Newspeak- November 2017
 

Search for Cure of Melanomas in the Gray Horse

New treatments and research concentrated on stopping the growth of melanomas in the gray horse are continuing.

 

For those buying a gray horse today, chances are any melanoma it might develop would be more a nuisance than a death sentence, thanks to one or more vaccines on the horizon.

When a horse “grays out,” its coat hairs lighten while the skin beneath remains black. Pigments called melanins are what give skin and hair their color. As the hair coat loses its dark pigmentation, something—possibly ultraviolet light that can now reach the skin beneath—triggers the melanin-producing cells to begin reproducing abnormally and eventually produce these tumors we call melanoma.
 
Compared to melanoma in other species, particularly humans and dogs, the condition in horses is less serious. The human and canine versions are aggressive and often fatal. In horses most melanoma lesions are benign and slow-growing, and while it’s common for them to appear nastier than their effects, they’re not harmless. Some tumors grow internally, but usually the first sign of a problem is the appearance of external dark, rounded masses.

Melanoma tumors can develop anywhere on the horse’s body, but they are most commonly found under the tail, in the sheath, in the croup region, in the throatlatch and around the jaw, or around the lips or eyes.

Generally, as long as a melanoma lesion isn’t affecting the horse negatively, veterinarians leave it alone. But even a benign tumor can cause problems if it grows so large it interferes with the horse’s ability to urinate, manure, or eat; or if it hinders its movement or otherwise impedes performance. When this happens, everybody wants the lesion gone—and this is where melanoma management gets tricky.

Gene therapy advances in both human and veterinary medicine could potentially lead to development of vaccines that actually “tell” the body to attack and destroy cancer cells. Unlike “shock and awe” treatment methods such as traditional chemotherapy, which causes undesirable collateral damage to healthy cells as it carpet-bombs the cancer cells, a cancer vaccine is a therapeutic vaccine “that creates a response so the animal’s body targets only the cancer cells,” says Hoyt Cheramie, DVM, MS, Dipl. ACVS, senior manager of large animal veterinary services at Merial Inc., who is based in Lexington, Kentucky.

Therapeutic vaccines differ from immunisations, such as those children receive to prevent diseases like polio and measles. In the case of a horse with melanoma, a targeted therapeutic vaccine would “create an immune response to the melanoma the horse already has,” Cheramie says.

Another advantage of a therapeutic melanoma vaccine would be that it’s just one injection given systemically, not what Phillips calls “the whack-a-mole approach” involved in surgical removal of lesions (although whack-a-mole has its merits: “I’ve never seen a horse re-present with a lesion in the same place” where one was removed, he says).
Researchers are pursuing several equine melanoma vaccine development avenues using gene therapy.

Results from two separate studies of other potential treatments for equine melanoma, both conducted in Germany, have been published in the past year. In the first, at the University of Veterinary Medicine Hannover, Foundation, researchers vaccinated 27 melanoma-positive gray horses with DNA vectors encoded with interleukin-12 and interleukin-18 (both proteins that help regulate the body’s immune response), alone or in combination with human glycoprotein or human tyrosinase—the “active ingredients” in ImmuneFX and Oncept, respectively. The researchers found that tumor size decreased significantly in response to the vaccine regimen, but they saw that the horses in the study did not demonstrate a measurable immune response against either glycoprotein or tyrosinase. 

In the second study, conducted at Martin Luther University, in Halle, researchers found that in vitro (in the lab) injections of a chemical compound and two derivative substances were effective against two equine melanoma cell lines, as well as against human melanoma. The compound, betulinic acid, is found in the bark of several plant species, notably white birch trees.

Existing surgical treatments still have an important place in the treatment regimen as the search for cures is clearly far from over.


 

 


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