Recurrent Airway Obstruction (Heaves)

Summary

Recurrent airway obstruction (RAO) is an inflammatory, obstructive airway disease that becomes clinically evident in middle-aged horses. Attacks of airway obstruction are induced by exposure of susceptible animals to organic dust (typically hay dust). Following dust exposure, there is a massive influx of neutrophils into the airways. This is accompanied by bronchospasm and mucus accumulation. The obstruction and inflammation resolve when dust exposure is eliminated. Affected horses are susceptible to recurrent bouts of obstruction throughout their lives and therefore need to be carefully managed. RAO-affected horses must be maintained in a low-dust environment, fed low-dust feeds and bedded on low-dust bedding. Administration of corticosteroids, either systemically or by inhalation, will reduce inflammation and airway obstruction. Bronchodilator drugs should be used as needed to relieve respiratory distress.

Introduction

Recurrent airway obstruction (RAO) is an inflammatory, obstructive airway disease that becomes clinically evident in middle-aged horses. The disease, also known as heaves, is most prevalent in the northern hemisphere where horses are stabled for large parts of their lives and are fed hay [1]. A similar syndrome, summer pasture-associated obstructive pulmonary disease (SPAOD) occurs in the southeastern United States, Britain and California in horses that are kept on pasture when the weather is warm and humid [2,3]. Evidence to date suggests that the two syndromes are the same disease but with different initiating factors. Recurrent airway obstruction was formerly known as chronic obstructive pulmonary disease (COPD). However, because of differences between equine and human COPD, a recent workshop recommended that the terms RAO or heaves, rather than COPD, be used for the horse disease [4].

Recurrent airway obstruction is initiated by the inhalation of organic dusts. The most common source of such dusts is hay and bedding [5-7]. Summer pasture-associated obstructive pulmonary disease is most likely a result of inhalation of organic dusts occurring in pastures in hot humid climates [2]. Organic dusts contain a variety of components that can initiate lung inflammation. These include specific allergens, endotoxin, components of molds such as beta-glucan, and small particulates. Elevated levels of specific IgE in bronchoalveolar lavage fluid (BALF) [8] favor the hypothesis that RAO is initiated by an allergic response to thermophilic molds and actinomycetes such as Faenia rectivirgula. A similar increase in specific IgE could not be documented in horses with SPAOD [9]. More recent studies examining the cytokine responses in both RAO and SPAOD lean toward a TH2 (allergic) immune response [10]. However, it has proved impossible to induce the heaves syndrome simply by administration of molds to susceptible horses [11-13]. Hay and stable dust contains endotoxin [7] and it is known from work in other species that administration of endotoxin into the airways can also induce many of the changes typical of RAO such as neutrophilic inflammation and mucus hypersecretion [14]. Small particles and fibers landing on the surface of the epithelium also can initiate the release of pro-inflammatory cytokines such as IL-8 [15,16]. Because organic dusts contain such a mixture of materials, RAO and SPAOD probably are initiated by several of these factors acting in concert. A genetic susceptibility to these diseases is suggested by the observation that many horses are housed without apparent problems in environments that can provoke airway obstruction in a RAO susceptible horse. Evidence in support of such a genetic component does exist [17].

Pathophysiology

When a horse with a history of RAO is moved from pasture to a stable where it is fed hay, airway inflammation develops [18,19]. Under the influence of IL-8 [20,21] perhaps leukotriene B4, and ICAM-1 [22], neutrophils accumulate in the lung and invade the airway lumen within six to eight hours [23]. Concurrently, airway obstruction develops. This obstruction is a result of bronchospasm, mucus accumulation, and inflammatory changes in the wall of the airway [1]. Inflammatory mediators acting on cholinergic nerves and airway smooth muscle are responsible for bronchospasm [24-27]. Mucus accumulation is due to augmented mucus production and increased mucus viscoelasticity which impairs mucus clearance [28]. Changes in glycosylation of mucus may contribute to its reduced clearability [29]. Inflammatory changes in the airway wall occur both acutely (edema) and chronically (airway wall remodeling). Airway wall remodeling, including mucus metaplasia, smooth muscle hypertrophy, peribronchial fibrosis, and peribronchial inflammation, is responsible for the chronic, intractable development of airway obstruction in RAO [30,31]. This remodeling likely is a result of the repeated bouts of airway inflammation and the accompanying release of proteases [32,33] and other mediators that occur in the RAO-susceptible animal.

