Equine Multinodular Pulmonary Fibrosis and Lymphoma in a Horse Associated with Equine Herpesvirus-5

A 9-year-old mare presented with a 2-week history of partial anorexia, multiple swellings in the area of the throatlatch, and purulent nasal discharge. On initial presentation, the horse had submandibular and retropharyngeal lymphadenopathy, mild ventral edema, and weight loss. Thoracic radiographs revealed a pulmonary interstitial pattern. Necropsy revealed enlarged lymph nodes throughout the body. The lung parenchyma contained multiple random, well-circumscribed nodules, which, on cut section, were pale, tan colored, and very firm with a distinct line of demarcation from the surrounding normal parenchyma. The subendocardium of the left ventricle, left atrium, and, multifocally, the right ventricle contained white, gritty areas of mineralization. There was marked subintimal mineralization of the aorta and pulmonary artery. Histopathology of the lymph nodes revealed effacement of the parenchyma by a neoplasm composed of large numbers of small mature lymphocytes, fewer large lymphocytes, and scattered moderate numbers of histiocytes. Immunohistochemistry tests for CD3, CD79a, and CD20, confirmed the lymphoma was T-cell-rich B-cell lymphoma. The lungs contained marked interstitial fibrosis with alveolar histiocytosis. Polymerase chain reaction test results of lymph node and lungs were positive for equine herpesvirus-5 (EHV-5), a gammaherpesvirus. Gammaherpesvirus infection has been associated with lymphoma and pulmonary fibrosis in other species. This report describes the association between EHV-5 and both pulmonary fibrosis and lymphoma.