Melanomas are a type of skin tumor, or neoplasm, most commonly observed in aging grey horses. Up to 80% of gray horses aged 15 or older develop melanomas during their lifetime. [1][2][3]

While human melanomas are typically cancerous, melanomas in gray horses are often benign blemishes that remain dormant without causing problems. However, these tumors can metastasize in later life, posing a fatal risk without intervention. [3]

There are several treatments available for melanomas in horses, including surgical removal, laser therapy and cryotherapy. Not all melanomas respond the same way to treatment, and new nodules may appear later. [4]

If you have a grey horse, regularly check their skin for any signs of melanomas. Should you find any, promptly consult your veterinarian for a definitive diagnosis and treatment.

Melanomas in Horses

Melanomas are a common type of tumor in horses that manifest as small nodules on glabrous (non-hairy) skin. They can appear individually or in clusters at different sites, such as beneath the tail in the perineal region, around the eyelids, or on the lips.

These tumors arise from melanocytes, which are cells responsible for producing the pigment melanin. [10] While melanomas in horses are typically benign and grow slowly, they can become malignant and invasive in some cases.

In some rare cases, these nodules may develop into open sores or ulcers. [2][3][5]

Although the exact cause of melanomas in horses is still under investigation, there is a genetic predisposition in gray horses.

Benign Melanomas

Benign melanomas are considered non-cancerous. This means that the tumor does not spread to other parts of the body, does not invade nearby tissue, and, in most cases, does not pose a serious threat to the horse’s health.

These melanomas often appear as small, firm, dark nodules under the skin and can occur anywhere on the body. While these tumors can grow slowly over time, they often remain localized and do not spread to internal organs. They are less likely to cause issues such as ulceration or bleeding. [3]

Malignant Melanomas

Malignant melanomas are considered cancerous, meaning they have the potential to metastasize (spread) to other parts of the body. In horses, these melanomas often present with irregular borders, rather than appearing as small, distinct nodules.

These cancerous tumors can infiltrate surrounding tissues and impair organ function, leading to health complications. In severe cases, malignant melanomas can be fatal for horses. [3]

Some researchers suggest that all melanomas, including benign ones, should be categorized as malignant, because they all have the potential to become malignant. [2] According to one study, an estimated 66% of melanomas eventually become malignant with enough time. [10] However, another study reported that just 14% become malignant and suggest that may be an overestimate. [26]

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Types of Melanomas

There are four main types of melanomas that can occur in horses. Understanding these different types is important because recommended treatments and outcomes vary among them. [4][7][9]

Melanocytic nevus

A melanocytic nevus, commonly referred to as a mole, is a type of benign growth on the skin formed by the accumulation of melanocytes. These masses can develop in horses of all breeds, colours and ages.

These lesions are usually well-defined and can be flat or slightly raised, presenting as dark pigmented spots or patches on the skin.

While these melanomas generally do not pose a health risk, any sudden change in appearance or size is a cause for concern and warrants a veterinary examination.

Dermal Melanoma

Dermal melanomas are the most common type of melanoma seen in horses. These benign tumors are found on the skin (dermis) and vary in size.

They are typically found in mature grey horses and frequently occur under the tail and around the anus.

Even though many dermal melanomas in horses remain benign, some have the potential to turn malignant and spread to other tissues.

Dermal Melanomatosis

Dermal melanomatosis is used to describe a condition where multiple melanomas or melanocytic tumors appear on the skin.

While dermal melanoma involves one or two discrete masses on the skin, horses with dermal melanomatosis usually have multiple lesions converging into a large mass.

Both types share histological similarities, meaning affected tissues exhibit common characteristics when examined under a microscope. [10]

Anaplastic Malignant Melanoma

Anaplastic malignant melanoma is a rare and particularly aggressive form of cancerous melanoma characterized by the presence of anaplastic (undifferentiated) cells. These cells grow and divide rapidly, making tumors of this type more aggressive and challenging to treat.

Because anaplastic tumors metastasize and invade nearby tissues, the prognosis for affected horses is generally poor.

