From Keratoma to Anaplastic Malignant Melanoma in a Horse's Hoof

Simple Summary Melanocytic origin neoplasms are most common in older and gray horses, but their nature may differ from those of non-gray horses. In this case report, we describe the repeated excision of a hoof lesion, which was initially diagnosed as a keratoma with melanocytic grains, and evolved over time into an anaplastic malignant melanoma. Melanomas in horses are most often associated with gray, older horses with an average age of over 16 years. Anaplastic malignant melanoma, however, can very rarely affect non-gray horses. Herein, we report a case of a 16-year-old Wielkopolski gelding with a chronic lameness caused by a mass in the hoof. The first resection of the lesion and histopathological examination confirmed the presence of a keratoma. The regrown mass and persistent lameness resulted in another mass resection. The second histopathological examination result suggested a neoplastic growth of melanocytic origin with a low histological malignancy. Less than 2 years after the first resection, the horse returned to the clinic with deformation of the hoof capsule and severe lameness. The result of the third histopathological examination indicated low-differentiated malignant neoplasm. The result of the immunohistochemically test indicates a tumor of melanocytic origin with high malignancy.