Latent papillomavirus infection in cattle.

During a long term experiment designed to identify the contribution of bovine papillomavirus type 4 (BPV-4), environmental mutagens and immunosuppressants to the development of carcinomas of the upper alimentary tract of cattle, there was evidence of latent papillomavirus infection. Papillomatosis-free animals, when immunosuppressed either by feeding bracken fern or by azathioprine treatment, developed skin warts containing either BPV-1 or BPV-2. Skin warts appeared also in an immunocompetent animal at sites of damaged skin. It was concluded that the animals harboured latent papillomavirus which was reactivated by immunosuppression and/or physical trauma, causing skin warts. Papillomavirus DNA was also detected in lymphocytes of both experimental and control animals, suggesting that one of the sites of latency may be the circulating lymphocyte.