Treatment options for melanomas in grey horses

It was the vet, Derek Knottenbelt, he of Liverpool Cream fame, who said it was a scandal that there had not been more research and progress made in treating melanomas in grey horses. These tumours affect up to 80% of greys over 15 years old, making it a significant health concern for owners of this coat colour.

What are melanomas?

Melanoma is a type of cancerous tumour that appears primarily in grey horses and ponies. Unlike melanomas in humans, equine melanomas tend to grow very slowly and remain benign for many years. They typically develop around the tail dock, under the tail, near the anus, genitals, eyes, and sometimes inside the skin or internal organs like the abdomen or guttural pouches.

While many melanomas do not interfere with a horse’s quality of life, some can spread and invade surrounding tissues, potentially causing problems with bodily functions. Horses that develop multiple clusters at a younger age (under 10 years) are at greater risk of aggressive disease than older horses with isolated lesions.

What causes melanomas in grey horses?

Melanomas arise when melanocytes – the pigment-producing cells containing melanin – multiply abnormally. This condition is linked to a specific genetic mutation called STX17 found exclusively in grey horses. This mutation causes the distinctive coat colour but also predisposes greys to developing melanomas.

Horses of other colours rarely get melanomas, and if they do, those are more likely to be malignant and require urgent treatment. This genetic link explains why melanomas disproportionately affect grey horses and why benign behaviour is more common in them.

How is a melanoma diagnosed?

Diagnosis usually requires a biopsy, where a vet removes a small sample of cells using a fine needle for laboratory analysis. This confirms whether a lump is a melanoma or another condition such as a sarcoid that might require a different treatment approach.

Regular monitoring of slow-growing tumours is common practice, although predicting which will become malignant remains a challenge. Research continues to aim at identifying cellular markers that will help classify melanomas and inform treatment decisions.

Current UK treatment options

The contemporary veterinary consensus for treating melanomas in grey horses in the UK in 2025 combines several established and innovative approaches, depending on tumour accessibility, number, and location:

• Surgical Removal: When melanomas are accessible and operable, surgery is the preferred option. Complete excision with wide margins can reduce the chance of regrowth and complications.
• Locoregional Hyperthermia and Intratumoural Chemotherapy: A novel treatment available at the Royal Veterinary College Equine Referral Hospital utilises chemotherapy injections directly into the tumour combined with targeted microwave heat therapy. This is especially suitable for inoperable locations such as deep around the salivary glands or near the jawbones. Horses undergo three weekly treatments under sedation with promising outcomes for tumour reduction and quality of life improvement.
• Cryosurgery: Freezing off tumours with liquid nitrogen is effective for clusters where surgical removal is difficult.
• Chemotherapy Injections: Direct injection of chemotherapy agents may be used when surgery and cryotherapy are impractical. Although side effects typical of human chemotherapy are minimal, there are safety and cost considerations requiring clinic visits.
• Immunotherapy: The ONCEPT™ vaccine, originally developed for dogs, can be used off-label by specialised vets in horses. This vaccine stimulates the horse’s immune system to fight melanoma cells with some preliminary evidence of tumour shrinkage, though more research is needed.

Practical considerations in treatment decisions

Not all melanomas require immediate treatment, especially slow-growing isolated tumours causing no discomfort or functional problems. Treatment can sometimes provoke tumour spread, so decisions are individualised based on tumour size, number, location, and the horse’s age and general health.

For example, tumours under the tail may cause injury and attract flies, making treatment desirable. Similarly, tumours in areas where tack fits may interfere with riding.

Early treatment generally slows tumour progression and prevents complications affecting the horse’s wellbeing or performance. Working closely with your vet to create a tailored management plan is essential.

Genetic risk and breeding advice

Genetic testing is recommended for breeders to identify grey horses at higher melanoma risk due to specific gene alleles. Responsible breeding strategies can help reduce prevalence over time by avoiding pairing horses homozygous for high-risk genes.

Conclusion

Melanomas are common in grey horses but usually slow-growing and benign. Advances in veterinary medicine, including novel targeted chemotherapy with hyperthermia and immunotherapy, offer hope for better management of these tumours, especially in difficult locations. Ongoing research and responsible breeding practices promise to improve outcomes for grey horses affected by melanomas.

Discuss any concerns with your vet to choose the best plan for your horse.