Breed susceptibility to melanoma may not be totally true and it is the colour grey which is the most significant predisposing factor.

Most grey horses over 10-12 years of age will have at least some melanoma lesions. The large majority of affected horses have multiple lesions.

Where the horse is flea-bitten grey with chestnut or brown flecks, the susceptibility for melanoma appears to be much reduced. However there is a still a higher risk in these colours than in chestnuts or horses of other colourings.

Melanoma also occurs in non-grey horses, and cremello/albino horses are sometimes severely affected. When melanoma occurs in non-grey horses it tends to be more dangerous than in grey horses.

The Arabian, Lipizzaner, Andalusian and Percheron breeds appear to be more susceptible, whilst the naturally grey Erisakay pony appears to be a less affected breed.

Melanomas occur in the skin, ear and eye. The cells responsible for them (melanocytes) are normally responsible to the colour of the skin and therefore most lesions are found in the skin.

If a melanoma is found internally it is due to spread from a malignant melanoma of the skin

The most common site for melanoma is the skin of the perineum (around the anus and the base of the tail). More than 50% of melanoma tumours are situated in and around the perineum.

Melanoma can also occur in the following areas:

• Eyelids, iris and retina
• Mouth, in particular the lips
• Parotid salivary glands and lymph nodes
• Penile and vulval skin
• Internal organs, including the intestine, heart and lungs.

Melanoma may have little effect on an individual organ until it reaches a size that interferes with the function of that organ, however in some sites such as the eye and the spinal cord, even small tumours can have a devastating effect.

 

In humans melanoma is associated with exposure to sunshine and the risk is increased with sunburn. However in grey horses the locations where melanomas develop are usually not exposed to much sun! Whilst we know there appears to be a genetic susceptibility that is linked to the grey genes, little else is known about the cause of melanoma in horses.
Typically melanomas start as one cell changes that result in an expanding clone of abnormal cells. The trigger for cancerous change seems to be an abnormal accumulation of melanin pigment in otherwise normal cells. Changes to the genetic structure of the cells seems to be triggered by the abnormal and sustained presence of the pigment in the cells.

These early “tumours” are probably very benign.
Melanoma of non-grey horses, humans and dogs appear to be induced by a more conventional cancer mechanism involving a sudden genetic mutation in one cell following exposure to UV light or some other cancer inducing factor.

Melanoma’s main effect are due to their location and change to the structure of the affected tissue. Their size and location will influence how dramatic the effect is. Melanoma within the spinal cord will have dramatic clinical consequences even if the tumour is benign in nature as there is little room for growth. Melanoma in the heart can cause severe heart rhythm issues which may result in sudden death.

Massive melanoma in the perineal region can affect defaecation and urination. In mares melanomas may create reproductive difficulties with both mating and parturition.
Melanomas can result in an unacceptable distortion of the local anatomy in various organs. The most common site for this problem is in the parotid (throat) region below the ear. There may be enormous expansion of the tumour masses. Whilst melanomas at this site seldom cause significant functional problems, airway distortion and pressure can arise.

Melanoma masses are commonly found in the throat/parotid region where they may involve the lymph nodes and parotid salivary gland. They can become extremely large without causing any airway obstruction. These lesions usually represent secondary spread from a melanoma in the skin.

Any horse with melanoma in the skin should have its guttural pouches examined using an endoscope prior to treatment. The presence of melanosis or melanoma within the pouches indicates that multiple areas may be affected and the treatment plan may have to be radically altered.

Many grey horses will have at least some melanosis (black areas) visible on the lining of the guttural pouch. These look like tiny black spots and are most often on the outer surface.

Melanoma is most commonly found overlying the maxillary artery or other major blood vessels within the guttural pouch. It will often look like a splash of black paint. If the melanoma invades the blood vessels this can result in rapid and usually fatal bleed.

The affected areas may expand slowly over time or regress and “re-appear” at other sites. Any evidence of melanoma in the guttural pouch is a bad sign as it indicates that the melanoma has spread throughout the body.

