Melanoma in a Horse

The images below are from a 20 year-old gelding that presented for the complaint of straining to defecate. Examination of the horse's perineum revealed multiple, lobulated soft tissue masses that were surrounding the anus and under-side of the tail head region (Figures 1 and 2). These soft tissue masses are consistent with cutaneous melanomas in a horse.

Figure 1

Figure 2

In addition, the owner had reported that the horse appeared to be having difficulty seeing from the right eye. Careful exam of both eyes noted the presence of a soft tissue mass within the anterior chamber of the right eye (Figure 3). Although this mass was not biopsied for obvious reasons, it is likely that the mass is a melanoma. Scientific studies report that approximately 14% of cutaneous tumors in horses are melanomas making them relatively common, especially in grey horses. In addition, studies report that 80% of grey horses that are over the age of 15 have multiple melanomas.

Figure 3

The most common site for melanomas is under the tail head and perianal region. Less common sites include base of the ear, periorbital (around or near the eye), lips, and lower limbs. Diagnosis is typically made by simple physical exam however some melanomas grow below the skin surface and a biopsy may be necessary to confirm the diagnosis. Unfotunately, melanomas continue to grow in size and multiply in number. As that happens, melanomas may become an significant problem as in this case in which they are obstructing the horse's anus. If diagnosed early, complete surgical resection is strongly recommended! Alternative therapies include intra-lesional chemotherapy treatment and systemic treatment with cimetadine.