Treatment options for melanomas in grey horses

It was the vet, Derek Knottenbelt, he of Liverpool Cream fame, who said it was a scandal that there had not been more research and progress made in treating melanomas in grey horses. After all, these can affect 80% of greys.

What are melanomas?

Melanoma is a type of cancerous tumour and one of a few different types of skin conditions which can affect horses and ponies. Melanomas seem to only occur in grey horses and not on the white patches of skewbalds and piebalds.

Melanomas in horses behave differently to melanomas in people in that they grow very slowly and can cause no interference to the horse’s life for many years. They may never spread or metastasize and are commonly benign but they should still be monitored carefully.

Sometimes melanomas can spread and invade the surrounding tissue interfering with normal physiological functions. Sometimes they multiply rapidly and spread throughout the body. A key determining factor seems to be the age at which they first appear; a horse in their mid to late teens with a few random lumps and bumps which are not very large is at far less risk than a horse under ten who is already exhibiting several clusters. 80% of grey horses over fifteen years of age will have at least one melanoma.

What is a melanoma made up of?

Melanomas occur when the cells that contain melanin which is a dark pigment multiply. These cells are called melanocytes.

In appearance, melanomas are usually a smooth solid nodule and they favour certain locations on the horse – the dock, around the rectum, the genitals or around the eyes and mouth. Melanomas can however also grow within the skin – dermal melanomas – and they can be present internally so in the abdomen and the guttural pouches at the back of the pharynx although this is less common.

Why greys and no other colour?

Theories about super sensitivity to sunlight have long been discarded by scientists as geneticists have discovered a single gene, STX17, which is a mutation present only in the skin of a horse with a grey coat colour. This grey mutation means there is almost no risk in horses of other colours but perversely, lumps appearing in horses that are not grey in colour are more frequently malignant and will need prompt investigation whereas melanomas on a grey horse are commonly benign.

How to confirm if a lump is a melanoma?

Confirmation of the identity of a lump is done by means of biopsy. Accurate diagnosis is important even with a grey horse as the lump could be something else like a sarcoid with superficial pigmentation. Treatment options will vary so don’t assume a lump on a grey horse is a melanoma without a veterinary diagnosis. A biopsy simply draws out some cells from the lump using a very fine needle and these are then sent off to a laboratory for analysis.

Isolated melanomas that are slow-growing are commonly benign and should be monitored although there is no way currently of determining if they will become malignant in the future. Scientists are currently researching ways to identify markers within the cells so that they can classify melanomas and track those who are likely to develop a malignancy.

Treatment options

With slow-growing melanomas that are small in number, it is a moot point whether to treat immediately or to leave them alone.

There is a school of thought which maintains that treatment can encourage spread. But a lot of it depends on how many there are and where they are. For example, a small melanoma in the girth area is likely to be treated simply because its location is going to cause difficulties with fitting tack. Or isolated nodules under the tail can be troublesome as they can ulcerate with the friction of the tail movement and are a huge attraction to flies in the summer months which in turn can cause infections.

One of the reasons why some melanomas are left alone is that they may have begun to spread into the surrounding tissue. Complex melanomas are difficult to remove in their entirety and incomplete removal can sometimes act as a trigger for rapid growth or spread of the tumour. However, early treatment of melanomas is usually deemed to be a good thing as it can slow down their rate of progress and prevent them becoming an obstructive problem in either the horse’s ridden work or his natural bodily functions such as passing droppings.

Melanomas can be removed physically if they are in a location where there is good access and the entire melanoma can be taken off. Cryosurgery, where the tumours are frozen off with liquid nitrogen, is useful for clusters where individual removal may not be possible. Chemotherapy agents are another option if access is difficult for surgery; these are injected straight into the melanoma but have yielded mixed results. Despite the nature of the drugs, they do not produce the harmful side effects associated with chemotherapy in humans. There are also safety issues surrounding the use and administration of these chemicals for people which can impact on practicality and also cost i.e. the horse has to be injected in a clinic rather than at home.

There is a vaccine licensed in the States called Oncept which is currently used for treating melanomas in dogs. It works by stimulating an immune response in the body encouraging the animal’s body to fight the tumours. It has the advantage of a whole-body approach rather than just targeting one particular melanoma. The vaccine has been used in horses who are vaccinated four times with two weeks between each injection. If the horse responds to the initial course then injections are repeated at intervals of six months.

Clinical trials in America are showing some promising results with a reduction in the growth of some tumours and even regression in certain horses.

Discuss treatment options with your vet and plan your approach but it is good to see that there are now some serious options undergoing research for this very common issue in grey horses.