Parotid melanomas are rarely ulcerated unless they are interfered with by surgical biopsy. The more serious and aggressive forms appear as spherical or unevenly spherical shaped tumours. Usually they have a shiny surface and are most often found on the walls of blood vessels.

Despite their size and aggressive behaviour, they seldom ulcerate. However on occasions they can bleed considerably and be confused with a diagnosis of guttural pouch mycosis (fungal infection).

The malignant forms of melanoma are usually highly aggressive with rapid spread of multiple tumours in all major organs and body cavities. Malignant abdominal melanoma may result in colic that is likely to require surgery to resolve. There may also be large accumulations of fluid in the abdomen and/or chest (peritoneal and/or pleural effusion). The fluid may be blood tinged and contain obvious melanocytic cells.
Symptoms will reflect the organ that is infiltrated. For example, kidney involvement may cause blood in the urine and melanocytic cells may be found in the cellular deposit from a urine sample.
Involvement of the spleen may cause bleeding into the abdomen with abnormal melanin producing cells present in the fluid.
Malignant melanoma in the chest may limit the space available for the lungs to inflate and may damage the nerves affecting the face and eye which run through the chest area.

Many melanoma lesions may not alter significantly for many years and may have no material effect on the horse. However, horses with melanoma should be checked at regular intervals. This should include careful assessment of the parotid salivary gland and parotid and retropharyngeal lymph nodes (often best checked by endoscopic examination of the guttural pouches). The melanoma lesions themselves should be measured to ensure that any change in size is accurately identified at an early stage. Using a tumour map will help ensure that you can identify and measure all existing lesions and identify new lesions.

It may take years before the melanoma lesions become unacceptable in appearance or start to affect the horse’s function or welfare. Many horses with melanoma die of an unrelated cause. However you cannot predict which horses will be fine with the melanoma being left untreated and which will quickly develop spread and serious disease.

Surgical excision

Surgical removal of melanomas is recommended when they can be removed without significant skin damage or subsequent scarring. Individual, ulcerated or pedunculated tumours are usually amenable to surgery. Tumours in the eye or eyelid that are dramatically limiting vision or causing significant discomfort are usually better removed. However where removal of the melanoma would result in complications (such as eyelid or mouth) it may not be possible to remove them due to an inability to close the resulting wound. Extensive penile melanomas can however be successfully  removed from breeding stallions without any secondary consequences.

Cryosurgery

Cryosurgery involves freezing the lesion by a series of cycles. However it is rarely effective in melanoma lesions. Melanomas that have ulcerated and which bleed significantly can sometimes be managed by careful and repeated cryosurgery. Cryosurgery of eyelid melanoma is not recommended because of scarring. Combining cryosurgery with local chemotherapy may provide a better overall effect.

Intralesional chemotherapy

Some chemotherapy drugs can be injected into the tumour. The location and size of the tumour determines if this is option is suitable.

Melanoma vaccines

Vaccines are available which may help promote the horses immune response to the tumours. However the vaccine is expensive and requires specialised equipment for its administration.

Radiation

Radiation treatment is surprisingly poor at treating melanomas. High doses of gamma radiation delivered via a linear iridium ¹⁹² source had no material effect on one case treated. Similarly, beta radiation also had no effect when applied to melonomas on the lower eyelids of a horse.

Grey horses with uncomplicated skin melanoma have a good prognosis. Many affected horses live normal lives and it is often simply the cosmetic and social appearance of the tumours that result in the animal being destroyed. However, the disease is unpredictable. Even small tumours in “inconvenient” sites could have a profound effect on the animal; for example, a large melanoma in the thoracic inlet could cause recurrent choke and/or neurological effects.

Spinal, ocular, cardiac, or brain tumours may be catastrophic while others may cause clinical disease (colic or cardiac dysrrhythmia) and reduced athletic performance, that is not immediately attributable to the melanoma. Not all these are treatable and so may threaten the life of the horse. A few cases are highly malignant and it is currently impossible to predict which horses will be affected.