One of the characteristic features of horses with RAO is increased non-specific airway hyperresponsiveness [34,35]. This means that airways of RAO-affected horses narrow in an exaggerated fashion in response to a wide variety of stimuli including neurotransmitters [36], inflammatory mediators such as histamine [34,35,37], and non-specific stimuli such as citric acid [36]. The airway hyperresponsiveness is most pronounced during acute exacerbations of RAO when inflammation is most severe and hyperresponsiveness wanes when animals are out at pasture [34,36] and inflammation is less severe. Even quite brief exposure of a RAO-susceptible horse to a stable environment can induce hyperresponsiveness that persists for several days [38]. The causes of the hyperresponsiveness include airway wall thickening, smooth muscle hypertrophy, a reduction in some of the inhibitory mechanisms that limit smooth muscle contraction [39,40] and actions of inflammatory mediators on cholinergic nerves and smooth muscle to facilitate smooth muscle contraction [25,26]. Clinically, airway hyperresponsiveness is important because it means that RAO-susceptible horses are prone to develop bronchospasm in response to levels of stimuli that would not affect a normal horse. Reducing the level of airway inflammation best controls hyperresponsiveness.

Many apparently normal horses develop low levels of airway inflammation when housed in the environment that causes a massive influx of neutrophils into the airways on RAO susceptible animals [41,42]. The reasons for the up-regulation of the inflammatory response following a dust challenge and the persistence of inflammation when RAO susceptible horses are returned to pasture are under active investigation. There is evidence for depletion of endogenous antioxidants in the airways of RAO susceptible animals [43,44] and for prolonged activation of NFkB, a transcription factor that initiates the production of many pro-inflammatory cytokines [22]. Activation of NFkB may be due to a positive feedback loop involving the persistent production of TNFa and IL-1b by neutrophils [45]. Inflammation also persists because apoptosis of neutrophils is delayed [46].

Because of the diffuse obstruction of the peripheral airways, horses with RAO have a mismatching of ventilation and blood flow that leads to inefficient gas exchange and hypoxemia [47]. In order to compensate for the poor gas exchange, RAO-affected horses increase their minute ventilation by increasing respiratory rate [47,48]. Tidal volume does not change. Inhaling the same tidal volume in less time requires that the horse with RAO develop a higher mean airflow rate in the face of airway obstruction [47-49]. This is why the horse adopts the breathing pattern characteristic of heaves.

Pathology

Recurrent airway obstruction is classically described as a bronchiolitis but both functional and pathological changes can also be observed in the larger airways [30,39,40]. In the bronchioles, there is plugging by mucus and accumulation of neutrophils in the airway lumen. Other inflammatory cells including lymphocytes, monocytes, and occasionally eosinophils are found in large numbers in peribronchial connective tissue. There is mucus metaplasia in the bronchiolar epithelium, thickening of the airway smooth muscle, mucus flooding of adjacent alveoli, and peribronchial fibrosis [31,50]. All these changes provide evidence of chronic inflammation. The pathological changes in SPAOD and RAO are similar [51]. The older term for RAO was pulmonary emphysema and this term is still used in some parts of the world. Even though the lungs are usually hyperinflated on post-mortem examination, hyperinflation is a result of gas trapping which is a consequence of peripheral airway obstruction rather than being due to alveolar emphysema.

History

In both RAO and SPAOD, the environment in which the horse is being kept affects the onset of clinical signs. Typically, both RAO and SPAOD develop in horses 7 years of age or older. RAO occurs in animals that have spent a considerable period of their lives in a stable where they are fed hay. The clinical signs become less severe when the horse is put out to pasture but, even there, acute bouts of respiratory distress can occur. This is especially true when the weather is hot or when pastures dry out and become dusty. Clinical signs of SPAOD occur in summer in horses on pasture and the problem recurs each summer but resolves in winter.

