Bronchiolo-alveolar adenocarcinoma in a horse.

A bronchiolo-alveolar adenocarcinoma was diagnosed in the lungs of a horse which was euthanased after protracted respiratory disease and radiological evidence of pulmonary neoplasia. Multifocal, large, firm neoplasms occurred throughout both lungs. Neoplastic lesions were not found elsewhere. Histologically the bronchiolar and alveolar architecture was retained. The cuboidal cells lining neoplastic alveoli had very vacuolated cytoplasm, while some were ciliated. Electron microscopy identified the cells as Type II pneumocytes. Numerous distended myelinoid bodies in the tumour cells accounted for the vacuolated appearance seen by light microscopy. Special stains for fat, mucin, mucopolysaccharides and glycogen failed to elucidate the nature of the substance in these vacuoles.