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Basics ⬇

Outline

Basics

Overview!!navigator!!

  • A noninfectious condition affecting the lower airways of horses. This terminology has been introduced because of the numerous and confusing terminologies that have been used in the past, and because of the similarities that these conditions share with human asthma
  • Mild and moderate equine asthma is now used to describe the condition previously known as inflammatory airway disease, while severe equine asthma is used instead of heaves or recurrent airway obstruction
  • This new terminology does not imply a common pathophysiology
  • Results from the inhalation of antigens present in the environment of horses, primarily those found in hay (classic form) or pasture (summer form)
  • Systems affected—respiratory
  • Worldwide distribution. The severe form is rare in warm climates

Signalment!!navigator!!

  • No breed or sex predisposition
  • All ages for mild and moderate equine asthma; horses with severe asthma are usually 7 years or older

Signs!!navigator!!

  • Suggestive of lower airway disease
  • Exercise intolerance, cough, nasal discharge, increased breathing frequency
  • Episodes of labored breathing at rest only in severe equine asthma
  • Thoracic auscultation may be normal in the mild form, but wheezes and crackles are frequent when using a rebreathing bag, especially in the more severe cases

Causes and Risk Factors!!navigator!!

  • Immune-mediated reaction to inhaled antigens suspected
  • Genetic predisposition well documented in some families
  • Stabling and hay feeding, or pasture in the summer form

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

Other causes of lung diseases.

CBC/Biochemistry/Urinalysis!!navigator!!

Within normal limits.

Other Laboratory Tests!!navigator!!

Arterial blood gases may reveal hypoxemia in the severe form.

Imaging!!navigator!!

Thoracic radiography may reveal a bronchointerstitial pattern.

Other Diagnostic Procedures!!navigator!!

  • The diagnosis should ideally be based on the presence of lower airway obstruction, but is rarely performed due to the lack of portable and sensitive equipment
  • Bronchoscopy—tracheal mucus is common
  • Bronchoalveolar lavage cytology—increased neutrophils, mast cells, or eosinophils
  • Tracheal aspirates—reveal inflammation but not considered diagnostic

Pathologic Findings!!navigator!!

  • Chronic active bronchiolitis
  • Airway remodeling characterized by smooth muscle thickening and increased collagen, at least in the severe form

Treatment ⬆ ⬇

TREATMENT

  • Outpatient care
  • Avoid hay feeding, straw bedding, and stabling when possible
  • Favor pasture/paddock, pelleted hay, haylage, or pasteurized hay in the classic form
  • Favor stabling and pelleted hay in the summer form

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

Corticosteroids

  • The most potent drugs to control clinical signs
  • Inhaled drugs are preferred for prolonged administration
  • Improvement in clinical signs and airway obstruction within days (systemic) or days to weeks (inhaled). Cough may take longer to respond
  • Few residual effects unless combined with antigen avoidance strategies

Bronchodilators

  • Should not be used as sole therapy. To be combined with antigen avoidance strategies or corticosteroids
  • Short-acting bronchodilators may be administered by inhalation prior to exercise, or orally for more sustained effects

Other Drugs

  • Inhaled sodium cromoglycate (cromoglicic acid) with inflammation associated with mast cells
  • In the absence of antigen avoidance strategies, no drugs normalize airway neutrophilic inflammation

Contraindications/Possible Interactions!!navigator!!

  • Corticosteroids should not be administered systemically to horses with a history of laminitis or concurrent bacterial infection
  • Vaccination against botulism when feeding haylage is recommended in some areas

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

Repeat BAL fluid cytology and lung function (when possible) 6–8 weeks after implementation of antigen avoidance strategies to assess effectiveness.

Prevention/Avoidance!!navigator!!

Avoid exposure of all horses to moldy hay, especially when they are genetically susceptible.

Expected Course and Prognosis!!navigator!!

  • Mild and moderate equine asthma may be self-limiting
  • Severe equine asthma is incurable
  • Removing the offending antigens normalizes BAL fluid cytology within 6 weeks but complete control of the clinical signs may take up to 3 months without drug therapy
  • Currently it is not possible to predict which horses with mild/moderate asthma will progress to the severe form

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Age-Related Factors!!navigator!!

Severe asthma is rare in horses <7 years of age.

Pregnancy/Fertility/Breeding!!navigator!!

Avoid systemic administration of corticosteroids and β2-agonist bronchodilators during pregnancy.

Abbreviations!!navigator!!

BAL = bronchoalveolar lavage

Suggested Reading ⬆ ⬇

Pirie RS, Couëtil LL, Robinson NE, Lavoie JP. Equine asthma: an appropriate, translational and comprehendible terminology?Equine Vet J 2016;48:403–405.

Author(s) ⬆

Author: Jean-Pierre Lavoie

Consulting Editors: Mathilde Leclère and Daniel Jean