PREMATURELY GRAY by Ken Keckler DVM
(As seen in the OPHA newsletter)
Does a silver-grey horse with an amazing white flowing mane and tail send thrills of fantasy through you? The thought of a chivalrous knight or maybe a powerful king riding that shining steed (or maybe it’s a fair maiden, or even Lady Godiva) has been your ideal since childhood. You’re not alone. It seems that very early in the domestication of the horse, humans began to selectively breed for grey equines. Unfortunately, this manipulation has also led to tumors in a majority of older gray horses.
A research team at Uppsala University in Sweden has identified a genetic mutation that causes premature graying in horses. This has been intentionally passed along through breeding. “It is a fascinating thought that once upon a time a horse was born that turned gray and subsequently white and the people that observed it were so fascinated by its spectacular appearance that they used the horse for breeding so that the mutation could be transmitted from generation to generation,” said geneticist Leif Andersson of the University of Uppsala in Sweden.
Usually born black, bay or chestnut, gray horses lose pigmentation very quickly and are white by seven or eight years of age. Different from a true “white” horse that has pink skin and blue eyes, gray horses have black skin and brown eyes. The black skin is colored by a pigment called melanin, produced by cells called melanocytes in the epidermis (skin). The mutation that causes the premature graying also causes increased deposition of melanin in the cells, which causes a chain of events leading to production of neoplastic (cancerous) melanocytes producing even more pigment. Soon there is a mass protruding from the skin, or seen under the skin. These are more prevalent at the perineum (around the tail, anus and vulva), eyelids, in the sheath, and inside the mouth. Also they can be found in the salivary gland regions and some lymph nodes. Usually these masses are benign, slow growing, and locally invasive. Occasionally melanomas can be metastatic, and spread to lymph nodes, lungs and other organs. Masses can be found in the abdomen, potentially causing abnormalities with nutrient absorption, movement of the GI tract and possibly leading to colic.
The typical melanoma is found in clusters under the tail and around the anus. These are usually not life threatening, but can rupture and drain black/gray material, or bleed. They can grow to enormous sizes, invading the normal tissue and potentially damaging the sphincter muscles. A single mass can be surgically removed, especially when small, but in most instances the masses come in numbers that make it impractical to take them off. When removed, the tumor is often spherical, with a thin capsule surrounding a dense black mass of pigment.
One situation where early surgical intervention is strongly recommended is a mass on the eyelid margin. If small, the melanoma can be removed and the lid margin, which plays a vital role in eye health, can be saved. As the mass grows, it distorts and pulls the lid away from the eye and it becomes impossible to correct.
It’s been estimated that greater than 75% of gray horses over the age of 15 years will develop benign melanomas, and a small number of those will become malignant. Unfortunately, there is not much one can do to prevent or stop melanoma production. There has been at least one study that suggested oral cimetidine may reduce the growth of melanomas, but clinical results have been mixed at best. Ask your veterinarian if you want to give cimetidine a try.
So, when considering a gray horse, remember that chances are high the beautiful coat could have an ugly mass growing somewhere. Most will not be a huge cause for concern, but will be unattractive to see, and difficult to deal with. If you are aware of the risk, and the rest of the horse is what you are looking for, melanomas do not have to be a deal breaker. They are just a reminder that with beauty comes a price.