Ultrastructural findings in horses with chronic obstructive pulmonary disease (COPD). II: Pathomorphological changes of the terminal airways and the alveolar region.

Extensive light and electron microscope studies (transmission and scanning electron microscopy) of the bronchioles and alveolar region, in 28 horses suffering chronic obstructive pulmonary disease (COPD) and eight control horses, revealed good correlation between clinical severity and morphological changes. In the bronchiolar epithelium the non-ciliated bronchiolar epithelial (Clara) cells, in particular, showed ultrastructural alterations and, even in the mild stages of disease, these presented degenerative changes and lack of differentiation. Together with loss of granulation in the Clara cells and metaplasia of the goblet cells, cells were seen with unusual intracytoplasmic lamellar inclusion, the number of which increased sharply with clinical severity. The focal changes in the alveolar region were necrosis of type I epithelial cells, alveolar fibrosis of varying degrees with type II epithelial transformation and emphysema or hyperinflation, with an increase in Kohn's pores. Some horse also showed morphological signs of interference with the surfactant system, in the form of marked cysts with lamellar structure. The alveolar changes were mostly in the peribronchiolar region and were, therefore, interpreted as reactive processes. No conclusions as to the aetiology of equine COPD can be derived from these morphological investigations.