Clinical signs of RAO and SPAOD are similar. The first noticed by the owner is usually a cough. In RAO, coughing typically occurs when dust levels are increased, e.g., during feeding and cleaning out, or when the horse begins to exercise. At the same time, depending on the level of performance expected from the animal, owners might notice a decreased exercise tolerance. As the disease advances, the owner may describe prolonged recovery from exercise, some nasal discharge, and occasional bouts of respiratory distress exhibited as an abdominal effort during breathing or nasal flaring that is inappropriate for the level of activity. Once the disease becomes severe, respiratory distress may be present at all times especially if the horse is stabled and most horse owners can recognize these severe signs of heaves.

Clinical Signs

A horse with severe RAO or SPAOD is easily recognized by its signs of respiratory distress. The nostrils are flared, respiratory rate is increased, the horse uses its abdomen to assist expiration, and it often appears anxious. Abdominal effort can be so marked that the horse many rock to and fro during breathing. If respiratory distress is very severe, the horse may be unable to eat adequately and therefore loses weight. The horse may have a nasal discharge. Clinical signs in the less severely affected animal include coughing associated with activity or during feeding and cleaning out, reduced exercise tolerance and delayed recovery from exercise.

On physical examination, clinical signs are restricted to the respiratory system. The nostrils may be flared and there may be a milky mucus discharge from the nose. Compression of the cranial trachea may reveal an increased sensitivity of the cough reflex. Depending on the severity of airway obstruction, the horse may use its abdominal muscle for exhalation to an exaggerated degree and, if the animal has had respiratory distress for some time, a heave line may be obvious. The heave line is due to hypertrophy of the external abdominal oblique muscle.

Abnormal lung sounds are heard to varying degrees depending on the severity of airway obstruction. In some severely affected animals, the lungs can be quite silent despite very strong inspiratory and expiratory efforts. This is because the airways are so obstructed that there is insufficient air movement to generate audible breath sounds. Usually however, breath sounds are increased at all levels of the airways but particularly over the peripheral lung fields. Wheezing is heard quite commonly but it is wise to listen for several breaths at many points over the lung because wheezing can be intermittent. Wheezes referred from deeper in the lung may be heard over the trachea and sometimes simply by listening at the nostrils. In horses that are less severely affected, ventilation may have to be increased by the use of a rebreathing bag or exercise in order to hear abnormal lung sounds. Percussion will reveal increased size of the lung fields in severely affected animals.

Diagnostic Procedures

The complete blood count (CBC) and routine blood chemistry screen are within normal limits in most horses with RAO and SPAOD. Measurement of blood gases can be used to evaluate the magnitude of gas exchange compromise and the response to treatment. Depending on the severity of airway obstruction, PaO2 will be depressed to varying degrees but PaCO2 is normal or only slightly elevated. The magnitude of gas exchange abnormality correlates with the severity of bronchiolitis and clinical signs [47]. An increase in PaO2 should be expected in response to treatment.

The severity of lung inflammation can be evaluated by cytological evaluation of bronchoalveolar lavage fluid (BALF; see chapter by Viel and Hewson for lavage and cytology techniques). In normal horses, lymphocytes and macrophages form the majority of cells in BALF and neutrophils comprise less than ten percent of cells. In horses with RAO or SPAOD, there is an increase in the percentage of neutrophils and, in severely affected animals, neutrophils comprise over 50 percent of cells and are not degenerate. Despite the large number of neutrophils in BALF, there is no evidence of bacterial infection.

Aspiration of tracheal mucus or a tracheal lavage can also be used to evaluate lung inflammation but it is less reliable than BALF. Because there can be increased numbers of neutrophils in the tracheal wash but not in BALF [52], it is wiser to base evaluation of peripheral lung inflammation on the cytology of BALF. Presumably increased numbers of neutrophils in the tracheal secretions reflect local tracheal inflammation that does not extend deeper into the lung. Mixed populations of bacteria are common in a tracheal wash and usually are of no significance.

