Abstract
Ultrastructural changes observed in the acute and repair stages of experimental PI3 virus pneumonia in young calves are described. In the early acute stage of the pneumonia, paramyxovirus replication was observed within epithelial cells of the respiratory tract and within alveolar macrophages. There was loss of cilia and of ciliated cells in small bronchi and bronchioli. Proliferation of non-ciliated bronchiolar cells (Clara cells) produced zones of hyperplastic bronchiolar epithelium and, within alveoli, widespread proliferation of Type 2 pneumonocytes led to alveolar epithelialization. Necrosis was noted in both bronchiolar and alveolar epithelium. Early evidence of repair was the appearance of macrophages on the surface of damaged bronchiolar and alveolar epithelium. Organization of bronchiolar exudate involving fibroblasts and macrophages was widespread in calves killed between 7 and 12 days after infection and, in calves killed at 12 days and subsequently, bronchiolitis obliterans was widespread. Re-epithelialization of damaged bronchiolar mucosa and alveoli also occurred.
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