Lesions of ovine progressive pneumonia: interstitial pneumonitis and encephalitis

Lesions in sheep with ovine progressive pneumonia are described. Lungs were mottled grayish-pink, firm, and large (two to three times normal weight). There was chronic diffuse interstitial pneumonitis characterized by extensive hyperplasia of lymphoid cells around airways and blood vessels and by accumulation of mononuclear cells in the interstitium. In some lungs, this reaction was accompanied by excessive fibrous tissue and smooth muscle and less frequently by hyperplastic epithelium of terminal bronchioles and alveoli. Secondary bronchopneumonia was common. Lymphocytic meningitis, choroiditis, and leukoencephalitis were seen in a few sheep. Meninges and choroid plexuses were infiltrated with lymphoid cells, and subependymal blood vessels were cuffed with lymphoid cells. Much subependymal white matter was demyelinated and had foci of necrosis and gliosis.