Genetics

Melanomas in grey horses are believed to have a genetic component. These tumors stem from melanocytes, the pigment-producing cells in the horse’s body. As horses age and their coats turn grey, melanin metabolism is disrupted. [10]

In 2008, researchers identified a mutation in the STX17 gene responsible for the greying process in horses. Horses that are homozygous for this mutation, meaning they carry two copies of this gene, not only grey faster but also have a heightened risk of melanoma and vitiligo. [5]

This genetic mutation is especially prevalent in breeds such as Andalusians, Arabians, and Lipizzaners, which are frequently affected by melanomas. [6]

Melanomas in Non-Gray Horses

While melanomas are most commonly observed in gray horses, they can occur in horses of all coat colours. Horses with darker coat colours, such as chestnuts or bays, are more prone to developing aggressive, malignant tumors. [11]

This increased risk is likely due to the higher melanocyte activity in their more pigmented skin. Tumors typically develop around the trunk, abdomen, or legs of young horses. [7][8]

Clinical Signs

Clinical signs of melanomas in horses vary depending on the size and location of the tumor. Melanomas commonly appear as dark, firm nodules on the non-hairy parts of the horse’s body, including the following areas: [6][7]

  • Perineum (hairless area under the tail)
  • Tailhead
  • Lips, ears, and eyelids
  • Neck
  • Genitals

Some horses may also develop internal melanomas near the parotid lymph nodes. These lymph nodes are located in the region below the ear, extending toward the throat. [7]

Melanomas can grow larger and become ulcerated, resulting in the discharge of a dense black fluid. [11] If large melanomas develop around the perineum and anus, they can interfere with the horse’s ability to defecate.

Risk Factors

Recognizing risk factors associated with melanomas in horses is crucial for proactive management and early intervention. The main factors that influence a horse’s risk of melanomas include:

As grey horses age, their coats progressively lose pigmentation due to the depletion of melanin in theur hair shafts. This can impair the regulation of melanocytes, leading to their abnormal proliferation and the formation of tumors. [12][13]

Additional proposed risk factors include: [3][13]

  • Genetics and family history
  • Sun exposure (debated by researchers)

Diagnosis

Your veterinarian will diagnose melanomas through a clinical examination, utilizing imaging techniques and, in some cases, a skin biopsy. Additionally, they may examine common melanoma sites on your horse’s skin, such as under the tail, and palpate for any nodules or masses.

A procedure known as fine-needle-aspiration may be conducted to confirm the diagnosis and determine if the melanoma is malignant. In this procedure, a small tissue sample is obtained and submitted for cytological examination. [2][6]

Based on these results a veterinary pathologist will classify the melanoma as benign or malignant.

Differential Diagnosis

As part of the process for diagnosing melanomas, your veterinarian will rule out other skin conditions that have similar clinical presentations.

  • Cutaneous lymphosarcoma: Also referred to as cutaneous lymphoma, this is a type of cancer that originates in the lymphocytes (a kind of white blood cell) and primarily affects the skin. This malignancy can manifest as nodules, plaques, or ulcers on the skin. [8][14]
  • Sarcoids: These non-cancerous tumors appear as raised, scaly, or ulcerated skin growths of varying shapes and sizes. Sarcoids commonly affect areas such as the head, neck, chest, and limbs, and are typically seen in young horses. [8]
  • Pyogranulomatous Disease: This rare systemic inflammatory disease involves the formation of granulomas or clusters of immune cells in the horse’s organs and tissues. It can lead to severe skin inflammation and tissue wasting. [15]

Treatment

The best treatment for melanoma is determined by factors such as tumor location, size, behavior, and the horse’s overall health. Treatments may involve conservative management or more aggressive interventions. [16]

In many cases, melanomas may not require treatment. If the tumor is not causing discomfort or interfering with the horse’s bodily functions (i.e. defecation), it may be left untreated at your veterinarian’s discretion. [6]

In such cases, it is important to closely monitor the melanoma and notify your veterinarian of any changes in size or shape. Such changes could indicate that the melanoma has become malignant and requires treatment.

