<p>Ovine pulmonary adenocarcinoma (OPA), also known as jaagsiekte, is a transmissible lung tumor of sheep caused by jaagsiekte sheep retrovirus (JSRV). OPA is first described in the early 19^th century" however, it has still not been possible to devise effective methods for controlling its spread and it remains an important problem in most countries where sheep are farmed. JSRV induces neoplastic transformation of alveolar and bronchiolar secretary epithelial cells and the resulting tumors can grow to occupy a significant portion of the lung. Tumor growth is frequently accompanied by the overproduction of fluid in the lung, which further compromises normal respiration. Lungs develop few multifocal consolidated slightly elevated gray to white masses. The presence of Jaagsiekte sheep retrovirus (JSRV) is usually demonstrated in the lungs by immunohistochemistry. JSRV protein is detected in all tumor epithelial cells, histologically normal alveolar type II cells, and few bronchiolar epithelial cells, alveolar macrophages, lymphocytes, and plasma cells. The period between infection and the appearance of clinical signs may be several months or years and many JSRV infected sheep do not exhibit clinical signs at all during their lifespan. This allows the spread of OPA into new flocks through contact with infected but apparently normal animals. This is due to the absence of an immunological response to JSRV in infected animals, which has hindered the development of serological diagnostic tests and vaccines. It can cause about 80% loss of flock for first exposure and 20% for endemic area.</p>