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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • A nonseptic inflammation of the lower airways caused by an accumulation of a variety of inflammatory cells leading to excess mucus production, coughing, and a reduction in exercise performance
  • Some clinical features are similar to heaves/RAO
  • The term “equine asthma” has recently been used when describing nonseptic inflammation of the lower airways with “severe equine asthma” for what was known as heaves/RAO, and mild to moderate equine asthma for IAD. This new terminology does not imply a common pathophysiology

Pathophysiology!!navigator!!

Horses develop an increased sensitivity to dust, molds, pollens, and other irritants, resulting in inflammatory cells such as mast cells, neutrophils, eosinophils, lymphocytes, and alveolar macrophages to release potent mediators. The relationship between the presence of increased populations of mast cells/eosinophils and airway hyperreactivity has been reported. Peribronchiolar inflammatory cell infiltration, goblet cell hyperplasia, and accumulation of mucus and neutrophils in the airway lumen has been demonstrated in lung biopsies.

Systems Affected!!navigator!!

  • Respiratory
  • Pathologic changes more severe in the peripheral airways (i.e. bronchioles); alveoli are not affected

Genetics!!navigator!!

Breed and sex-specific genetic linkages have not been consistently identified.

Incidence/Prevalence!!navigator!!

Widespread where horses are stabled or trained indoors.

Geographic Distribution!!navigator!!

Identified in North America, the UK, Australia, Iceland and Europe.

Signalment!!navigator!!

  • Any breed
  • Typically first recognized in horses <12 years of age but older than 1 year

Signs!!navigator!!

Historical Findings

  • Reduced exercise tolerance or poor performance, with a prolonged recovery period after exercise is the most common reported clinical sign in performance horses
  • Other observations include intermittent to frequent coughing while the horse is eating or early in exercise and nasal discharge

Physical Examination Findings

  • Vital signs are within the normal range, except on occasion, when the resting respiratory rate may exceed 18 breaths/min or when a prolonged return to normal respiratory rate after exercise is observed
  • Nasal discharge is uncommon
  • A significant increase in bronchial sounds over both lung fields easily heard on auscultation
  • With a rebreathing bag, wheezes are frequently detected over the dorsal area of the lung field and coughing may be elicited in performance horses
  • Lung-field percussion may identify dorsal and caudal areas of hyperresonance, detectable beyond the 16th rib

Causes!!navigator!!

  • A combination of airborne environmental allergens and endotoxins is considered to be the primary causative agents—mold and dust from hay and bedding and noxious gases such as ammonia from stagnant bedding
  • The role of respiratory viral infections in the developmental of IAD remains under investigation
  • Exacerbation of airway inflammation following a bacterial bronchitis in weanlings and occasionally yearlings
  • Development and progressive airway inflammation following onset of exercise-induced pulmonary hemorrhage is a suspected cause

Risk Factors!!navigator!!

  • Predisposing and exacerbation of specific risk factors include respiratory viral infections (e.g. EIV and equine rhinitis viruses) and bacterial bronchitis (Streptococcus zooepidemicus) in young horses
  • Higher incidence during hot summer days with high humidity
  • High-intensity training and racing

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Respiratory viral infections (e.g. EIV and equine herpesvirus) generally affect several horses in the same stable within a defined period of time, whereas IAD affects and persists in individuals and worsens over time
  • Bacterial tracheitis and bronchitis, usually secondary to respiratory viral infection
  • Localized pulmonary abscess may present with a similar history. However, pulmonary abscesses may be diagnosed based upon characteristic clinical signs including fever, inappetence, and pain on chest percussion over the anteroventral area of the lung fields

CBC/Biochemistry/Blood Gas Analysis/Urinalysis!!navigator!!

CBC and serum biochemistry profile are within normal limits.

Other Laboratory Tests!!navigator!!

  • Bronchoscopy and bronchoalveolar lavage to retrieve cells of the lower airways and alveoli
  • Total count and differential of cell counts harvested from bronchoalveolar lavage fluid. These cells are analyzed quantitatively and qualitatively to determine major changes in the inflammatory cell population—neutrophils, mast cells, eosinophils, lymphocytes, and exfoliated epithelial cells
  • Bronchoprovocation with histamine is a specific test that determines the degree of airway hyperresponsiveness (increased sensitivity to a variety of agents)
  • Lung biopsies to evaluate histologic changes of the small airways and provide information on severity and prognosis. Lung biopsies are not routinely performed

Imaging!!navigator!!

Thoracic radiography has little value except to demonstrate noncharacteristic, small, 2–4-mm diameter, donut-shaped lesions in the periphery of bronchioles in some horses.

Other Diagnostic Procedures!!navigator!!

Pulmonary scintigraphy may have some potential as an adjunct diagnostic tool but is limited to most academic institutions or specialized equine clinics.

Pathologic Findings!!navigator!!

Persistent small airway inflammation has led to the term “airway remodeling,” referring to dynamic pathologic changes during the course of persistent ongoing inflammation. Such changes have been recognized in heaves but have not been well characterized in IAD, where:

  • Limited evidence suggests that lesions are generally restricted to the peripheral airways (<5 mm)
  • Accumulation of inflammatory cells in the airways and mucus plugging of the airways caused by goblet cell hyperplasia may be early changes
  • Increased smooth muscle mass suspected

Treatment ⬆ ⬇

Outline

TREATMENT

Aims!!navigator!!

