A melanoma is one of most common skin tumours seen in a horse or pony.
Equine melanomas occur most often in grey horses, and it is expected that at least 80% of grey horses will get at least one melanoma in old age.
The chance of a melanoma appearing in a horse does increase with age - melanomas are more common in horses and ponies who are over 9 years old.
Melanomas can be divided into two different types, benign or malignant, depending on their appearance and activity.
Most melanomas found in horses are benign - once present these benign types of melanoma are not aggressive in their growth and may progress very slowly over several years requiring no treatment.
Most melanomas do not metastasise, or spread through the body, although they can occasionally change to a more rapid period of growth in aged grey horses.
A small percentage of melanomas are aggressive, with rapid increase in size, and spread to other parts of the body - these are described as malignant melanoma.
Melanoma in a grey horse is caused by excess melanin in the affected horse's skin.
Because the horse has the grey gene this excess melanin is not dispersed through the horse's hair as it normally would be, so it is formed into clumps of cells which are the skin tumours that we know as melanomas.
A melanoma can be soft, hard, single or occur in a cluster of multiple melanomas. They are usually foun around the horse's ears, eyes, sheath, face and under the tail and are easy to see.
Melanomas are usually coloured black or dark brown to gray, however some melanoma appear unpigmented.
The tumours are often located under normal haired skin, however they sometimes become ulcerated over and become infected.
Melanoma in horses can cause severe problems when they are malignant, or when they become of a size that they ulcerate, bleed and become infected.
When they are present internally equine melanoma sometimes grow so large they can cause severe weight loss in a horse or pony and/or colic.
Malignant melanomas do cause a threat to the life of a horse.
Benign melanomas although not life threatening can cause problems. If a melanoma is situated on the head in an area where a bridle, saddle, headcollar or rug might rub, as well as being uncomfortable for the horse, which can cause behavioural problems, infections can occur.
Benign equine melanomas can also cause problems with defecation and urination.
Not all melanomas require treatment - they may cause no problems for many years.
Treatments for more troublesome melanoma in horses include:
Your vet will be able to advise you on the most suitable treatment
Research is being carried out in the United States for a vaccine to combat equine melanoma.