Radiographs of the lung are useful to rule out other types of lung disease but radiographic changes are not pathognomonic for RAO or SPAOD. There can be increased bronchovascular and interstitial changes and sometimes lung hyperinflation.

Reduction of respiratory distress after administration of a bronchodilator confirms the presence of bronchospasm, the major cause of airway obstruction in heaves. Intravenous atropine (0. 02 mg/kg) should relieve respiratory distress within 15 minutes in a horse with RAO or SPAOD. A single atropine dose is safe, but the dose should not be repeated or there is a risk of intestinal stasis. In rare cases, horses with chronic interstitial pulmonary disease will present with classical signs of RAO but these animals will not respond to a bronchodilator.

Lung function tests such as measurement of the maximal change in pleural pressure during tidal breathing (DPplmax), pulmonary resistance (RL) and dynamic lung compliance (Cdyn) can be used to document the severity of airway obstruction and to follow the response to treatment [48,53,54]. However, the variability in these function tests makes them of little value for confirmation of diagnosis unless the animal is so severely affected that the confirmation is unnecessary [48]. Function tests such as nitrogen washout [55], capnography [56,57], and scintigraphy [58], that can detect early disease and regional airway obstruction may be of more value for evaluation of less severe disease. Of these, scintigraphy offers the most hope because it can detect abnormalities of lung function when conventional tests do not [58].

Diagnosis

The diagnosis of RAO or SPAOD is based on the history and typical clinical signs. Other chronic pulmonary diseases that may cause similar clinical signs include chronic interstitial lung diseases that lead to lung fibrosis or diffuse granuloma formation [59]. These and chronic pneumonia or pleuritis generally can be ruled out by radiographic examination. If lung disease is diffuse and the diagnosis is still in doubt, a lung biopsy can be safely obtained via thoracoscopy [60,61].

Management and Prevention

Management and prevention of RAO and SPAOD involves three principles, environmental control, use of corticosteroids to reduce inflammation, and administration of bronchodilator drugs to relieve respiratory distress. The same principles must be applied at all stages of the disease from the horse with a chronic cough to the animal with obvious severe respiratory distress. In addition, it may also be useful to assist in the removal of mucus from the airways.

Environmental Control

Involves reduction or elimination of dust exposure. In most horses with RAO, the principal source of dust is from hay and bedding and those should be tackled first [5-7]. It is vital to realize that many RAO-affected horses are exquisitely sensitive to the agents that provoke airway inflammation. Many horse owners insist that they are keeping the horse outdoors but, when questioned more intensely, they will admit to bringing the horse in during inclement weather, overnight, or for grooming. In an RAO-susceptible horse, a few minutes contact with hay may be sufficient to induce attacks of coughing and heaves that lasts for days [38].

Several types of management change are effective in reducing the clinical signs and airway obstruction of RAO. Green pasture is the best [62,63]. Horses must remain out of doors at all times and receive a complete pelleted diet if there is insufficient grass. Where winters are cold, horse owners are reluctant to keep their horse outdoors year-round. Horses do well outside with temperatures as low as -30ºC, as long as they have shelter from wind and precipitation. Horses do not need to be kept in a warm building to stay healthy.

If pasture is not available, other means can be used to reduce exposure to dust. Traditionally, hay has been sprinkled or soaked with water [64,65]. Studies in progress in our laboratory are comparing dry hay with hay soaked for 2 hours for the management of RAO. Dry hay makes lung function worse whereas soaked hay prevents the worsening of lung function but does not result in improvement. Sprinkling water on the surface of the hay is unlikely to be very effective because it will not soak into the center and will rapidly evaporate. Grass silage is very effective in maintaining normal airway function [66,67]. However, RAO-affected horses fed grass silage and kept in a stable still have airway hyperresponsiveness, which suggests that the airway inflammation is not totally resolved.

