Equine Melanoma: Managing Skin Tumours in Grey Horses

Melanoma is the most common skin tumour in grey horses, typically benign initially but capable of malignant transformation, requiring monitoring and welfare-focused management decisions.

Key Facts

Up to 80% of grey horses develop melanoma by age 15, making it the most common equine neoplasm by far.
Melanomas in horses typically appear under the tail, around the anus, genitalia, and parotid salivary gland.
Most equine melanomas are initially benign dermal melanomas; malignant transformation with metastasis does occur.
Grey horses have reduced melanin packaging in skin cells — melanocytes accumulate and proliferate abnormally.
Small, slow-growing nodules may require no intervention; large or rapidly growing tumours need assessment.
Treatment options include cimetidine (variable efficacy), surgical excision, cisplatin chemotherapy, and newer immunotherapies.
Tumours affecting defaecation, urination, or eating significantly impair quality of life.
Regular monitoring for rapid growth, ulceration, or functional impairment guides management decisions.

Welfare Considerations

Equine melanoma welfare impacts range from negligible (small subcutaneous nodules causing no functional problem) to severe (large tumours obstructing defaecation, urination, or swallowing). The perinanal and perineal location of most melanomas means that growing tumours can impair fundamental bodily functions, causing chronic pain, difficulty defaecating, and secondary skin inflammation from pressure and ulceration. Horses with advanced melanomas affecting quality of life face difficult management decisions about intervention versus palliation. The insidious progression over years means owners may underestimate how much functional impairment has accumulated. Regular veterinary assessment with functional scoring is important for grey horses from middle age onward.

What You Can Do

Monitor grey horses regularly for new or growing melanoma lesions, especially around the tail and anus.
Have new or rapidly growing tumours assessed by your vet rather than assuming they are benign.
Assess whether tumours are causing functional problems — difficulty defaecating or urinating requires prompt management.
Discuss treatment options with a veterinary oncologist for progressive or problematic tumours.
Consider cisplatin chemotherapy or intralesional treatment for accessible tumours before they reach critical size.
Make end-of-life decisions based on functional quality of life, not tumour size alone.