Development of multiple pigmented viral plaques and squamous cell carcinomas in a dog infected by a novel papillomavirus.

Canine viral plaques are uncommon skin lesions that are induced by papillomaviruses (PVs). Plaques are usually of little clinical significance in dogs, although they have been reported rarely to progress to squamous cell carcinoma (SCC). Here is described a 7-year-old mixed-breed dog that developed numerous darkly pigmented plaques up to 8 cm in diameter. Multiple ulcerated nodular masses were visible within plaques on the ventrum and axilla. The dog showed no clinical evidence of immunodeficiency and appeared otherwise healthy. Over the next 2 years, five surgeries were performed to remove 23 ulcerated masses that ranged in size from 2 to 5 cm in diameter. Five masses were submitted for histology, and all were SCCs. Each was surrounded by epidermis that contained histological features consistent with those described in canine plaques. Suggestive of a PV aetiology, massive numbers of large keratohyaline granules were present throughout the thickened epidermis. Additionally, koilocytes were focally present, and one sample contained a band of keratinocytes within the superficial epidermis that contained pale cytoplasm and marginated chromatin. From two samples, DNA sequences from a previously unreported PV were amplified, and immunohistochemistry confirmed the presence of PV antigen in both. The PV DNA sequences were most similar to those of canine PVs previously associated with plaque formation. The plaques observed in this case were unusual owing to their rapid growth, large size and frequent malignant transformation. It is unknown whether this unusual behaviour was due to the specific PV detected in this case or to host factors within the dog.