To control the airway inflammation, leading to improvement in pulmonary function and reduction in airway hyperreactivity and mucus production.

Appropriate Health Care!!navigator!!

  • Environmental management is imperative to successfully treat and manage cases of IAD. Make all possible attempts to avoid environmental dust from low-quality hay and bedding. In susceptible horses, hay alternatives (e.g. pelleted/cube hay, haylage, and pasteurized hay (“hay steamer” such as Haygain)) are effective in reducing dust level
  • Encourage outdoor living with opened shelter access

Nursing Care!!navigator!!

N/A

Activity!!navigator!!

No limitations with proper therapeutic plans and environmental management control.

Diet!!navigator!!

See Appropriate Health Care.

Client Education!!navigator!!

  • IAD can be managed, allowing the horse to maintain an active and maximal performance athletic career
  • Environmental management is imperative and includes a well-ventilated stall environment, rubber flooring with high-efficiency urine-absorbing materials and complete pelleted ration with small amount of steamed or soaked hay
  • Turn out horses as much as possible
  • Long-term therapy with an MDI is preferable as the medication is directly delivered to the lungs with minimal systemic effect. Alternatively, systemic therapy with corticosteroids (prednisolone/dexamethasone) at the minimum effective dose
  • Maintain an up-to-date vaccination schedule for respiratory viral pathogens

Surgical Considerations!!navigator!!

N/A

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

Corticosteroids (Mature Horses)

  • Oral—prednisolone (400 mg BID for 15 days, then 300 mg once a day for 15 days, and a maintenance dose of 300 mg once a day on alternate days for as long as needed) or oral dexamethasone at a starting dose of 0.05 mg/kg daily or on alternate days
  • MDIs with special delivery devices—fluticasone propionate (250 μg/puff; 8–12 puffs BID for 2 weeks, then on alternate days for as long as needed) or beclomethasone dipropionate (beclometasone dipropionate) (250 μg/puff; 12 puffs BID for 2 weeks, then on alternate days for as long as needed)

Mast Cell Stabilizer

Nedocromil sodium (2 mg/puff; 12 puffs BID for 2 weeks, then on alternate days for as long as needed).

Bronchodilators

  • Clenbuterol (see the label recommendation; dosage varies from country to country)
  • Ipratropium bromide (20 μg/puff; 5 or 6 puffs given 10–15 min before exercise)
  • Albuterol (salbutamol) (100 μg/puff; 5–10 puffs given 10–15 min before exercise)

Corticosteroid and Bronchodilator Combination

Fluticasone propionate/salmeterol combination (2500/250 µg BID) has proven superior to fluticasone alone at improving lung function in heaves.

Contraindications!!navigator!!

Do not use high doses of corticosteroids in cases with suspected concomitant viral or bacterial infection, or when the administration of corticosteroids is contraindicated.

Precautions!!navigator!!

  • Haylage has not been widely utilized in North America due to the fear of botulism
  • Oral corticosteroids are not recommended in mares while in late gestation; however, inhaled corticosteroids appear safe because of their very low systemic effect
  • Verify medication regulations for withdrawal times before racing and/or competition

Possible Interactions!!navigator!!

N/A

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

N/A

Prevention/Avoidance!!navigator!!

  • Avoid moldy hay and bedding
  • Maximize turnout periods and reduce stabling time. Outdoor living with shelter is an ideal environment
  • Change diet—haylage, complete feed, or hay cubes with low-dust content and steamed/soaked flakes of hay

Possible Complications!!navigator!!

Acute exacerbation similar to heaves.

Expected Course and Prognosis!!navigator!!

Prognosis largely depends on early diagnosis and owner compliance with maintaining a low-allergen environment and an appropriate therapeutic maintenance regimen.

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

N/A

Age-Related Factors!!navigator!!

Presently there is little evidence in the scientific literature that the condition progresses with age.

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

See Medications and Precautions.

Synonyms!!navigator!!

  • Nonseptic inflammatory airway disease (NSIAD)
  • Allergic lower airway disease
  • Allergic small airway disease

Abbreviations!!navigator!!

  • EIV = equine influenza virus
  • IAD = inflammatory airway disease
  • MDI = metered dose inhaler
  • RAO = recurrent airway obstruction

Suggested Reading ⬆ ⬇

Couëtil LL, Cardwell JM, Gerber V, et al. Inflammatory airway disease of horses—revised consensus statement. J Vet Intern Med 2016;30:503–515.

Leguillette R, Tohver T, Bond SL, et al. Effect of dexamethasone and fluticasone on airway hyperresponsiveness in horses with inflammatory airway disease. J Vet Intern Med 2017;31:1193–1201.

Robinson NE. Inflammatory airway disease: defining the syndrome. Conclusions of the Havemeyer workshop. Equine Vet Educ 2003;15(2):61–63.

Author(s) ⬆

Authors: Mary C. Furness and Laurent Viel

Consulting Editors: Daniel Jean and Mathilde Leclère