If a RAO-affected horse is stabled with other animals that are being fed hay, it is still beneficial to provide the affected animal with silage, or a complete cubed [68] or pelleted [63] diet. This greatly improves lung function within a few days but there is still some residual airway obstruction [63] (Fig. 1). The improvement in lung function provided by silage or a pelleted diet may reduce the dose of corticosteroids or bronchodilator necessary to return the horse to useful function.
Figure 1. Effect of feeding pelleted diet and bedding on shavings on the effort of breathing (DPplmax) in heaves-affected horses. The heaves-affected animal was stabled with three other horses. The environment and diet in the stall of the heaves-affected animal was changed, management of horses in the three other stalls remained unchanged (hay and straw). Lung function improved significantly by day 3 in the heaves-affected animal and reached a plateau by day 7. Administration of atropine (0. 02 mg/kg IV) on days 7 and 14 resulted in a further improvement in lung function. This demonstrates that there is still some bronchospasm that remains after the environmental modification has caused an improvement in lung function (From: Jackson et al., [62]).

In the horse with severe RAO or SPAOD, initial improvements in lung function resulting from a change in management may not be immediately clinically obvious. Persistence with environmental management is essential. Horse owners will often ask about the dust content of various feeds and bedding. Respirable dust content and mold spores of common feedstuffs and bedding are presented in Table 1 [6]. New unpublished information indicates that cardboard bedding and specially processed wood shavings are very low in dust (Lekeux, personal communication, 2001). Rolled oats are surprisingly dusty but adding molasses dramatically decreases the amount of dust. More research is necessary to determine if it is the overall level of inhaled dust or the levels of specific dust components that are important in the initiation of airway inflammation in heaves.

The management of the horse with SPAOD requires removal from the offending pasture and stabling the horse in a cool stall with clean hay or pellets for feed. In California, a form of SPAOD is observed in horses eating alfalfa cubes. These horses are treated by changing to another form of forage.

Anti-inflammatory Drugs

They are the second line of treatment for RAO. Nonsteroidal anti-inflammatory drugs have no value [69] and may even be contraindicated. Non-steroidal drugs decrease the production of prostaglandin E2 (PGE2), a good prostanoid that inhibits inflammation and prevents bronchospasm. Levels of PGE2 are elevated in BALF of horses with both RAO and SPAOD [70,71].

The anti-inflammatory drugs of choice for treatment of RAO are corticosteroids. They inactivate NFkB, thus blocking transcription of genes encoding for pro-inflammatory cytokines, reduce the production of pro-inflammatory eicosanoids, and prevent down regulation of b2-adrenoceptors [72]. Corticosteroids can be administered either systemically or by inhalation. Systemic administration is easy, particularly if a drug is given by mouth, but side effects are more likely to occur. Topical administration of corticosteroids onto the airway epithelium is accomplished by inhaling the drug that is delivered from a metered dose aerosol canister. This puts the drug where it is needed with less risk of side effects.

Most systemically administered corticosteroids are highly effective for the treatment of RAO (Table 2). Dexamethasone (0. 1 mg/kg, IV) dramatically improves lung function within 3 days and, after 7 days, lung function is as good as that of the horse kept on pasture [73,74] ((Fig. 2). This high dose of dexamethasone also reduces the number of neutrophils in BALF concurrent with the improvement in lung function.

Figure 2. Effect of dexamethasone treatment (0. 1 mg/kg q24h) on effort of breathing (DPplmax) in heaves-affected horses. Dexamethasone significantly improved lung function within three days of treatment and improvement continued until Day 7. (From Robinson et al., [73]).

* Dose determined with a highly effective inhaler not yet available on the market. Higher doses may be necessary with the Aeromask.

Lower doses of dexamethasone (e.g., dexamethasone 21-isonicotinate 0. 04 mg/kg IM) improve lung function but have a lesser effect on the inflammatory cell population in the airways [73]. When administered by mouth, dexamethasone is approximately 50 percent bio-available [74] so oral doses of 0. 1 to 0. 2 mg/kg are appropriate to rapidly improve airway function in a horse with severe RAO.

