In recent years, aerosolized drug therapy in the horse has transitioned from a curiosity to a well established treatment modality. Practitioners and owners alike have recognized the benefit of topical application of bronchodilator and glucocorticoid drugs, thus avoiding the side effects and even toxicities associated with the systemic delivery of these drugs. Although several publications regarding aerosolized drug therapy in the horse have been published in the past 5 years, a dearth of information concerning efficacy, pharmacokinetics, and pharmacodynamics of aerosolized drugs in the horse exists. Often, treatment rests upon extrapolation from discoveries made in the treatment of human asthma or chronic obstructive pulmonary disease (chronic obstructive pulmonary disease) to the horse. Further complicating the matter is the confusion that still exists concerning various manifestations of inflammatory, non-septic lower respiratory disease in horses. For the purposes of this discussion, this author will adhere to the recommendations of the recent international workshop on equine chronic airway disease, which recognized two distinct entities: recurrent airway obstruction (RAO, “heaves”) and inflammatory airway disease (inflammatory airway disease). Heaves is a familiar disease, whereas inflammatory airway disease is less well defined but encompasses the signs of cough, exercise intolerance, mucus in the airways, and varying degrees of lower airway inflammation in younger horses (see “Inflammatory Airway Diseases: Definitions and Diagnosis in the Performance Horse”). Most practitioners agree that these are two very different clinical entities that clearly demand different treatment recommendations. Nonetheless, it is now generally recognized that inflammation is of vital significance in both conditions. Antiinflammatory treatment is therefore the cornerstone of therapy for each.
The goals of treatment must be clear in order for client, patient, and veterinarian satisfaction, which entails a team approach and acceptance that treatment may be a lifelong issue that may be modified but is unlikely to disappear. Goals in treating recurrent airway obstruction should include: (1) immediate relief of the bronchospasm that causes dyspnea, (2) reduction of lower airway inflammation that causes cough and mucus hypersecretion, (3) long-term prevention of episodes of heaves by control of lower airway inflammation and airway obstruction, and (4) return to limited or even full athletic potential. The goals for treatment of nonseptic inflammatory airway disease are similar, as follows:
- 1. Eliminate bronchoconstriction that impairs performance.
- 2. Reduce mucus production and airway plugging.
- 3. Reduce coughing.
- 4. Reduce airway reactivity.
- 5. Prevent recurrences.
Aerosol therapy has its place in each of these goals, although systemic corticosteroids are usually necessary for initial reduction of airway inflammation, and environmental control is paramount in long-term control of recurrent airway obstruction (RAO; see Chapter 8.4: “Heaves [Recurrent Airway Obstruction]: Practical Management of Acute Episodes and Prevention of Exacerbations”). To achieve success, the veterinarian and client must plan for regular check-ups and be prepared for changes in treatment strategy that might be necessary. Owner education is critical in achieving compliance. Establishing a reasonable definition of “return to athletic use” is critical to client satisfaction. Looking forward to returning a mildly affected, young racehorse to full racing potential is reasonable, whereas a reasonable goal for a horse with recurrent airway obstruction might be a much more modest return to light pleasure riding. When available, lung function testing before and after administration of a bronchodilator can be very useful for identifying the horse that is less likely to respond to conventional therapy. Horses with poor initial responses to either bronchodilators or steroids may not respond as readily in the long term. Horses with inflammatory airway disease that exhibit intense airway hyperreactivity or high numbers of inflammatory cells also may be less responsive to therapy.
Monitoring Response To Therapy
It is important to have a baseline assessment of the horse before initiating therapy. Ideally, this includes careful physical examination, auscultation with and without a re-breathing bag, observation during exercise, baseline pulmonary function testing and measure of airway reactivity (inflammatory airway disease), or, in the case of horses with RAO, the effect of bronchodilation. Bronchoalveolar lavage cytology should be evaluated in either case. Although pulmonary function testing is currently available only at a few specialized veterinary clinics, user-friendly systems for field-testing may become available, thus making objective baseline assessments available to practitioners. A clinical scoring system has been devised; however, it does not discriminate inflammatory airway disease from normal sufficiently for clinical use. Horses with heaves can be scored on abdominal lift (normal, mild, moderate, severe increase: 1-4) and nasal flaring (normal, mild, moderate, severe increase: 1-4). The goal of a thorough baseline assessment is to facilitate a treatment regimen tailored to the individual horse and to monitor response to therapy. This author offers free lung function testing one month after initiation of therapy to assess response and fine-tune therapy for the upcoming months. Communication with owners and referring veterinarians is encouraged to facilitate this process.
Patient Nonresponse To Therapy
If response to therapy is poor, detective work to determine why treatment has been unsuccessful is important. It is essential to check the client’s technique for using the drug delivery device. Simple issues — such as the use of canisters with no drug, holding the canister upside down, poor mask fit, failure to shake the (metered-dose inhaler) metered-dose inhaler before using chlorofluorocarbon (chlorofluorocarbon) formulations, or giving repeated puffs of drug too quickly — may interfere with successful treatment. Occasionally, horses may react to a certain formulation of drug; switching to a different formulation within the same class usually will help. Failure to modify the environment may, in some horses, negate any attempts at drug therapy. Some horses with chronic, severe pathologic processes may be resistant to corticosteroids or may have irreversible changes in the lungs that prevent response to bronchodilators. As noted previously (with short-acting β2-agonists), lung function testing with albuterol challenge can successfully identify these horses. Finally, lack of response to therapy may be due to underlying infectious disease and may indicate the need for further diagnostics and perhaps an entirely different approach or concomitant antibiotic use.