Melanomas are often associated with older grey horses: many affected horses continue to enjoy long and successful careers with the tumors having little effect on their quality of life.
The melanomas are usually benign and grow very slowly, although in a small number of horses they can be annoying and ultimately can be fatal. Typical candidates are grey horses older than 5 or 6 years old and approximately 80% of all grey horses will develop a melanoma when they are over 15 years old. Usually they are noticed as firm, grey or black growths on one of the following body areas :
- 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.
Melanomas around the tail and anus may become ulcerated and keeping these areas clean and hygienic is important. In addition, effective fly control in the summer months can help.
Types of melanomas
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.
Treatment varies according to the type of the melanoma. Over the years several different types of treatment have been tried, with varying levels of success. Surgery can be used for small solitary nodules but this is not always an option due to the size and position of the growth.
Cryosurgery , radiotherapy, chemotherapy and injections of BCG vaccine into the melanoma have all had mixed results. A practical, successful and reliable method of treating melanomas is yet to be found.