The terminology that surrounds Severe Equine Asthma can be confusing. Originally referred to as chronic obstructive pulmonary disease (COPD), chronic obstructive lung disease, alveolar emphysema, chronic bronchitis, or chronic bronchiolitis. These terms are either inapplicable or outdated and should not be used. Recurrent airway obstruction (RAO) is still commonly used by many veterinarians, but severe equine asthma (SEA) has been adopted due to the primary feature of bronchospasm (active airway narrowing), reversible with bronchodilators and corticosteroids. Somewhat ironically, most people, including vets, still refer to the condition as heaves.
Causes
Although not completely understood SEA is probably an allergic/inflammatory response to inhaled allergens. The list of allergens is large, with more than 50 different potential allergens identified, with the primary source being hay, straw, or pasture. Examples include Aspergillus fumigatus, Faenia rectivirgula, Thermoactinomyces vulgaris, and endotoxin.
The result of the exposure to allergens is the release of a variety of pro-inflammatory products, such as histamine, leukotrienes and cytokines. These in turn result in bronchoconstriction, production of mucus, neutrophil influx, and thickening of the walls of airway bronchi.
Signs
The signs of severe asthma can include cough, weight loss, difficulty breathing (dyspnoea), exercise intolerance, and a persistent or intermittent nasal discharge. The combination of bronchoconstriction and mucus production produces signs that are much worse on expiration. When affected horses breath in the narrowed airways may dilate marginally, but when they breath out the narrowed airways reduce further in diameter such that there may be small airway obstruction. This leads to a dramatic abdominal component to breathing, ultimately causing hypertrophy of the abdominal muscles (heave line).
Listening to lung sounds with a stethoscope can often reveal a range of abnormal (adventitial) sounds, including expiratory wheezes, and inspiratory and expiratory crackles. The wheezes are caused by air being forced through narrowed airways and the crackles are caused by air moving through and over mucus.
The abundance of mucus in the airway can impede the normal mucociliary clearance leading to occasional secondary bacterial infections. This could result in a further exacerbation of signs, and may include fever. Fever is not a feature of uncomplicated asthma.
Diagnosis
In many regions of the world severe asthma is seasonal. In cold climates it is frequently diagnosed in stalled horses over winter. The barn environment is typically filled with fungi and molds associated with stored feed, heating and poor ventilation. Horses become sensitized to these particles and develop the classical signs of cough, breathing difficulty, and mucus production. In regions with hot and humid conditions, such as the tropics and sub-tropics, severe asthma is typically seen in pastured horses during the wet summer months. In these regions the moisture and heat favour growth of similar molds and fungi on the pasture. Severe asthma is also sporadically in other regions, presumably associated with specific environmental allergens or feeds.
The clinical diagnosis is based on history (previous seasonal episodes), age (mature horses >4 years of age), an increased expiratory effort, reduction in exercise tolerance, and nasal discharge. Chronic cases commonly experience weight loss. Auscultation is also very helpful.
Bloodwork (complete blood count and SAA) may be unremarkable unless secondary infection is present. Airway cytology is very helpful as both the tracheal and bronchoalveolar samples typically demonstrate an abundance of non-degenerative neutrophils and excessive mucus.
Treatment
For most horses the focus of treatment is management of the environment. This can be relatively straightforward for horses with typical seasonal severe asthma. For example, horses stalled during winter can be turned out, and horses with summer pasture-associated asthma can be relocated to other regions or housed indoors. It is often more difficult when atypical SEA is present, where identification of specific sources of allergen may be challenging.
Generally, improvement in ventilation in housed horses is very important. Reducing the amount of dust of hay is also helpful – this can achieved using a hay steamer or by soaking the hay for 90 minutes. Alternatively, you could use pelleted hay or haylage. Avoid using straw bedding – use sand, wood shavings, shredded paper or cardboard, or peanut hulls. Note that some wood shaving supplies can be very dusty, particularly when spread.
Corticosteroids are the most effective therapy for horses with severe asthma. Typically, treatment is for a minimum of 2-4 weeks, and often longer to control severe asthma, assuming that environmental control is neither possible nor effective. It may take up to 4 days to see a good clinical response after beginning therapy.
There are multiple modes of delivering corticosteroids to affected horses. The simplest (and least expensive) is to use dexamethasone for injection over the tongue (orally) daily on an empty stomach. The drug can also be efficiently delivered by daily injection into the vein or muscle, or by nebulization. In our region it is common to use a Flexineb wet nebulizer to deliver corticosteroids, including dexamethasone or, more commonly, budesonide. Dry inhalation devices can be used to deliver corticosteroids, such as fluticasone or beclomethasone. Recently, a commercial inhalation device was released to the market, the Aservo EquiHaler, that contains the corticosteroid ciclosenide. This drug is effective in controlling airway inflammation, but each device only lasts for 10 days. Therefore multiple devices may be necessary to control a horse with SEA.
Oral prednisolone (“Preddy granules’) is well absorbed in horses but efficacy is lacking, and it is certainly not as effective in severe asthma as other corticosteroids.
Bronchodilators are also commonly used in conjunction with environmental management and corticosteroids. These can be given systemically (e.g., clenbuterol) or by inhalation (e.g., salbutamol, ipratropium bromide). When used by inhalation it is recommended to proceed corticosteroid therapy with a beta-2 adrenoreceptor agonist, such as salbutamol, in order to increase distribution of the steroid.
Allergy testing and desensitization remains a controversial topic in the management of severe equine asthma. There are conflicting data, but some veterinarians believe that intradermal testing may provide a useful method to identify specific allergens in the horse’s environment. Others would challenge this opinion, and in the absence of solid data it remains controversial.
Note: Corticosteroids in horses and ponies with suspected or confirmed Equine Metabolic Syndrome. It is generally accepted that corticosteroids increase the risk of laminitis in animals with EMS. As such it is preferred to use preparations and routes of delivery that reduce the systemic exposure of the drug. This usually means using inhaled preparations, such as fluticasone or ciclosenide.
Tags: Respiratory