How to manage and pharmacologically treat allergic respiratory disease

“Allergic respiratory disease” can have two different presentations in the horse, anaphylaxis (1) and Equine Asthma (2, 3).

Anaphylaxis, also known as anaphylactic or allergic shock, is an acute, severe allergic response that affects the entire body. This reaction is characterized by narrowing of the airways, resulting in dyspnea, and can also be accompanied by non-respiratory signs like hives or cardiovascular shock. In severe cases, swelling of the upper airways (throat, laryngeal edema) and/or lower airways (bronchospasm, primary or secondary pulmonary edema) can obstruct air passage to a potentially life-threatening degree. Immediate therapeutic action is indicated and depending on the severity and presentation may include pharmacological and physical intervention (1): Epinephrine (1 : 1000), 3 - 5 ml/450-500 kg slowly IV in 20-30 ml NaCl (also IM or SQ in less severe cases); holding the head of the patient high; applying ice-cooling; lidocaine spray, epinephrine 2% or phenylephrine spray (0.1%, 20 - 30 mL) locally to decrease swelling; furthermore NSAIDs, such as flunixin, 1.1 mg/kg i.v. BID, steroids, such as dexamethasone, 0.05 - 0.2 mg/ kg IV SID, antihistamines, such as Ceterizin 0.2-0.4 mg/kg PO, q12h; and if pulmonary edema is present, furosemide (1 mg/kg IV slowly, repeat after 30 minutes if necessary) may be used; in severe cases of upper airway obstruction, tracheostomy can be life-saving and O2 insufflation (10 - 20 ml/kg/min.) may be given by nasal tube or tracheostomy; in severe allergic shock, crystalloid and colloid fluids may be indicated for cardio-vascular support.

While anaphylaxis is rare and poorly documented in the horse, Equine Asthma is very common in some regions and its clinical manifestations and management are well-described. However, the exact etiology and pathogenesis, particularly the role of allergy in Equine Asthma, are still unclear. Nevertheless, this syndrome is commonly regarded as an “allergic respiratory disease” in the broader sense (i. e. a hypersensitivity to external “normal” stimuli). This concept is indeed helpful to explain to owners of affected horses that pharmacological treatment is only symptomatic, and the condition should be treated and controlled by avoiding or decreasing external triggers. Apart from “summer-pasture associated Equine Asthma” (prevalent in some regions with hot, humid climates and suspected outdoor “allergens”), most affected horses are hypersensitive to indoor stable environments, specifically irritants and potential allergens from hay dust and bedding. The best documented potential allergens in indoor environments are of fungal origin (i. e. mold spores, amplified by non-specific irritants like endotoxin and B-glucans), but currently no proven allergen testing and desensitization protocols exist. Accordingly, management changes directed at improving stable air hygiene to decrease dust exposure (e. g. steaming of hay; replacing by haylage, hay cubes or pelleted feeds; pasture with grass as roughage etc.) are the most important aspects of Equine Asthma therapy.

Despite measures to improve air hygiene, or when these cannot be fully instituted, many horses still require medical therapy to control clinical signs. Systemic and aerosolized formulations are available for various medications, and choice will depend on clinical presentation, availability local regulations, and veterinarian/ owner preferences. In severe exacerbations, bronchodilators can provide relief of airway obstruction. Muscarinic receptor antagonists, like hyoscine butylbromide (0.2 - 0.3 mg/kg IV), also known as scopolamine butylbromide (brand name Buscopan), is only short-acting but highly effective, and can also be used diagnostically to demonstrate reversibility of bronchospasm induced signs. It is well tolerated and does not have the side-effects and risks of atropine. B-agonist bronchodilators can be administered by inhalation (e. g. Salmeterol or Albuterol) or orally (Clenbuterol, 0.8 -3.2 μg/kg PO q12h; gradual increase of dose; drug tolerance with prolonged use). Bronchodilators should not be used as the sole treatment and are often combined with corticosteroids given systemically (e. g. prednisolone 0.5-2 mg/kg or dexamethasone, 0.03 - 0.1 mg/kg) or by inhalation (e. g. fluticasone, budesonide, beclomethasone, ciclesonide). Ciclesonide (brand name Aservo EquiHaler) provides the best documented therapeutic efficacy with the least systemic effects and is registered for the treatment of severe Equine Asthma in many countries. Further options include sodium cromoglycate, a mast cell stabilizer, for horses with mastocytic mild-moderate Asthma, and supplementation with polyunsaturated omega-3 fatty acids as an adjunct treatment. Mucolytic agents are often used, but evidence for efficacy, at least as a sole medication, is lacking. Non-steroidal anti-inflammatory drugs, leukotriene-receptor antagonists and antihistamines do not appear to be helpful in Equine Asthma therapy.