How dangerous are melanomas? [H&H VIP]

The tendency for grey horses to develop melanoma has long been recognised. Typically discovered during grooming, melanomas generally appear as small, isolated and slow-growing skin nodules. This is perhaps why they are often ignored unless they start to interfere with the horse’s lifestyle in some way.

But is it safe to assume that melanomas are of little consequence, or can they become cancerous if left unmonitored and untreated?

These “melanocytic” growths are an abnormality of skin cells that contain melanin, a natural pigment. Also termed neoplasms, melanomas are variably pigmented and may be grey, brown or black.

While they can develop in horses of any age or coat colour, they most commonly appear in mature greys. One survey indicated that more than 80% of greys over 15 years old are affected. It has been suggested that all grey horses would develop them eventually if they lived long enough.

Despite its widespread nature and long history — these tumours have been recognised for more than 200 years — relatively little is known about the condition.

The key question of whether melanocytic growths in horses are benign or malignant is surprisingly complicated and contentious.

Benign or malignant?

Historically, melanomas have been described as non-neoplastic pigment cell dysplasia (abnormal growth).

One theory is that the condition may be a pigment storage disorder. An ageing, greying horse is less able to produce pigment and pass it on to growing hair, so the pigment is instead stored in a slow-growing mass.

But the fact that there is a wider variability in growth presentation and progression than would be expected for a pigment disorder has led this theory to be redundant.

In recent times it has generally become accepted — and it is my opinion — that equine melanomas are neoplastic (new growths) with malignant potential.

That is, any melanoma can progress to a cancer that can spread, but not all of them will.

The percentage that might develop into malignant cancerous growths remains uncertain, although one author cites that at least 66% eventually do so.

As yet there is no clear way to predict which melanocytic growths will go on to spread through the body, a process called melanomatosis. My hypothesis is that progression from a discrete (individual) melanoma to melanomatosis depends on additional genetic events occurring.

Cancer is relatively rare in horses as a species, so it could be suggested that they are comparatively resistant to genetic mutation. The fact that the additional, sequential mutations necessary for progression to melanomatosis are less likely to occur in the horse may give us a treatment window.

Early removal of discrete tumours may prevent development of more aggressive forms of the disease.

A lesion of typically pigmented skin that develops on a grey horse in a commonly associated site may well be recognised as melanoma by the vet without further investigation.

Diagnosis is more difficult, however, in non-greys or when location or appearance is uncharacteristic.

If this is the case, a sample of cells can be taken from the lump with a hollow needle (aspirate) and sent to a clinical pathologist for analysis.

If an aspirate is not possible or fails to achieve diagnosis, a biopsy needs to be taken. The precise type and method of biopsy will depend on the individual circumstances of both tumour and horse.

A question of timing

When and whether a melanoma should be treated is one of the most difficult questions facing vets and owners. The disease has stages when the masses are small and solitary, during which little growth occurs. Lesions may exist for years, sometimes for the lifetime of the horse, and cause no clinical problems.

But lumps that grow and/or start to coalesce (form a mass) can be problematic if left unchecked.

Parotid melanoma in the throatlash area, for example, can grow so large that an affected horse is unable to flex at the poll, turn his head from side to side or, eventually, eat and drink in comfort.

Coalescing perianal tumours, under the tail, frequently become large enough to limit defaecation and cause faecal impaction. Perirectal tumours can outgrow their blood supply, leading to death of part of the tumour and subsequent ulceration and secondary bacterial infection.

Your vet will weigh up the growth rate and anatomical position of the tumour with possible risks and benefits of any potential treatment. The method chosen will depend on the availability of viable treatments and any potential complications, along with the horse’s age and proposed use.

Choosing a treatment

A range of treatments is available, but none is accepted as being ideal. Options include:

• Surgical removal: limited to smaller masses and certain anatomic locations. Modern imaging methods such as ultrasound, computed tomography (CT) and magnetic resonance imaging (MRI) now allow more detailed pre-operative tumour resection planning, enabling more complicated surgeries to be undertaken. This may not be possible, however, with advanced cases or those in trickier areas.
  A multi-disciplinary approach of specialists in surgery and internal medicine — akin to what happens in human oncology — enhances success rates in more complicated cases.
• Laser removal: excellent for discrete and generally small lesions under the tail and in other superficial locations.
• Cimetidine: limited evidence suggests that this human anti-histamine tablet may help treat lesions that cannot be removed surgically. I have used this as a last resort, but I am always guarded in how much it may help.
• Cisplatin: beads impregnated with a human chemotherapy agent are implanted surgically or injected in a sesame seed oil emulsion. A useful adjunct where an entire tumour cannot be surgically removed.
• Mitomycin C: another human chemotherapy agent that can be injected into the tumour.

Treatment complications

Complications can occur with treatment, the most common being infection, bleeding and the slow healing of surgical wounds. Operating around the anus may lead to faecal incontinence or retention if the anal sphincter is damaged.

Generally, the smaller the lesion the easier it is to deal with. When combined with the hypothesis that any melanoma could potentially become malignant, it is my opinion — as long as removal is relatively safe and in the horse’s welfare interest — that it is best to get rid of a mass before it becomes a problem.

Ref: H&H 19 February, 2015