Melanotic Tumors in Horses

Melanomas are reported to make up 3.8% of the total number of neoplasms diagnosed in horses. Melanomas occur in horses of all colors, but they are seen most commonly in gray and white horses older than six years of age. Approximately 80% of gray horses older than 15 years of age are affected by melanomas. Rarely, the tumors are congenital. A sex predication for melanomas has not been determined definitely. Melanomas can occur anywhere on the body, however, they have an externam predilection for the undersurface of the tail near its root, the perineal and perianal regions, the male genitalia, the head below the entrance to the pinna, the ear margin, and the paratoid salivary gland.

Our study wants to see incidence and histological type of equine melanoma in our studied cases.

INTRODUCTION

The first report of a horse melanoma belongs to GOHIER in 1813 (5). Primary locations are almost constant in black skin, with a smooth aspect and scarce fine hairs. The most affected body regions are: the inferior side of the tail, the lips, the anus, the vulva, the prepuce, the perineum (1, 3, 5, 6). Few cases with locations at the level of the ear concha (4) or in the external auditory canal (6) are reported. Usually melanic tumors occur in adult horses and especially in old one. Tumor formations with a nodular aspect are well circumscribed, adherent to deep tissues and less to skin. The diameter of nodules varies from several millimeters to several centimeters. Nodules are hard, they are not painful, and in time the skin that covers them becomes ulcerated, exposing a black mass and a black oily liquid (2).

Histological, early changes occur at the level of pilose follicles melanic pigment gradually accumulates and extends to the sweat glands; cells have a melanic content, they are either fusiform or dendritic, in chronic forms multinucleated cells occurs. Melanocytes appear as dendritic cells with a large, clear nucleus and cytoplasm with melanin granules; melanophages are large reticuloendothelial cells. At regional lymph nodes sinus level metastasis are obvious by lymphoendothelial cells, dendritic cells and multinucleate cells are found, whose cytoplasm contains melanic pigment.

MATERIAL AND METHOD

500 horses was clinically examined, from this 30 horses was white or grey; melanic tumors was identified in 5 subjects from this 4 in white or grey horses and one in sorrel. From necropsy department cases 6 was diagnosed with melanic tumors. Histologic exam was made in 8 subjects, 6 from necropsy and 2 biopsies. Sample staining was made in 10 % neutral formalin, and then processed by paraffin technique; 3 – 5 μm sections were colored HE, TM, PASS reaction, and in some cases discoloured with H2O2. Slides was examined to fix the diagnose and characteristic images were photographed.
Microscopic aspects where classified by OMS criteria.

RESULTS AND DISCUSSIONS

There were examined 30 white and grey horses and were identified 4 cases with melanocitic tumors. In one sorrel horse was identified a melanic tumor in external auditory canal. Blanc and grey horses presented melanocitic tumors in 13,33% of cases. Tumor localisation in external auditory canal is very rare, that in fact is noticed in specialty literature too (2, 6).