Pathologic Changes in 3-methylindole-induced Equine Bronchiolitis

The pathologic features of bronchiolitis were studied in horses and ponies from 30 minutes to 27 days after an oral dose of 3-methylindole (3MI). From 30 minutes to 3 hours, lesions were limited to nonciliated bronchiolar epithelial (Clara) cells, which lost apical caps and cytoplasmic granules and had dilated smooth endoplasmic reticulum (SER). At 12 hours, necrotic Clara cells were exfoliated; degeneration and necrosis were evident, in bronchiolar ciliated cells. Rare epithelial cells with hyperplastic SER appeared on the denuded basal lamina at 24 hours. Inflammatory cells, epithelia, fibroblastlike cells, collagen, and debris occluded many bronchiolar lumens from 3 to 6 days. Reorganization resulted in a simple columnar bronchiolar epithelium with relatively normal ciliated cells and fewer fibroblastlike cells. However, mature Clara cells were rare at 27 days, and collagenous bands still divided bronchiolar lumens. Thus, 3MI toxicosis is a persistent model of equine bronchiolitis with many morphologic features of the spontaneous disease.