Hello Brenton, I recently heard that grey horse melanomas are the subject of research for human cancers, as the equine melanomas do not develop into life threatening cancers, also that the grey horses that develop them are all related to one grey Arab that passed the gene on for developing grey horse melanomas. I have no idea if this is true as I did not know the person and did not question his evidence, and I can't find anything relevant when I did a quick search (not a scientific search).
Ruth, they certainly can't all be related to one gray Arab. There is zero Arab blood in most strains of Andalusian, for example -- yet Brenton and I have a friend in Adelaide whose Andy horses have been riddled with melanomas. And nor is it likely that gray QH's, TB's, or any other breed get their propensity for melanoma from one or more than one Arabian horse, since there is very little Arabian blood in these breeds. The prevalence of melanoma in gray horses is, rather, due to peculiarities of pigment-bearing cells in gray horses (of whatever lineage) that make them particularly susceptible to precancerous changes.
It is true that human cancer researchers have been interested in horse melanomas because they progress very slowly and are usually not the primary cause of the animal's death.
A vaccine against the development of these tumors in horses is nonetheless very welcome news. I myself would be reluctant to buy any gray horse, simply because of the high chance in most areas of the world and in most latitudes that the animal will develop melanomas. An effective vaccine would allay my concerns. -- Dr. Deb