Work closely with your veterinarian to develop the best treatment plan for your horse, based on your goals and your horse’s condition. If the melanoma becomes cancerous, your veterinarian will help you decide whether to pursue curative or palliative therapy.

Surgical Removal

Localized melanomas that are accessible on the skin may be removed surgically, especially if they are causing discomfort or obstructing the horse’s anus and affecting defecation. This procedure involves excision of the tumor and surrounding tissue to prevent recurrence. [4][17]

To remove the melanoma the horse may be placed under general anesthesia or standing anesthesia while sedated. [27] For larger or internally situated tumors, advanced surgical methods might be required.

All surgical procedures carry a risk of side effects. Before proceeding with surgery, your veterinarian will conduct a thorough assessment to determine if surgery is warranted and help you understand potential risks.

Laser Therapy

Laser therapy is a precise, non-invasive medical procedure used to treat melanomas in horses. It uses high-energy light beams from a carbon dioxide (CO2) laser to effectively target and treat the tumor. [10][18]

These lasers emit a high-energy beam of light that is absorbed by water molecules in the targeted tissue. This causes tissues in the tumor to heat up and vaporize, resulting in the elimination of melanoma cells.

Laser therapy has a lower risk of causing trauma or infection in the horse. It results in less hemorrhaging (bleeding) than traditional surgical methods, contributing to a smoother recovery.

Cryotherapy (Cold Therapy)

Cryotherapy, or freezing, involves applying extremely cold temperatures to the tumor to freeze and destroy the melanoma cells. This procedure is typically performed by applying liquid nitrogen directly to the tumor. [19][20]

The liquid nitrogen freezes the tissue, damaging the cells and blood vessels within the tumor. This leads to cell death, or necrosis, and triggers the immune system to eliminate damaged cells.

This treatment is effective for small, superficial melanomas and for reducing the size of large tumors. Persistent or larger nodules may need multiple treatment sessions in conjunction with other therapies. [2]

Intralesional Cisplatin

Cisplatin is a chemotherapy medication that is used in the management of melanomas in horses. This cytotoxic agent disrupts the DNA of fast-dividing cells, inhibiting their growth and proliferation. [10]

Cisplatin is administered by direct injection into the tumor, enabling targeted delivery while minimizing systemic effects. Horses typically undergo four treatments spaced two weeks apart. [17]

The response to cisplatin varies. Some horses see a reduction in tumor size or a slower rate of growth, while others do not respond as effectively.

Cimetidine

Cimetidine is an H2 histamine receptor antagonist (H2 block) that stimulates the immune system and may slow down melanoma growth in horses. It is typically used with other treatments, such as surgical removal. [2]

Cimetidine is administered orally as tablets or granules mixed with the horse’s feed. In one study, it was shown to reduce the size and number of melanoma tumors in horses over several months. [21]

The response to cimetidine treatment can vary, with some showing limited or no significant response. More research is needed to understand the benefits of cimetidine for horses with melanoma. [22]

Melanoma Vaccine

Oncept is a canine melanoma vaccine used for the treatment of oral melanoma in dogs. The vaccine targets tyrosinase, an enzyme produced by melanocytes and melanomas. By targeting tyrosinase, the vaccine aims to stimulate the dog’s immune system to recognize and attack melanoma cells. [23][24]

The Oncept vaccine was the first USDA-approved therapeutic vaccine for the treatment of cancer in animals. It is used in conjunction with surgery and/or radiation therapy to improve outcomes in dogs with oral melanoma.

Horse melanomas also express the tyrosinase enzyme, and there has been some research into the off-label use of Oncept in horses. [24] However, the efficacy and safety of Oncept in horses are still topics of research. [25]

Prognosis

The prognosis for horses with melanoma depends on the type, location, size, and aggressiveness of the tumor. The age and overall health of the horse, as well as the treatment pursued, will also impact the outcome.

