Trouble spots

Melanomas affect grey horses almost exclusively. they occur, and how veterinary thinking on treatment is shifting

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A MELANOMA is a form of skin cancer most commonly seen in grey horses.

These horses (even when they have turned white) have black skin containing the black pigment melanin. Cells that produce this pigment - melanocytes - can multiply abnormally, producing the skin masses called melanomas. They can develop at any age and may even very rarely be present at birth. It is estimated that up to 80% of grey horses will develop at least one melanoma by 15 years of age.

In certain breeds the prevalence is very high, for example Lipizzaners and Pura Raza Española (PRE) horses, in whom the grey colour predominates. But in contrast to skin cancers occurring in people, there is no connection between solar radiation and melanomas in horses.

THE GREY COLOUR

THE mutation in the syntaxin-17 gene causing the grey colour probably arose after horses were domesticated. Because wild horses are prey animals, it is likely that there was strong natural selection for camouflage, and strong selection against the white colour.

It is thought that all greys worldwide trace back to a single common ancestor in whom the mutation arose. An analysis of 700 horses from eight breeds found that this genetic mutation was shared by all of them.

History is full of references to white horses, which were highly valued and often ridden by princes and kings. One of the earliest written records is by the Greek historian Herodotus, who wrote that the Persian emperor Xerxes kept “sacred” white horses 2,500 years ago.

Grey horses are usually born non-grey. Foals may be black, dark bay, dun or even chestnut. Progressive depigmentation of the hair coat will cause them to slowly “grey out” as they age, eventually becoming white.