Melanoma in Grey Horses: Welfare Management of a Common Tumour

Melanoma is extremely common in grey horses, with the majority of grey horses developing tumours by age 15. While often slow-growing, melanomas can cause significant welfare problems when they compress vital structures or ulcerate.

Key Facts

Approximately 80% of grey horses over 15 years old develop melanoma; the condition is linked to the grey gene's progressive depigmentation mechanism.
Melanomas in horses most commonly develop around the perineum (under the tail), parotid salivary glands, sheath, and eyelids.
Most equine melanomas are benign and slow-growing; the main welfare concern arises when tumours grow large enough to impair defaecation, urination, swallowing, or eye function.
Malignant transformation occurs in approximately 10% of grey horse melanomas, with metastasis to internal organs causing systemic illness.
Cimetidine (an H2-receptor antagonist) was used historically to treat equine melanoma, but controlled evidence of efficacy is weak.
Cisplatin chemotherapy (injected directly into tumours) has demonstrated effectiveness for localised melanoma treatment.
Electrochemotherapy (combining bleomycin injection with electrical pulses) shows promise for treating equine melanoma with reduced systemic drug exposure.
Surgical removal is possible for small, accessible tumours but regrowth is common; careful patient selection and timing are important.

Welfare Considerations

Grey horse melanoma welfare concerns are determined by tumour location and size rather than the presence of tumours alone. A horse with small, stable perineal melanomas may have no welfare compromise at all. However, large or rapidly growing tumours around the perineum impair defaecation and can cause severe discomfort — constipation-like symptoms and straining are painful. Periglandular melanomas compressing the oesophagus or salivary gland cause dysphagia. Orbital melanomas threaten vision. Ulcerated tumours are painful and vulnerable to infection. Welfare monitoring requires regular physical examination of melanoma-bearing regions, with veterinary assessment triggered by any change in tumour size, appearance, or associated function. Proactive treatment of accessible, growing tumours before they reach sizes that impair function is the most welfare-positive approach.

What You Can Do

Examine all grey horses regularly for melanoma development, particularly around the perineum, under the tail, and near the parotid gland.
Request veterinary assessment of any melanoma that is growing rapidly, ulcerating, or causing signs of functional impairment.
Discuss electrochemotherapy or intralesional cisplatin with an equine oncologist for growing, accessible melanomas.
Monitor bowel function in grey horses with perineal melanomas — straining or infrequent defaecation warrants prompt veterinary assessment.
Include melanoma assessment in annual health examinations for all grey horses over age 8.