Prednisone tablets (1 mg/kg SID) are ineffective in the prevention or treatment of RAO [63,73,76]. To be effective prednisone must be absorbed and converted by the liver to prednisolone. After administration of prednisone tablets to horses, little prednisone appears in blood and, in most horses, concentrations of prednisolone never exceed the limit of detection [77]. This lack of prednisolone explains why prednisone tablets are relatively ineffective. In contrast to prednisone, orally administered prednisolone is 50 percent bio-available. There have been no clinical trials on the efficacy of prednisolone for treatment of heaves but based on its bioavailability, oral prednisolone (1 mg/kg) should be highly effective. The other corticosteroid that has been clinically tested is triamcinolone [78]. A single dose (0. 09 mg/kg IM) improves lung function for up to three weeks.

While the efficacy of short courses and high doses of corticosteroids has been demonstrated for the treatment of RAO, there have been no investigations of long-term efficacy and systemic side effects of corticosteroids. Traditionally, the dose of corticosteroids has been reduced progressively once control of the disease has been achieved. It is optimal to maintain horses on alternate-day therapy in order to avoid adrenal suppression and other side effects of corticosteroids.

Treatment with systemic corticosteroids tends to be the last resort in horses with RAO. In human medicine this was also true until the development of inhaled topically active corticosteroids. With the availability of these drugs, corticosteroids are now used much earlier in the treatment of asthma [79-82]. This is because corticosteroids prevent the remodeling of the airways, that is, airway smooth muscle thickening and epithelial metaplasia. It is thought that prevention of remodeling is important in maintaining the long-term health of the lung. There are no data on the effect of corticosteroids on the airway remodeling in horses but it is probably useful to use corticosteroids early in the treatment of RAO. For example, once a history of a horse with RAO is known, corticosteroids can be used to prevent acute bouts of airway obstruction. If, for example, a horse always develops airway obstruction during spring, it should be treated with a corticosteroid during that season to prevent airway inflammation.

Systemic corticosteroids are no longer widely used for the treatment of human airway obstructive diseases. The development of topically active corticosteroids that can be inhaled into the lung has revolutionized the treatment of asthma [79,82,83]. These topically active corticosteroids, such as beclomethasone dipropionate, fluticasone propionate, budesonide, and triamcinolone acetonide, are available in metered dose inhalers or as inhaled powders. Most of these drugs are metabolized to an inactive form on first pass through the liver so that any drug that is swallowed or absorbed from the lung is rapidly inactivated. The Equine Aeromask [84] and Equine Haler are available for delivery of drugs from metered dose canisters to horses. These systems consist of a mask that is attached to a spacer and the metered dose canister. A spacer provides a reservoir to hold the aerosol until next inhalation. It also improves the quality of the aerosol by allowing larger particles to sediment out so that the remaining particles are all of the size that can be inhaled deep into the lung. With these devices, any inhaled corticosteroid available in a metered dose canister can be delivered to the horse. Inhaled corticosteroids need to be administered twice-daily and once treatment is ended, their effect rapidly wears off.

It is not useful to initially use a topically active corticosteroid for the treatment of a horse with severe RAO. The severity of the airway obstruction prevents the deposition of the inhaled steroid into the peripheral airways, which are the major site of inflammation. There are several ways to administer corticosteroids to such a horse. Intravenous or oral administration of dexamethasone avoids the obstructed airways and delivers the corticosteroid throughout the lung. Once a horse has responded to this initial dose of dexamethasone it should be possible to adequately deliver the inhaled corticosteroid. Alternatively, the horse can be treated with the bronchodilator drug such as intravenous atropine or an inhaled b2-adrenergic agonist 15 minutes before administering the inhaled corticosteroid [85]. The bronchodilator opens the airways and allows adequate distribution of the inhaled steroid.

The inhaled corticosteroid that has been most extensively investigated for use in the horse is beclomethasone dipropionate. When used with the Equine Aeromask, doses of 3,750 micrograms q12h have been used with variable effect [86]. Smaller doses of beclomethasone have been used successfully with a newer equine inhaler developed by 3M Corp. [74,87,88]. Like most inhaled corticosteroid [85]. The bronchodilator opens the airways and allows adequate distribution of the inhaled steroid.

