While calves are most commonly infected, and develop lesions that spontaneously resolve as described above, there is a definite correlation of types of tumours with strains of virus. In addition there is clear evidence that in areas where cattle graze bracken both epithelial and fibroblast papillomas may undergo malignant transformation. Quercetin has been identified as the co-mutagen present in bracken fern.

A similar role has been postulated for UV light acting as a co-carcinogen with papillomaviruses in the squamous cell carcinoma of the eye seen in Hereford cattle, and in carcinoma of the ear in merino sheep, particularly in the tropics [2].