Melanomas are most commonly associated with grey horses: many affected animals will enjoy long and successful careers with the tumours having little effect on their quality of life.
The tumours are usually benign and grow slowly, although in a small number of animals they will be an ongoing nuisance and can ultimately prove fatal.
Grey horses who are more than five or six years old are typical candidates for melanomas and approximately 80% of greys older than 15 years will develop a growth.
Usually they are noticed as firm, grey or black masses in one of several typical sites including:
- under the tail and around the anus
- on the head below the ear and behind the jaw bone
- sometimes on the genitalia
- less frequently on the limbs and neck
- occasionally on the eyelid or within the eye
Often owners will notice a small solitary nodule beneath the skin but over time, sometimes many years, other nodules may appear and the growths join up to form a multi-nodular mass that may ulcerate through the skin.
Types of melanoma
Melanomas are caused by a disturbance in melanin metabolism, but it is difficult to predict how a particular nodule is going to progress over time.
The vast majority of melanomas remain benign throughout the life of the horse and grow slowly over a number of years. They rarely spread, although the lump itself may increase in size and become a problem in some areas.
The less common sequence of events is that the slow-growing melanoma suddenly becomes malignant and spreads via the blood or lymph to other sites within the body such as the liver, spleen and lungs.
Very rarely, melanomas can be malignant as soon as they form and in these unfortunate cases, the condition progresses rapidly.
Most melanomas require no treatment at all, as long as their size and shape does not alter, and their position does not interfere with tack.
If they do require treatment; options include surgical removal and cryosurgery (freezing) depending on the location of the melanoma and the extent. There can be varying success with these treatments. We (your veterinary surgeons) will be able to advise you of the appropriate option.