TUMOURS are generally uncommon in domestic livestock, but small ruminants are unusual because two neoplastic diseases - ovine pulmonary adenocarcinoma (OPA) and enzootic nasal adenocarcinoma of sheep and goats - can affect many animals in countries and flocks where they are endemic. These diseases can be transmitted experimentally, but do not seem to affect cattle or other animals. They are contagious and caused by two distinct but closely related betaretroviruses. Both types of tumour arise from secretory epithelial cells that retain their secretory function after transformation. OPA is produced by a tumour in the lungs of sheep; the tumour has also been seen in goats and wild moufflon, but more rarely. The disease has been recognised in over 20 countries spanning Europe, Africa, the Americas and Asia, and in a wide variety of breeds. Studies in Britain and South Africa show that OPA accounts for almost 70 per cent of all sheep tumours. As discussed in this article, recent studies have thrown light on the aetiology, pathogenesis and epidemiology of OPA and this, in turn, has led to current research efforts to develop preclinical diagnostic tests for the disease.
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