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.

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).

Melanomas can also occur inside the eye. They develop from the iris which is the area of the eye around the pupil that gives the eye its colour. Intraocular melanoma is rarely malignant, but the physical size of the melanoma may compromise vision and function of the pupil and the surface of the eye (cornea) resulting in corneal oedema (cloudiness) and non-ulcerative keratopathy (inflammation).

Ciliary or iridal melanomas are the most common forms. They are often located in the side of the eye closest to the nose (nasal quadrants) at the base of the iris. Melanoma in the eye often has a blue-black appearance rather than pure black (this may distinguish them from iris cysts and granulae irridica that characteristically have a very black or dark brown appearance).

Primary iridal melanomas are usually very slow growing and can sometimes remain symptom free for life. In a few cases, contact between the tumour and the corneal lining (endothelium) can cause a significant keratopathy and with pain and opacity across the surface of the eye.

In a few cases the melanoma may expand into the vitreous (jelly within the eye) and cause serious visual defects, glaucoma (excessive pressure in the eye) or bleeding in the back of the eye.

Lesions on the retina which is reponsible for vision at the back of the eye are very rare and there is no information on the effects these small aggregations might have on vision.

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.