Farm Ireland

Tuesday 20 February 2018

Confronting the grey area of melanomas

How to recognise and treat melanin-linked tumours

Caitriona Murphy

Caitriona Murphy

As the owner of a grey horse, the link between greys and tumours under the skin has always been at the back of my mind, and even more so as my hunter Max gets older.

Veterinary studies have shown that around 80-85pc of grey horses over the age of 15 will develop tumours known as melanomas.

Their development is thought to be linked with melanin, the pigment that makes some skin darker than others and which is abundant in the skin of grey horses.

Melanomas are solid, hard, usually round black tumours that are typically found under the tail and around the anus, on the head below the ear and behind the jaw bone. They are sometimes found on the genitalia, less frequently on the limbs and neck and occasionally on the eyelid or within the eye. While melanomas in humans, often referred to simply as skin cancer, are extremely serious, spread rapidly internally and have a poor prognosis, equine melanomas are less worrying.

Vet Sue O'Doherty, from the Horse Happy Veterinary Service in Co Wicklow, is often asked about melanomas in grey horses.

"Melanomas in horses are not to be confused with melanomas in humans as they are rarely malignant," says the vet. "Generally, melanomas in horses do not metastasise or spread in the same way.

"The significance of melanomas in horses is more to do with their location on the body and what impact they have on the body's function.

"For example, melanomas in the back passage might eventually affect the horse's ability to pass droppings.

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Similarly, a melanoma in the throatlash or parotid area of the throat might affect the horse's ability to swallow, eat or produce saliva. In mares, melanomas can affect their ability to strain during foaling.

"However, most of the time, equine melanomas are slow-growing tumours that do not affect the horses' lifespan," Sue adds.

Melanomas are recognised by horse owners as generally smooth, hard and non-painful lumps under the skin. Over time, these lumps can increase from the size of a pea to that of a large apple or even bigger.

The lumps are sometimes clustered together, particularly in the region under the tail and around the anus.

"The lumps start to cause problems when there is a big coalescing mass of them together and they interfere with bodily functions," says Sue.

When it comes to treating melanomas, the majority of vets are inclined to leave them alone. However, this is dependent on the growth rate, size and location of the melanoma.

If treatment is required, then there are a number of options, including surgery and drug treatment.

"Surgery depends on the size of the tumour and whether the location is amenable to surgery," adds Sue. "You need quite a wide margin around the tumour and this can be difficult in some areas, for example in the throat."

Treatment with Cimetidine, a drug used for treating stomach ulcers in humans, is another possibility.

"One study found that it reduced the number and size of tumours over a four-month period," says the vet.

"However other studies have not validated the findings and the drug is quite expensive."

Melanomas have also been treated with cryosurgery or freezing, which has variable results and is only suitable for certain locations on the body. Yet another treatment is to inject the drug Cisplatin directly into the tumour in an effort to kill off the problem cells.

"Generally speaking, however, most people leave melanomas alone unless they are causing a physical problem," adds the vet.

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