The percentage of neutrophils in bronchoaveolar washes of heavey horses is increased 4-6 fold over healthy horses. Airway obstruction results from bronchospasm, and the accumulation of mucus and exudates in the airway lumen associated with the inflammation.

Aspergillus fumigatus, Faenia rectivirgula, Thermoactinomyces vulgaris and Micropolyspora faeni are among the etiological antigens. Clinical improvements of COPD can be produced by aerosol provocation tests using antigens derived from given fungi.

The main lesion in COPD-affected horses is bronchiolitis marked by accumulation of intraluminal neutrophils and peribronchial inflammatory cells that are mostly lymphocytes.