A Vaccine for Melanomas?

Some 80 percent of grey horses will get melanomas at some point during their lives. Megan Rust takes her chances with a new vaccine called Oncept.

Top photo via Wikimedia Commons

From Megan:

In my first article about melanomas — tumors which grow on the hairless areas of a horse — I said that 80% of grey horses will be affected by them at some time in their lives. My grey Canadian Warmblood mare, Juno, has some. The tumors are usually seen in mid-life or later, though I have a friend with a grey Connemara mare who had some removed when she was only eight. Juno will be 18 this summer, but I’ve been fighting the tumors since she was 11 or 12. Last fall I took Juno to a vet hospital for a laser debriding of more than 17 of them, after my vet had used a scalpel to remove some on three different occasions. My vet said she could not keep up with the  growth of the tumors, hence her suggestion for the laser surgery.

But the laser surgery barely slowed the growth of the melanomas. I was running out of ideas when my vet advised me to use a new melanoma vaccine as a “Hail Mary” pass at getting rid of the tumors. Originally proven effective for treating melanomas in dogs, Oncept was the first approved therapeutic vaccine for the treatment of cancer — in either animals or humans. Last year Lincoln Memorial University (Tennessee) proposed a study wherein the vaccine was used in gradually escalating doses in horses that had been diagnosed with melanoma. The results were promising: From a LMU report, “Virtually every horse has demonstrated substantial improvement following treatment. In some cases, benefit has been seen as early as two weeks after initiation.”


I’d tried Cimetidine, I’d tried an herbal supplement that actually has worked on some horses, and I tried having the melanomas removed — the solution suggested by the vet school at UC, Davis — to no avail. I had nothing to lose, so I asked my vet to order the vaccine.

Well, I guess I did have something to lose: the contents of my wallet. The melanoma vaccine is new, and it’s not cheap. Each dose costs $500, and four doses, given at intervals of two weeks, runs $2,000. Add to that the vet’s farm call and administration fees and you’re looking at $2,600. But I could not leave the tumors to run wild. In the past, the tumors were left alone, but the new medicine says to get rid of them as soon as they are seen because the smaller tumors are easier to remove. If they grow, they can affect bodily functions that could lead to death. I had no choice.

I wouldn’t have minded paying so much for a surefire vaccine, but this one is not 100% preventive. Twenty-five percent of horses who get the vaccine have the tumors vanish entirely, but in 50% there’s only a halting of the tumor growth. The horse doesn’t get any more tumors, but they’re still there. In the remaining 25% there is absolutely no effect at all. That means that there’s a 75% chance that SOMETHING positive will happen, but there is still a 25% that Juno won’t see any help from the vaccine.

The vaccine is administered by a needle-less syringe called a jet injector on one of the pectoral muscles of the horse. A small patch of the hair on Juno’s chest was shaved, and my vet applied the vaccine with the Vet-Jet syringe. My vet said that the administration of the vaccine feels like a rubber band being snapped over your skin, and yes, she did it to herself with some saline solution before she tried it on a horse. The air syringe is noisy, so I had to talk to Juno rather loudly right before she got the shot to prepare her for the sound. She was fine with it — as she is with all of her shots, good girl! — and she got cookies during and after the application of the vaccine.




My vet takes photos of the melanomas every time she gives Juno a dose of the vaccine to keep track of what is happening. She says that results have been seen as early as after the first dose and as late as six months after the last dose, if it does work.

I’ve got my fingers crossed.

Editor’s Note: Post-publication, we were notified by reader Judy Nordmeyer about another exciting development in equine melanoma vaccine development. Judy writes, “Dr. Rowan Milner at the University of Florida College of Veterinary Medicine has developed a vaccine for treatment of melanoma in dogs, a highly fatal form of cancer. Reported results look promising after several years of studies. He has also created the vaccine used in horses WHICH IS A DIFFERENT VERSION OF THE VACCINE USED IN DOGS. Our expectation is that the melanoma(s) will not continue to enlarge after vaccination. It is our hope that they will in fact shrink or disappear.” You can read more about the study here.

Megan Rust lives in Port Townsend, WA, after spending 44 years in Alaska, where she worked as a professional pilot. As a teenager, she started and showed a Morgan-ish grade mare, but took a 30-year hiatus from horses after high school. She returned to a horsey life in 2003, and now she shares her time with a husband, a tuxedo cat, two Pomeranians, two WB mares, a Lusitano mare, and two miniature jennies.


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