Chronic obstructive pulmonary disease in the horse. 1: Nature of the disease.

The aetiology, pathophysiological changes, pathology and clinical signs of the disease as presently understood are discussed. The condition appears to be a hypersensitivity of the respiratory system in some horses to poor quality hay and straw. Micropolyspora faeni is the chief agent identified in the northern part of the United Kingdom. In other locations, the chief agent is probably different. The principal changes are spasm of the airways and bronchiolitis of the small airways. Onset may be acute or insidious. The chief clinical signs are well known but the disease process is reversible if the source of the antigen is removed. In early cases, determination of increased maximum intrathoracic pressure changes and co-existent subnormal arterial oxygen pressure are the best objective confirmation of diagnosis. The same measurements may be made on asymptomatic horses if they have been challenged 4 to 5 h earlier by inhalation of the appropriate antigen, or even exposed to poor quality hay or straw. Other methods available to the clinician are discussed.