Tail-base mass from a “horse of a different color”.

The study reports an unusual case of a non-grey horse developing an aggressive form of skin cancer known as amelanotic melanoma, primarily around the tail, eyes, and shoulders, and the subsequent treatments and observations.

Case Presentation

• The researchers studied a 14-year-old bay Thoroughbred gelding – a type of horse – that had developed a significant mass at the base of its tail. The unusual growth had been present for a year and had begun to enlarge recently. Additional unusual growths were observed around the horse’s eye and shoulder.

Diagnostic Findings

• The researchers took a fine-needle aspirate of the tail-base mass, a sample for microscopic examination. The sample showed highly abnormal cells, both round and polyhedral in shape, that contained dark green to black cytoplasmic granules. These were interpreted to be melanin, the pigment responsible for the color of skin, hair, and eyes.
• Further histological analysis showed the mass was composed of variably shaped, poorly pigmented cells identified as cancerous melanocytes, the cells responsible for producing melanin.
• Immunohistochemistry, a method of detecting proteins in cells, was applied. It showed the cells tested positive for vimentin and S-100 – markers typically associated with melanoma and neural tumors – but negative for pancytokeratin and Melan-A, markers typically found in melanocyte and melanoma cells.

Diagnosis and Treatment

• The cytological (cell-based) and histopathological (tissue-based) observations led to the diagnosis of amelanotic melanoma, a rare and usually highly malignant form of skin cancer that lacks the dark pigment melanin.

Treatment Outcome and Conclusion

• The horse received treatment with cimetidine, an antihistamine more commonly used to treat stomach ulcers in horses but which has shown some promise in managing equine melanoma. Despite this, the cancer continued to progress.
• The study reports a rare instance of amelanotic melanoma in a non-grey horse and highlights the unique correlation between cytological and histological findings. This case contributes to the broader understanding of cancer in horses and may aid in the future identification and treatment of similar conditions.