The inhaled corticosteroid that has been most extensively investigated for use in the horse is beclomethasone dipropionate. When used with the Equine Aeromask, doses of 3,750 micrograms q12h have been used with variable effect [86]. Smaller doses of beclomethasone have been used successfully with a newer equine inhaler developed by 3M Corp. [74,87,88]. Like most inhaled corticosteroids used in human medicine [82,89-92], beclomethasone dipropionate causes some suppression of serum cortisol concentration but this is quite limited at the low doses that can inhibit airway inflammation [88]. Fluticasone propionate also is effective for treatment of RAO with less cortisol suppression [93].

Bronchodilator Drugs

Relax airway smooth muscle and thereby relieve some of the respiratory distress experienced by the horse with heaves. Because bronchodilators do not treat the airway inflammation, they are essentially rescue medications. For this reason, they need to be available at all times and horse owners need to be instructed to administer the bronchodilator whenever the horse is in respiratory distress or before using the horse for exercise.

There are three classes of bronchodilator drugs that are used in equine medicine (Table 3): anticholinergics, b2-adrenergic agonists, and methylxanthines [24,94-97].

* Dose determined with a highly effective inhaler not yet available on the market. Higher doses may be necessary with the Aeromask.

Anticholinergic drugs block the binding of acetylcholine to the muscarinic receptor on airway smooth muscle. These drugs, which include atropine and ipratropium are highly effective in horses with heaves [24,27,98]. A single intravenous dose of atropine (0. 02 mg/kg) relaxes airway smooth muscle within 10 minutes (Fig. 3). This drug is very useful for relieving acute respiratory distress but should not be used for routine treatment because of other effects of anticholinergics such as tachycardia, intestinal stasis, and mydriasis.

Figure 3. Effect of bronchodilator treatment on effort of breathing (DPplmax) in heaves-affected horses and control horses. Animals were studied while at pasture (P) or in the stable (S). Atropine (0. 02 mg/kg IV) was administered and 15 minutes later 400 micrograms of pirbuterol (a large dose) were given by inhalation. Bronchodilator drugs had no effect in control horses either at pasture or in the stable. Neither was there any significant effect of bronchodilators when heaves-affected animals were at pasture. However, when the latter animals were stabled, the effort of breathing increased dramatically due to the airway obstruction. Atropine significantly improved lung function but addition of pirbuterol had no more effect. These data demonstrate the importance of cholinergically mediated bronchospasm in heaves. The data should not be taken to mean that pirbuterol has no effect in heaves-affected animals. If given without atropine pre-treatment, pirbuterol is a very good bronchodilator.

Ipratropium bromide is a quaternary ammonium topically active anticholinergic agent that is taken by inhalation. In humans, ipratropium causes bronchodilation without the other side effects. In heaves-affected horses, a single dose of 50 micrograms provides bronchodilation for four hours without side effects [27,98,99]. However, the effects of repeated administration of ipratropium bromide have never been reported in horses.

The β2-adrenergic agonists are the bronchodilators most widely used in equine medicine. The prime example of a β2-adrenergic agonists is clenbuterol (Ventipulmin) which is administered systemically to horses [100,101]. The usual dose is 0. 8 µg/kg, q12 h. Doses up to 3. 2 µg/kg, q12 h can be used as long as they are approached slowly. The side effects of high doses of clenbuterol are those reported for all b2-agonists that is, sweating, trembling, tachycardia, and excitement. These effects are generally short-lived. Clenbuterol also increases the rate of mucociliary clearance in horses with heaves [102]. Albuterol is another β2-adrenergic agonist that has been administered orally to horses. There are no data on its efficacy as a bronchodilator.

Delivering a β2-adrenergic agonist by inhalation using the Equine Aeromask [84] or other device [103,104] can eliminate many of the potential side effects. b2-adrenergic agonists are very effective bronchodilators but most are relatively short acting. For example, the duration of effect of albuterol and pirbuterol is one to two hours. Salmeterol is a long acting b2-adrenergic agonist. In people with asthma, salmeterol (Serevent) causes bronchodilation for 12 hours [105]. In horses, bronchodilation lasts about 8 hours after an aerosol dose of 210 µg [106].

