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]

Mad About Horses
Join Dr. Chris Mortensen, PhD on an exciting adventure into the story of the horse and learn how we can make the world a better place for all equines.
Apple Podcasts Spotify Youtube
Mad Barn Equine Nutrition Consultants

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