development of testing systems that can be used in the field.
Measurement of serum antibodies specific to respiratory irritants (e.g., molds) and skin testing procedures (tiny amounts of various allergens are injected into the skin on the neck) have been used to diagnose heaves and to determine whether a horse has "allergies." Unfortunately, these diagnostic tests are not very useful. Typically, both normal and heaves-prone horses will have "positive" blood or skin tests. Remember that all horses are exposed to the dusts, etc., and these test results simply reflect that exposure rather than a true allergy.
Fortunately, there are ways to provide relief from the airway obstruction that plagues horses with heaves and IAD/SAID, and in many cases allow their return to full athletic function. In some young horses, IAD/SAID appears to be due to repeated bacterial infections of the larger airways. As horses develop immunity to these organisms, the IAD/SAID is self-limiting.
By contrast, there is no cure for heaves (and, probably, IAD/SAID). Affected horses will remain prone to "heavey" episodes, particularly when we fail to minimize exposure to dust and other environmental pollutants. As well, realize that over time there can be permanent lung scarring and loss of function that is no longer reversible. This can be averted by recognition and proper management of these conditions as was stated early in this article.
The important components of treatment are drug therapy to relieve airway obstruction and inflammation, and environmental control to reduce the horse's exposure to the offending irritants and prevent recurrences.
Drug treatment must be directed by your veterinarian. In general, bronchodilator and corticosteroid (anti-inflammatory) drugs will be used. These can be given by mouth, by injection, or by aerosol (inhalation). The availability of systems for aerosol drug administration has greatly improved the safety and effectiveness of treatment by maximizing the amount of drug that reaches the airways while minimizing some of the undesirable side effects that can occur when these drugs are given by mouth or by injection.
Bronchodilator drugs are very effective for relaxing the muscle around airways and thus improving lung function, particularly in horses experiencing an acute "heaves" attack. Remember, though, that these drugs don't fix the underlying problem--airway inflammation--and most only provide short-term relief (one to four hours). In general, bronchodilators are used for the first two to three weeks of treatment, giving the horse breathing relief while other measures that deal with the inflammation are instituted (severely affected horses might require bronchodilator treatment indefinitely). Bronchodilators are sometimes used about 30-40 minutes before a conditioning session to allow the horse to train properly.
Corticosteroids are very effective at reducing airway inflammation. For mod-erate to severe cases, oral (prednisolone) or injectable (dexamethasone) steroids are given in an attempt to quickly dampen the inflammatory response. Possible side effects are a major concern with this type of corticosteroid treatment. These side effects include suppression of the immune system, laminitis, and Cushing's-like signs. To avoid these problems, your veterinarian will try to reduce the drug dose and frequency of administration as quickly as possible, depending on the speed of the horse's recovery.
These side effects are less likely with inhaled corticosteroids, and this method of treatment has also been shown to be effective in reducing airway inflammation in horses with heaves. For these reasons, it is recommended that the course of oral or injectable steroid therapy be kept to a minimum and be followed by use of inhaled steroids for longer-term prevention of the airway inflammation.
Response to drug treatment will be very poor unless efforts are made to "clean up" the horse's breathing zone. Keeping the horse at pasture, without exposure to hay and straw, is by far the most effective means of environmental control. In horses with signs of heaves, significant improvement in lung function can occur within three days of changing from a stable to a pasture environment (Jackson et al. 2000).
If the horse must be stabled, removing dry hay from the diet and keeping him in the best-ventilated area of the barn are the most important aspects of environmental control. In mild cases, soaking the hay prior to feeding is often effective in reducing dust levels in the horse's breathing zone. The hay must be thoroughly soaked, preferably by immersing the hay in a large tub of water for a 10-15 minute period. Dry areas in a poorly soaked hay portion can release enough mold spores to cause problems. This soaking process should be done no sooner than 20 to 30 minutes before feeding to avoid leaching of water-soluble nutrients from the hay.
For more severely affected horses, it is better to use an alternative forage source such as hay cubes or haylage, or to feed a complete pelleted diet. Haylage and silage are becoming more popular in the industry and are generally safe so long as a few precautions are taken. Do not use broken or damaged haylage/silage bags, and maintain freshness by using all of it within two to three days of opening; mold counts in haylage or silage can increase rapidly after exposure to air.
Straw bedding is the other major source of dust and mold spores in a barn. Shredded paper, wood shavings that are low in dust, rubber mats, or peat moss should be used instead. Recent studies have shown that clinical improvement occurs even when the feeding and bedding management adjustments are made only in the stall of the affected horse, so it is not essential to make changes throughout the barn (Jackson et al. 2000). However, improving the environment in the whole barn is even better. Ideally, affected horses should be turned outdoors during stall cleaning and re-bedding to minimize exposure to dust.
Finally, it should be stressed that even in the face of top-notch environmental controls, horses prone to heaves or IAD/SAID are at continual risk for occasional flare-ups that require drug treatments. Watch for the early, tell-tale signs (such as excessive coughing) and call your veterinarian as soon as possible so that therapy can be started without delay.
Hoffman, A.M. Inhaled medications and bronchodilator usage in the horse. Veterinary Clinics of North America: Equine Practice 13, 519-530, 1997.
Jackson, C.A.; Berney, C.; Jefcoat, A.M.; Robinson, N.E. Environment and prednisone interactions in the treatment of recurrent airway obstruction (heaves). Equine Veterinary Journal 32, 432-438, 2000.
Robinson, N.E. International workshop on equine chronic airway disease, Michigan State University, June 16-18, 2000. Equine Veterinary Journal 33, 5-19, 2001.
Woods, P.S.A.; Robinson, N.E.; Swanson, M.C.; Reed, C.E.; Broadstone, R.V.; Derksen, F.J. Airborne dust and aeroallergen concentration in a horse stabled under two different management systems. Equine Veterinary Journal 25, 208-213, 1993.
About the Author
Ray Geor, BVSc, PhD, Dipl. ACVIM, is professor and chairperson of Large Animal Clinical Sciences at the College of Veterinary Medicine at Michigan State University