Cutaneous viral papilloma with local extension and subungual cyst formation in a dog

A viral-induced digital cutaneous exophytic papilloma was diagnosed in a 2-year-old, spayed, female Siberian husky dog with lameness. Digital pain and lameness persisted after removal of the initial papilloma, and the fifth lateral digit was subsequently amputated. Upon histologic examination of the digit, a de novo digital, cutaneous, inverted, viral papilloma and subungual cyst were diagnosed. The inverted cutaneous papilloma, located at the junction of the digital paw pad and ventral nail, extended focally through the nail into the subungual space, where an expansile cyst was formed. Cellular changes suggestive of papillomavirus infection were present in the epithelium of the original exophytic papilloma, as well as the endophytic mass and subungual cyst. Cytopathic effects included ballooning degeneration of keratinocytes, koilocytosis, irregularity of keratohyalin granules, and margination of nuclear chromatin. Numerous faintly basophilic to eosinophilic intranuclear inclusions measuring 10-15 microm in diameter were present within keratinocytes of the exophytic, endophytic, and subungual cystic lesions. Electron microscopy was performed on tissues from all lesions and revealed numerous 40-45 nm diameter hexagonal virions characteristic of papillomavirus that were arranged in crystalline arrays and dense clusters within affected nuclei.