Chronic obstructive pulmonary disease: a review

Chronic obstructive pulmonary disease (COPD) is an inflammatory obstructive lower airway disease of the mature horse. It is a complex syndrome with variable clinical signs ranging from exercise intolerance in performance horses to chronic cough, mucopurulent or purulent nasal discharge, expiratory dyspnoea, and sometimes, weight loss in horses with severe disease.The term COPD was introduced by Sasse (1971) and is still the most commonly used name for the disease. The disease has also been known as heaves, broken wind, alveolar emphysema, chronic pulmonary disease, small airway disease, chronic obstructive lung disease, chronic bronchiolitis and recurrent airway obstruction. A certain amount of confusion exists with regard to terminology of lower airway diseases in the horse. Inflammatory airway disease (IAD) is a term that has been introduced recently to describe a milder clinical syndrome observed in young horses, particularly racehorses in training (Moore 1996). At present it remains uncertain whether or not this milder inflammatory airway disease progresses to the more severe syndrome of COPD in the older horse. This article is restricted to a discussion of COPD in the mature horse. Clinical signs of COPD are usually observed after exposure to hay and straw dust during stabling. Expiratory difficulty is caused by obstruction to airflow in both the large and the small airways. In the large airways this obstruction is caused primarily by bronchospasm, whereas in the small airways it is attributable mainly to inflammation and excessive secretions. The disease goes into remission when horses are pastured and not exposed to hay (unless they are also affected by summer pastureassociated obstructive pulmonary disease) (Derksen et al. 1985a). Although the exact pathogenesis of the disease is unclear, respiratory tract hypersensitivity to fungal and thermophilic actinomycete spores is believed to be involved. During periods of airway obstruction, affected horses demonstrate airway hyperresponsiveness (i.e. the airways show an abnormal tendency to narrow in response to a variety of stimuli), which wanes during periods of disease remission (Derksen et al. 1985a; Fairbairn 1993a).