Ovine pulmonary adenocarcinoma: A transmissible lung cancer of sheep, difficult to control

Ovine pulmonary adenocarcinoma (OPA) is a contagious lung cancer of sheep caused by jaagsiekte sheep retrovirus (JSRV), which induces the transformation of secretory epithelial cells of the distal respiratory tract. The disease has been recorded in the major sheep rearing areas with the exceptions of Australia and New Zealand, and is clinically characterized by a chronic respiratory process that often involves the production of pulmonary fluid that is discharged from the nostrils in final stages of the disease while the “wheelbarrow” test is performed. The respiratory route has been accepted to be the most important natural mode of transmission for JSRV but colostrum and milk can transmit JSRV to lambs. In OPA affected flocks, under endemic conditions, JSRV infected sheep may be present in a high proportion but very few of them develop clinical signs. Such apparently healthy animals are responsible for the spread of the disease because is difficult to identify infected animals or preclinical stages. There are not serological diagnostic tests due to the lack of a detectable immune response in JSRV infected animals, and although blood molecular tests are available their sensitivity is low. For this reason the disease has failed to be controlled so far and continues to cause significant economic losses. Control strategies based on colostrum and milk management have been demonstrated to be effective but expensive for most commercial flocks. In the recent years efforts for controlling the disease have focussed on searching methods for early diagnosis and the next future seems to be the establishment of low risk status for OPA in the flocks. Nevertheless, working on improving the identification of infected animals is essential for the implementation of effective strategies for OPA eradication.