Prevalence and Prognostic Impact of Equus caballus Papillomavirus Type 2 Infection in Equine Squamous Cell Carcinomas in Western Canadian Horses

Equus caballus papillomavirus type-2 (EcPV-2) has been proposed as a causal factor in equine genital squamous cell carcinoma (SCC). This study had 2 objectives: first, calculate the frequency of papillomavirus (PV) and EcPV-2 infection in papillomas, carcinomas in situ (CIS), and SCCs in Western Canadian horses; and second, determine if EcPV-2 status of equine SCCs is associated with overall survival (OS). EcPV-2 status of 115 archived tissue samples, spanning 6 years, was determined using broad spectrum (MY09/11) and EcPV-2-specific polymerase chain reaction (PCR) assays, EcPV-2-E6/E7 chromogenic RNA in situ hybridization (R-ISH), and amplicon sequencing. A retrospective survey gathered data on history, outcome, breeding, treatment, and rationales of referring veterinarians when managing PV-associated diseases. Histologic grade and completeness of surgical margins of SCCs were also considered. EcPV-2 DNA was identified in 10/58 (17%) SCC, 8/27 (30%) papillomas, 0/5 CIS, and 0/11 lesions identified as “other.” Overall, 18/101 (18%) of these lesions were positive for EcPV-2. EcPV-2 was identified in 10/35 (29%) SCCs arising from genital tissues but in 0/22 SCCs from other locations. There was no association between breeding history and EcPV-2 status of genital SCCs. EcPV-2 status of genital SCCs was not associated with OS (P = .76). The strongest negative predictors of OS were a lack of treatment (P < .01) and recurrence post-treatment (P < .01). Weaker predictors of OS included older age at time of diagnosis (P = .02). Completeness of margins at surgical excision, concurrent disease, treatment type, anatomic location of the SCC (anogenital vs other), and histologic grade of the SCC did not influence OS (P > .1).