A pedunculated exophytic mass developed on the rump of a dog. Fine-needle aspiration revealed keratin debris suggestive of a follicular tumor. However, histology revealed a pigmented viral plaque that contained numerous keratin-filled cystic cavities. Canine papillomavirus 18 DNA sequences were detected in the lesion. Viral plaques are typically multiple sessile lesions of dogs. A viral plaque appearing as a solitary exophytic keratin-filled mass has not been reported previously, to our knowledge. The novel clinical findings in this case expand the ways that viral plaques may appear in dogs. In addition, the histologic findings represent a novel pathologic entity of dogs. Given that canine viral plaques can be progressive, and dogs typically develop numerous plaques, it is important to differentiate between a viral plaque and a hair follicle tumor.
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