Sheep pulmonary adenomatosis: a contagious tumour and its cause.

Sheep pulmonary adenomatosis (SPA) is a contagious lung tumour that can be transmitted experimentally. Two viruses have been associated with the disease, a herpesvirus and a retrovirus. All ovine herpesviruses are related antigenically and have been isolated only from SPA tumour tissues. They do not appear to cause the tumour and their association with SPA appears to arise from reactivation of latent virus in the respiratory tract. SPA tumour tissue and lung fluid contain a retrovirus that has properties similar to those of type B and type D retroviruses. Homogenates of tumour that contain this retrovirus can transmit SPA to experimentally inoculated sheep. Various retroviruses have been cultured from such tumours. Only one of these has properties similar to that of the retrovirus detected in the tumour and it can transmit SPA experimentally. The others appear to be isolates of the non-oncogenic ovine lentivirus, maedi-visna virus, and play no part in the aetiology of SPA.