Recurrent Airway Obstruction

Chronic obstructive pulmonary disease (COPD)

COPD (Image sourced from Bristol Biomed Image Archive with permission)

COPD scanning electron micrograph (Image sourced from Bristol Biomed Image Archive with permission)

• Synonyms: Recurrent Airway Obstruction (RAO) (correct current terminology), "heaves", "broken wind"
• COPD is one of the most common conditions encountered in adult horses and is a major cause of loss of performance
• It is an asthma-like syndrome of the bronchioloalveolar region
• The aetiopathogenesis of COPD is not fully understood, but is thought primarily to be caused by an immune-mediated hypersensitivity response to inhaled allergens commonly found in mouldy hay or bedding and in stable dust (eg: spores from Faenia rectivirgula, Aspergillus fumigatus, and Thermoactinomyces vulgaris)
• The immune reaction is considered primarily to be a type III hypersensitivity (antigen-antibody complex mediated)
  • Type I and type IV hypersensitivity reactions may also play a role
• Repeated episodes of inflammation in the lower airways, especially the bronchioles, results in chronic changes that cause obstruction of the lower airways, sometimes followed by alveolar emphysema
• The histological features are what you would predict given this pathogenesis:
  • Luminal accumulation of exudate made up of thick mucus (goblet cell metaplasia), neutrophils and cellular debris
  • This can predispose to, and is often accompanied by, secondary bacterial infection eg: Streptococcus equi subsp. zooepidemicus
  • Thickened mucosa - epithelial hyperplasia, chronic inflammatory cell infiltrate with variable numbers of eosinophils in the airway wall
  • Peribronchiolar fibrosis
  • Bronchospasm
• When susceptible horses are exposed to inhaled allergens they develop airway inflammation and respiratory dysfunction
• Typically, susceptible horses are clinically normal when at pasture, but develop signs of disease shortly (hours to days) after being housed
• Clinical signs are what you would predict from a chronic inflammatory process in the airways:
  • Coughing (following activation of coughing reflexes due to inflammation in the airways, mechanical pressure on the airway walls associated with the accumulation of intraluminal exudate)
  • Nasal discharge (usually bilateral - mucopurulent tracheal exudate on bronchoscopy)
  • Increased respiratory rate (compensation for reduced tidal volume following the reduction in airway lumen diameter)
  • Pronounced effort during inspiration with an obvious abdominal component (may result in a "heave-line" in the flank due to hypertrophy of the abdominal muscles - reduced airway diameter results in increased resistance to air flow, so more inspiratory effort required for same airflow rates)
  • Wheezing and crackling lung sounds on auscultation at the end of inspiration and expiration - caused by bubbles in mucus collapsing