# Histopathological and Virological Findings of a Penile Papilloma in a Japanese Stallion with Equus Caballus Papillomavirus 2 (EcPV2)

## TL;DR

This paper reports the first case of a penile papilloma caused by EcPV2 in a Japanese stallion, providing insights into the virus's presence and diversity in Japan.

## Contribution

The study presents the first documented case of EcPV2-associated penile mass in Japan and reveals genetic diversity among EcPV2 strains in the region.

## Key findings

- EcPV2 E6/E7 mRNA was detected in the epidermis, confirming a viral papilloma.
- The detected EcPV2 strain was genetically similar to foreign strains but distinct from a previously reported Japanese strain.
- The penile mass was non-malignant and non-recurrent but posed a breeding challenge.

## Abstract

Equus caballus papillomavirus 2 (EcPV2) is known to cause genital neoplasms in horses. However, reports on EcPV2 in Japan and Asia are limited. Herein, we present the histopathological and virological findings of the first reported case of an EcPV2-associated penile mass in Japan. The patient was a 22-year-old stallion with a history of breeding in Japan and abroad. Histopathological examination contained RNA in situ hybridization targeting the E6/E7 region and an immunohistochemical approach, and whole-genome sequencing was conducted within the viral examination. Proliferating epidermal cells were observed, and EcPV2 E6/E7 mRNA was detected within the epidermis, which was interpreted as viral papilloma. The detected EcPV2 virus was genetically close to foreign strains and different from the strain previously reported from a Japanese mare. This suggests that various types of EcPV2 might already exist among horses in Japan. Although the mass reported herein was not malignant based on histopathological findings and the absence of recurrence, its presence on the penis would be an obstacle to breeding. These results provide a better understanding of the pathogenesis and diversity of EcPV2.