Cutaneous plaques associated with a putative novel papillomavirus type in a horse.

Case history and clinical findings: A 6-year-old Thoroughbred mare developed multiple flat plaques, < 1 cm in diameter, on the left front fetlock. These were treated topically using 5-fluorouracil and resolved after 4 weeks. However, additional similar plaques developed on the left front pastern 5 months later. These lesions resolved within 3 months without treatment.

Pathological and molecular findings: One plaque that developed initially and one plaque that developed later were examined histologically. Both consisted of well-demarcated foci of moderate epidermal hyperplasia. Scattered throughout both plaques were cells showing evidence of papillomavirus-induced cell changes and the same papillomaviral DNA sequence was amplified from both lesions using PCR. As the novel sequence had 79.1% similarity to a partial sequence previously amplified from an equine cutaneous wart and 67.9% similarity to Equus caballus papillomavirus type 1, these results indicate detection of a putative novel papillomavirus type.

Diagnosis: Multiple cutaneous plaques due to infection by a novel papillomavirus type.

Clinical relevance: Unlike more typical equine cutaneous warts which generally appear as pedunculated and filiform masses, the lesions in this horse appeared as raised plaques. With the exception of aural plaques that are confined to the ears, localised clusters of papillomaviral plaques have not been previously described in horses. The lesions contained subtle histological evidence of papillomavirus infection and careful examination is required to differentiate these plaques from pre-neoplastic lesions. The plaques contained a putative novel papillomavirus type. These results increase the spectrum of papillomavirus-induced skin disease in horses.