In general, grey horses with isolated melanomas that receive appropriate treatment have a good prognosis. For horses with other coat colours and multiple tumors, the prognosis is uncertain and guarded. [6]

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References

  1. Rieder S. et al. A Comparative Genetic Approach for the Investigation of Ageing Grey Horse Melanoma. J Anim Breed Genet. 2000.
  2. Moore J.S. et al. Melanoma in Horses: Current Perspectives. Equine Vet Educ. 2013.
  3. Seltenhammer M. et al. Equine Melanoma in a Population of 296 Grey Lipizzaner Horses. Equine Vet J. 2003. View Summary
  4. Billi T. et al. Surgical Excision of a Malignant Metastatic Melanoma Located in a Skeletal Muscle of the Lateral Thorax of a Horse. Vet. Med. Sci. 2020. View Summary
  5. Sundström E. et al. Copy Number Expansion of the STX17 Duplication in Melanoma Tissue from Grey Horses. BMC Genomics. 2012. View Summary
  6. Coyne C. et al. Skin: neoplasia – melanoma in Horses (Equis). Vetlexicon. Accessed Sep. 22, 2023.
  7. Poore L.A. et al. The Clinical Presentation of a Mid-Tail Melanocytoma with Sudden Malignant Transformation in a Bay Irish Draught Gelding: A Mid-Tail Melanocytoma with Sudden Malignant Transformation. Equine Vet. Educ. 2013.
  8. Villalobos A. Tumors of the Skin in Horses – Horse Owners. Merck Vet Manual. Accessed Sep. 22, 2023.
  9. Valentine B. Equine Melanocytic Tumors: A Retrospective Study of 53 Horses (1988 to 1991). J Vet Intern Med. 1995. View Summary
  10. Metcalfe L.V. et al. Malignant Melanoma in a Grey Horse: Case Presentation and Review of Equine Melanoma Treatment Options. Ir Vet J. 2013. View Summary
  11. Pimenta J. et al. Equine Melanocytic Tumors: A Narrative Review. Animals (Basel). 2023. View Summary
  12. MacKay R.J. Treatment Options for Melanoma of Gray Horses. Vet Clin North Am Equine Pract. 2019. View Summary
  13. Fleury C. et al. The Study of Cutaneous Melanomas in Camargue-Type Gray-Skinned Horses (2): Epidemiological Survey. Pigment Cell Res. 2000. View Summary
  14. Brown C. et al. Skin: neoplasia – lymphosarcoma in Horses (Equis). Vetlexicon. Accessed Sep. 30, 2023.
  15. Marsella R. et al. Sarcoidosis in Horses (Equis). Vetlexicon. Accessed Sep. 30, 2023.
  16. Spugnini E.P. et al. Electrochemotherapy for the Treatment of Multiple Melanomas in a Horse. J Equine Vet. Sci. 2011.
  17. Hewes C. and Sullins K. Use of Cisplatin-Containing Biodegradable Beads for Treatment of Cutaneous Neoplasia in Equidea: 59 Cases (2000-2004). J Am Vet Med Assoc. 2006. View Summary
  18. Groom L. and Sullins K. Surgical Excision of Large Melanocytic tumors in Grey Horses: 38 Cases (2001-2013). Equine Vet Educ. 2017.
  19. Jakobiec F.A. et al. Combined Surgery and Cryotherapy for Diffuse Malignant Melanoma of the Conjunctiva. Arch Ophthalmol. 1980.
  20. Coyne C. and Hollis A. Eyelid: neoplasia in Horses (Equis). Vetlexicon. Accessed Sep. 29, 2023.
  21. Goetz T.E. et al. Cimetidine for Treatment of Melanomas in Three Horses. J Am Vet Med Assoc. 1990. View Summary
  22. Laus F. et al. Evaluation of cimetidine as a therapy for dermal melanomatosis in grey horse. Israel J Vet Med. 2010.
  23. Pellin M.A. The Use of Oncept Melanoma Vaccine in Veterinary Patients: A Review of the Literature. Vet Sci. 2022.
  24. Lembcke L.M. et al. Development of Immunologic Assays to Measure Response in Horses Vaccinated with Xenogeneic Plasmid DNA Encoding Human Tyrosinase. J Equine Vet Sci. 2012.
  25. Phillips J.C. et al. Evaluation of Needle-Free Injection Devices for Intramuscular Vaccination in Horses. J Equine Vet Sci. 2011.
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