Vulvar squamous cell carcinoma in situ within viral papillomas in an aged Quarter Horse mare

Papillomas are among the most common equine cutaneous neoplasms (Baker and Leyland 1975; Sundberg et al. 1977). Acquired cutaneous papillomas in horses are viral-induced neoplasms which occur in 2 distinct forms: viral papillomatosis and ear papilloma (aural plaques) (Scott and Miller 2003). Viral papillomatosis most often occurs in horses aged <3 years, and spontaneous remission usually occurs within 3–4 months (Scott and Miller 2003). Lesions occur most commonly on the muzzle and lips, and less commonly on the eyelid, external genitalia and distal limbs. Papillomaviruses, the causative agents of acquired cutaneous papillomas in horses, have been shown to have malignant potential in several species of animals with neoplastic progression to squamous cell carcinoma in situ (Bowen’s disease) and invasive squamous cell carcinoma (SCC). Human cervical and vaginal papillomas and squamous neoplasms are highly associated with human papillomavirus (HPV). These are characterised by low to high grade cervical/vaginal squamous intraepithelial neoplasms (CIN and VAIN, respectively) and invasive squamous cell carcinoma. However, only a few subsets of HPV (HPV-16, HPV-18, HPV6/11) have been identified in invasive squamous cell carcinomas (Daling et al. 2002). In dogs, a novel canine papillomavirus (CFPV-2) with unique tropism has been cloned in persistent cutaneous papillomas of the footpad and interdigital tissue (Yuan et al. 2007). When this virus is transplanted into immunocompromised dogs with X-linked severe combined immunodeficiency (X-linked SCID), the resultant papillomas can progress to malignant invasive and metastatic squamous cell carcinoma (Goldschmidt et al. 2006). Cynomolgus monkeys have also demonstrated papillomavirus-associated cervical and vaginal papillomas, CIN/VAIN, and invasive cervical SCC (Wood et al. 2004). Although papillomaviruses have been implicated in the aetiology of SCC in the aforementioned species, papillomavirus antigen has not been previously detected by immunoperoxidase staining in equine SCC, despite the similarity of distribution to that of equine squamous papillomas (Junge et al. 1984). This report describes the clinical and histopathological findings and treatment of an aged mare with persistent vulvar papillomatosis and carcinoma in situ. To the authors’ knowledge this is the first report of vulvar papillomatosis undergoing malignant transformation to carcinoma in a horse.