|Mind The Miserable Melanoma|
By Stephen Champion, BSc (Hons) BVM&S MRCVS,
TUMOUR! A word guaranteed to strike fear into the heart of anyone who has a stake in the consequences. In veterinary medicine as well as human medicine, much is now known about the behaviour of tumours. Nevertheless, their diagnosis is almost inevitably followed by a period of misunderstanding, incomprehension and fear. In the last article in this series, we looked in detail at the sarcoid, in its many guises. This time, we shall concentrate on the miserable MELANOMA.Estimates suggest that more than 80% of grey horses over 15 will develop a melanoma! However, it affects many younger horses too, particularly the Percheron, Arab and Andalusian breeds, but not exclusively. Other coat colours can be affected too – but often in single sites over the body of legs. The melanoma is a tumour of the melanocyte cells¸ that live in the skin and produce the black skin pigment melanin. Like many ageing changes, the process involves a change (or mutation) in the cells, leading to tumour formation. One thing is pretty certain. Unlike the dreaded malignant melanoma in humans, (a product of the whole-body scorching achieved during the annual two weeks in Torremolinos), grey-horse melanomas do not appear to be linked to an overdose of the sun. (An unlikely consequence of a life in Scotland anyhow – this year excepting!) The tumours are usually obvious to see; are hard, soft or in clusters and found under the tail, or around the sheath, ears, eyes and face of the horse. They are usually black, but can manifest as the “great pretender”, appearing unpigmented or amelanotic, presenting a challenging diagnosis of what is supposed to be a black tumour! Sometimes the black variety can leak a copious amount of dark, sticky fluid, which can present a nightmare to extract from the tail hairs (personal experience!). Most of the time the melanoma remains a fairly benign, slow-growing tumour, which presents only cosmetic concerns. Oh, how reassuring it would be if they all remained that way. Within each tumour however, lurks the potential for a rapid change to malignant growth and the potential to invade the internal body organs of he horse – a life threatening and ultimately life-terminating event. And an event that can happen unheralded until the condition is too advanced for even palliative treatment to be appropriate. The author recollects his involvement in a particularly distressing case of a young mare with a melanoma of the skin of the mammary gland (the teats). Apart from the swollen teat and the gradual weight loss, there was very little else to see. Until she was humanely destroyed and a post-mortem examination was performed, that is. A horrendous sight lay before use, with hundreds of huge, vile, black masses invading the liver, spleen, lymphatics and internal body wall. How this mare could have been in a condition to be standing eating grass the day before, I shall never comprehend. So, beware the melanoma! Often benign and perhaps around for 10 plus years, they can suddenly, and so no reason apparent to us, convert to malignancy and spread rapidly around the body. A further caution! Some are malignant from their first appearance. I strongly recommend therefore that you obtain a veterinary opinion as soon as possible after a potential melanoma is sighted. And, assuming your horse has a melanoma, what can you and your vet do about it? MELANOMA MANAGEMENT Sometimes melanomas require no treatment at all, for the reasons described above. It may hang around for years causing no trouble. However, you must monitor it and check frequently for changes in size, shape, number and appearance. And don’t forget to get a good look under the tail in all grey horses. If your melanoma does, unfortunately, require treatment, here goes with some of the most common options. Surgical excision (removal): may be suitable for small, pedunculated (with a stalk) masses, say less than 3cm diameter. Some very small ones can even be “tied off” to fall off by themselves! Larger ones can be highly problematic as they frequently reoccur or the wound breaks down before it is healed and causes a real mess! Cryotherapy: that is, freezing to a very low temperature with liquid nitrogen. Usually the tumour is “debulked” first (surgical removal of a large bit of it) and then frozen to minus 20 degrees, two or three times. Often this procedure is repeated in the future to keep the tumours to a manageable size. Chemotherapy: using a drug called Cisplatin, which is injected into the tumour itself. Okay if the tumour is small but cannot diffuse far enough into the tumour if it is large. Often repeated every fortnight for a total of four applications, it can keep the tumour at bay for several months. Interestingly, as far as I know, horses do not develop “resistance” to the drug (as is common with many chemotherapy treatments) and so the treatment can be repeated as required, within reason. I attended an advanced session in equine cancer therapy at last-years’ Equine Veterinary Congress, as was made well aware, that chemotherapy for equine tumours is still in its very infancy! Here’s hoping things will soon advance in this field. Cimetidine therapy: one of the most promising advances in the treatment of melanoma since it was used first in the mid-1980’s. It is actually a human anti-ulcer drug,called Tagamet, so it could be of use if you are worrying excessively about your horse’s melanomas! It is estimated that up to 50% of horses will respond in part, and in these horses a reduction in size and progression tumours may be seen. I have several horses under treatment at present and subjectively at least feel that the disease progression has slowed down in some cases. We are not sure exactly how the drug works, but think it is likely to bolster the body’s anti-tumour defence mechanism, by blocking “T suppressor cells” – bet you’re glad you know that! Although offering a ray of hope for many owners, the drug is moderately (but not excessively) expensive and needs to be given orally in the form of (many!) tablets. One would hope to see an improvement after three months’ of therapy. If there is not, then other treatments may need to be attempted. If there is some response, then treatment may be discontinued once the tumour goes into remission and possibly other treatment modalities can be used alongside it. Vaccination: research is continuing in the States for a vaccine to combat melanoma. It is likely that it will be targeted against the metastases (the small bits of tumour that break off and spread around the body, causing further tumours), since they are not caused by a living bacteria or virus to target the vaccine against. I wouldn’t hold your breath just yet! Coming at the problem from a different direction, a research centre in Pennsylvania is offering an autochthonous vaccine, made from the cells of your horse’s own melanoma! The idea, I suppose, is to stimulate your horse to recognise and destroy its own tumour cells. The cost is $500 and according to the manufacturers “statistics have not yet been generated” to support its performance. Hmmm! I have no experience of this and it is mandatory to consult your own vet before contemplating anything of this nature! AND, IN SUMMARY I summary, melanomas are by no means a death sentence for your horse, and can hang around for many years causing no problems. However, they can convert to malignancy, which may spread both across the skin and to the internal organs. Suspicious lumps should always be checked by your vet, and monitored at least as frequently as your horse’s annual vaccinations. The best recommendation I can give in terms of treatment is to keep your expectations realistic and explore all the available options with your vet.