Bit & Bridle

Equine Melanoma in Grey Horses

Other Names

  • Dermal melanoma
  • Dermal melanomatosis
  • Melanocytic nevi
  • Anaplastic malignant melanoma
  • Grey horse melanoma

What is Equine Melanoma in Grey Horses?

Equine melanoma is a type of skin tumour that develops from melanocytes — the pigment-producing cells responsible for coat colour. In horses, melanoma is overwhelmingly associated with grey coat genetics, making it one of the most common tumours seen in horses of this colour. Studies suggest that up to 80% of grey horses over the age of 15 will develop some form of melanoma during their lifetime.

Unlike melanoma in humans, which is almost always malignant, equine melanoma exists on a spectrum. Many lesions are benign and slow-growing for years, while others can progress to become locally invasive or metastatic. The tumours typically appear as firm, rounded, dark nodules and can develop singly or in clusters. Common sites include under the tail (perineal region), around the anus, the sheath and genitalia, the parotid salivary gland, the lips, and around the eyelids.

Understanding the genetic risks associated with grey horses can help owners plan proactive care strategies. See also our overview of Equine Insect Hypersensitivity, another condition with strong breed predispositions.

Causes

The primary driver of equine melanoma is the genetic mutation associated with progressive greying. The most significant identified factor is a 4.6-kilobase triplication in the syntaxin-17 (STX17) gene, which is strongly linked to the greying-with-age phenotype. As grey horses age, melanocytes accumulate in the skin abnormally, increasing the risk of neoplastic transformation.

Melanoma development is considered polygenic — multiple genetic factors are likely involved beyond STX17 alone. Ultraviolet (UV) radiation may play a secondary contributory role, particularly in horses kept in high-sunlight environments with limited shade. The condition is not contagious and cannot be transmitted between horses.

Symptoms and Signs

Early-stage melanoma is often detected incidentally during routine handling or grooming. Key signs to watch for include:

  • Firm, rounded dark nodules — typically grey-black in colour, ranging from a few millimetres to several centimetres in diameter
  • Clusters of nodules under the tail or around the perineum
  • Swelling near the parotid gland that may interfere with bridling or chewing
  • Lesions around the eye causing discharge or changes to vision
  • Difficulty urinating or defecating if lesions obstruct relevant anatomy
  • Rapid increase in lesion size — a warning sign of malignant transformation
  • Ulceration or discharge from a previously stable lesion

Internal metastasis can affect lymph nodes, spleen, liver, and lungs, though this is less common in early presentations. Horses with multiple rapidly-growing lesions or those near vital structures warrant urgent veterinary assessment.

Diagnosis

Diagnosis is primarily clinical — a grey horse with characteristic nodules in typical locations is strongly suggestive. Your vet may employ the following:

  • Fine needle aspirate (FNA) — a minimally invasive procedure that draws cells from the nodule for cytological examination, confirming melanocytic origin without full surgical excision.
  • Biopsy and histopathology — provides definitive diagnosis and grading of malignancy. Recommended for rapidly growing or atypical lesions.
  • Ultrasound — useful for assessing depth, extent, and involvement of underlying structures, particularly in the parotid region.
  • Radiography or endoscopy — used to assess internal involvement if metastasis is suspected.

Staging the lesions — noting their size, number, location, and rate of change — helps guide treatment decisions and monitoring intervals. The British Equine Veterinary Association (BEVA) recommends regular veterinary review for all grey horses with confirmed melanoma.

Treatment

Treatment depends on the lesion’s size, location, number, rate of growth, and the horse’s overall health. Options include:

  • Surgical excision — the most common approach for solitary or small lesions. Success depends on complete removal with clear margins. Lesions in the perineal region can be technically challenging due to anatomy.
  • Cimetidine — an H2-receptor antagonist historically used as a palliative option. Evidence is mixed, but some horses show stabilisation or slight reduction in lesion size. It carries low risk and may be considered when surgery is not appropriate.
  • DNA melanoma vaccine — a xenogeneic DNA vaccine, originally developed for canine melanoma, has been used off-label in horses. A four-dose protocol administered fortnightly has shown promise in slowing progression and in some cases reducing lesion size. It is not curative, and results are variable.
  • Electrochemotherapy (ECT) — combines a chemotherapeutic drug (typically cisplatin or bleomycin) with electric pulses to enhance drug uptake by tumour cells. Shows good results for accessible lesions, though availability is limited to specialist centres.
  • Intralesional cisplatin — direct injection of cisplatin into the tumour. Used for lesions not amenable to surgery. Requires repeated treatments and careful handling of the chemotherapy agent.
  • Monitoring without intervention — appropriate for slow-growing, non-obstructive lesions in older horses. Regular documentation of lesion size and number allows early detection of change.

Prevention

There is no way to prevent melanoma in grey horses — the genetic predisposition is inherent. However, proactive management can improve long-term outcomes:

  • Begin regular inspection from the time a horse is identified as grey, ideally from two to three years of age
  • Photograph lesions at each inspection to track changes over time
  • Provide shade and UV-protective fly rugs to limit sun exposure
  • Consider elective early removal of accessible lesions with your vet before they become large or complicated
  • Factor the genetic background into breeding decisions — grey-to-grey matings increase the likelihood of grey offspring

Prognosis

Prognosis varies considerably. Many grey horses live comfortably for years with stable, benign melanomas that require minimal intervention. The long-term prognosis for horses with multiple lesions or evidence of malignant transformation is more guarded. Lesions near the anus, urethra, or airway can significantly affect quality of life as they enlarge. Metastatic melanoma carries a poor prognosis. Early detection and proactive management are the most important factors in achieving a favourable outcome.

When to Call the Vet

  • You notice new or rapidly growing lumps on your grey horse, particularly around the tail, anus, genitalia, or face
  • A previously stable lesion begins to ulcerate, discharge, or increase noticeably in size
  • Your horse shows difficulty passing droppings or urine
  • There is swelling near the jaw or parotid region affecting eating or bridling
  • You observe lesions on a young grey horse — early intervention yields better results

Frequently Asked Questions

Are all grey horses at risk of melanoma?

Yes, to varying degrees. The genetic mutation driving progressive greying also increases melanocyte proliferation, making melanoma a near-universal concern in grey horses as they age. Not all grey horses will develop clinically significant lesions, but the risk rises substantially after the age of eight, and most grey horses over 15 will have some form of melanoma. Learn more about grey horse breed characteristics in our horse breed section.

Is equine melanoma the same as melanoma in humans?

There are similarities in that both involve uncontrolled melanocyte growth, but equine melanoma behaves quite differently. A significant proportion of equine cases remain benign for extended periods, whereas melanoma in humans tends to be more immediately aggressive. That said, equine melanoma can and does become malignant, particularly with age and in horses with multiple lesions.

Can melanoma be cured in horses?

A complete cure is uncommon, but long-term control is achievable for many horses. Solitary lesions removed early and cleanly can be effectively treated. For horses with widespread lesions, the goal shifts to monitoring, slowing progression, and maintaining quality of life. Discuss options with a vet experienced in equine oncology for the best outcomes.

Should I buy a grey horse knowing they may develop melanoma?

Grey horses make wonderful animals, and melanoma should not be an automatic disqualifier. It is sensible to have any grey horse examined by a vet before purchase, paying specific attention to common melanoma sites. Factor ongoing monitoring into your management routine and discuss the horse’s age and existing lesions as part of a pre-purchase examination.