Methylxanthines (aminophylline and theophylline) have bronchodilator activity in horses with RAO but the plasma concentration necessary for bronchodilation varies considerably among individual animals [94]. In addition, at the plasma concentration that causes bronchodilation, horses also develop excitement. For these reasons, methylxanthines are not widely used for relief of bronchospasm in horses.

How should bronchodilator drugs be used? Obviously, when the horse with heaves is in severe respiratory distress it is in need of a bronchodilator drug. When treating such a horse it is advisable to use environmental management, a corticosteroid, and a bronchodilator drug until the horse shows much less respiratory distress. At this point, it is no longer necessary to give the bronchodilator drug several times daily. However, many RAO-affected horses that appear relatively normal have some residual airway obstruction and can benefit from bronchodilator treatment [63]. In this situation, one would use a bronchodilator drug just before the horse was used for exercise. Of course, the use of a bronchodilator before exercise pertains only to horses used in pleasure riding or during training. In most racing or show jurisdictions, use of bronchodilator drugs on the day of competition is forbidden.

Facilitation of Mucus Clearance

Mucus accumulates in the airways of horses with RAO [29] because synthesis and secretion are increased and clearability is reduced [28]. Attempts to improve mucus clearance involve increasing mucociliary transport rate with a b2-adrenergic agonist [102], administration of mucolytic drugs such as dembrexine hydrochloride and acetylcysteine that increase the clearability of mucus, and improving mucus hydration. With regard to the latter, intravenous infusions of large volumes of saline or nebulization of saline has been recommended. A search of the literature (Entrez PubMed) found no published evidence to document the benefit of use of mucolytic agents or of hydration in the treatment of RAO.

Other Drugs 

Many over-the-counter medications recommended for treatment of heaves contain an antihistamine. Although histamine is released during the onset of heaves [107], there have been no studies to evaluate the efficacy of antihistamines. In most inflammatory responses, the changes in organ function are the effect of a cascade of inflammatory mediators, and blockade of a single receptor is rarely effective for disease treatment. This is likely also to be the case with the use of antihistamines for treatment of heaves. Early reliance on such medications delays the introduction of more effective treatment.

Disodium cromoglycate (cromolyn sodium) is used in the management of human asthma and hay fever [108,109]. Doses of 80 to 200 mg given by inhalation are useful in the management of airway inflammation in horses [110-112].

Furosemide prevents some forms of human asthma [113,114]. In horses, furosemide (1 mg/kg) administered by aerosol or intravenously reverses airway obstruction in horses with heaves, probably by releasing bronchodilator prostanoids [115,116]. There is no information on the use of furosemide in the treatment of heaves under field conditions.

The alpha 2-adrenergic agonists such as xylazine, detomidine, and romifidine dilate the airways of horses with heaves by suppressing the release of acetylcholine from parasympathetic nerves [117,118]. This class of drugs should therefore be a useful premedicant to anesthesia for horses with bronchospasm.

Prognosis

The prognosis for RAO and SPAOD is highly dependent on the stage of the disease at which the diagnosis was made and the level of care provided for the horse. Prevention of recurrences of airway inflammation is essential. If this can be accomplished by rigorous continual prevention of exposure to organic dusts that are known to initiate inflammation, the prognosis for cessation of disease progression is excellent. Whether the chronic airway remodeling changes that are present in the lung can ever be reversed is unknown. If total environmental management is difficult or impossible, airway inflammation must be prevented and treated rigorously by use of corticosteroids in order to prevent progression of lung disease. Under these conditions, one can expect useful work from a horse with these diseases especially if bronchodilator drugs are used to ease breathing just prior to exercise. Despite these encouraging words about the prognosis, a recent study demonstrated that few horse owners are willing to undertake the rigorous measures necessary to maintain lung health in horses with RAO [119].