Melanomas are most commonly associated with mature 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.

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 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.

Treatment

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.

Small solitary nodules are often successfully treated by surgical removal, but in some cases, surgical removal is difficult due to the size or position of the masses.

Melanomas have also been treated with cryosurgery (freezing). This has had variable results and the treatment often has to be repeated. Other methods of treatment have been tried including radiotherapy, chemotherapy and injections of BCG vaccine into the lesion. All have met with mixed results.

A practical, successful and reliable method of treating melanomas is yet to be found. If any melanomas start to change unusually quickly, then it is advisable to contact your vet to assess the situation.

This article was originally published in Horse & Hound. Don’t miss this week’s vet clinic which focuses on headshy horses. Click here to subscribe and enjoy Horse & Hound delivered to your door every week and save up to 30%