Melanoma Warn­ing Signs

Learn how to rec­og­nize and treat the•e com­mon •kin tu­mor•.

Practical Horseman - - Contents - By Elaine Pas­coe with Chris By­ron, DVM, MS, Di­plo­mate ACVS

Learn to iden­tify these com­mon equine skin tu­mors, so you know when to start to worry about them and how to choose an ap­pro­pri­ate treat­ment.

Alyssa David­son’s gray geld­ing, Goose, was 13 when she first no­ticed a dark growth on him. The North Carolina rider didn’t re­call see­ing the lump lo­cated on his pe­nis be­fore, but she couldn’t be sure it hadn’t been there all along. Could it be melanoma, she won­dered? How wor­ried should she be? Melanomas, a type of can­cer­ous tu­mor, are com­mon in gray horses. If one of these tu­mors popped up on your skin, you’d rush to the doc­tor—and rightly so since hu­man melanomas are ma­lig­nant and can quickly metas­ta­size, spread­ing to dis­tant sites in the body. But, says Chris By­ron, DVM, “Melanomas in gray horses ex­hibit be­hav­ior that is quite dif­fer­ent from melanomas in hu­mans.” Gray-horse melanomas com­monly grow slowly, if at all, for years. They may never metas­ta­size. That doesn’t mean they can be ig­nored, how­ever.

Over time, many of these tu­mors (more than half, ac­cord­ing to some re­ports) in­vade sur­round­ing tis­sues. Some­times they be­gin to grow rapidly and mul­ti­ply, and some­times they do spread through the body. Here, Dr. By­ron, an as­so­ciate pro­fes­sor of large an­i­mal surgery at the Vir­ginia Mary­land Col­lege of Vet­eri­nary Medicine in Blacks­burg, Vir­ginia, helps ex­plain what to watch for and when to take ac­tion. Melanomas de­velop in most gray horses as they age, so if your horse is gray you’ll likely be fac­ing this prob­lem at some point. About 80 per­cent of gray horses older than 15 years have at least one of these tu­mors, some re­search shows.

Alyssa knew that, and she knew most gray-horse melanomas aren’t dan­ger­ous. In early sum­mer 2017, when she dis­cov­ered Goose’s growth, she had just moved to a new area and hadn’t yet found an equine vet. She de­cided to mon­i­tor the tu­mor for a while—but as the sum­mer went by, it grew big­ger.

What You See and Why

Melanomas de­velop when cells that con­tain the dark pig­ment melanin (called melanocytes) pro­lif­er­ate. “Most equine melanomas grow in the skin and are read­ily vis­i­ble,” Dr. By­ron says. You may find them as clus­ters of firm, dark nod­ules or as soli-

tary black bumps un­der the tail, around the rec­tum or the gen­i­tals or around the mouth and eyes, he adds. But they can grow on skin in other parts of the body as well. Der­mal melanomas (those lo­cated in the skin) can be flat or raised, smooth or warty, smaller than a fin­ger­tip or (in time) big­ger than a fist. Less often, melanomas may de­velop in­ter­nally in ar­eas such as the ab­domen and the gut­tural pouches (air-filled sacs lo­cated at the back of the phar­ynx). “These in­ter­nal tu­mors are not typ­i­cally de­tected un­less they cause prob­lems,” Dr. By­ron says.

Why do gray horses get melanomas? You know that ex­po­sure to ul­tra­vi­o­let light in­creases the risk of hu­man melanoma and other skin can­cers. That’s why you use sun­screen and avoid too much time in the sun. It was once thought that gray horses must be ex­cep­tion­ally sen­si­tive to UV light. But it turns out that the risk to these horses comes not so much from sun­light as from their genes—the coded in­struc­tions within body cells that con­trol how ev­ery liv­ing crea­ture ap­pears and func­tions.

Re­searchers have tracked both gray coat color and high melanoma risk to the same mu­ta­tion on a sin­gle gene (la­beled STX17), which ex­plains why these tu­mors ap­pear so often in gray horses but so rarely in bays or chest­nuts. Other fac­tors prob­a­bly help de­ter­mine the melanoma risk for an in­di­vid­ual horse. For in­stance, other genes may act to raise or lower the odds that a tu­mor will de­velop. But a horse born with the gray mu­ta­tion will turn gray as he ma­tures and he’ll be at in­creased risk for melanoma as he does.

Melanomas are rare in horses with other coat col­ors, but in those horses the tu­mors are more likely to be dan­ger­ous

and to spread. Read more about those cases in the box on

When to Worry

Your gray horse’s melanomas may never pose a se­ri­ous dan­ger to him. But, says Dr. By­ron, “It’s im­por­tant to rec­og­nize melanomas and keep track of their growth.” The risk that the tu­mor will turn ag­gres­sive is al­ways there and even without metas­ta­sis, a large tu­mor in a sen­si­tive lo­ca­tion can be dis­as­trous, in­ter­fer­ing with defe­ca­tion or other body func­tions.

If you no­tice lumps that could be melanomas on your horse, Dr. By­ron ad­vises, “Bring them to your vet­eri­nar­ian’s at­ten­tion dur­ing rou­tine check­ups. If there are many melanomas, melanomas are large or the masses are grow­ing quickly, the vet­eri­nar­ian should ex­am­ine the horse sooner to de­cide if treat­ment is nec­es­sary.”

Ap­pear­ance and lo­ca­tion are telling signs and they’re the keys to di­ag­nos­ing and eval­u­at­ing most gray-horse melanomas. A biopsy can con­firm that a lump is melanoma and not some­thing else, like a sar­coid with su­per­fi­cial pig­men­ta­tion. Some­times cells from the tu­mor are as­pi­rated (drawn out) with a fine nee­dle and sent off to the lab for eval­u­a­tion. But, Dr. By­ron says, “The melanomas in gray horses have such a typ­i­cal ap­pear­ance that they may be treated without biopsy.” Be­sides, while histopathol­ogy (ex­am­i­na­tion of tis­sue sam­ples) can iden­tify an un­com­mon but highly ag­gres­sive form of the dis­ease called anaplas­tic ma­lig­nant melanoma, it won’t tell you if a seem­ingly in­no­cent der­mal melanoma will be­come ma­lig­nant in the fu­ture. Re­searchers are search­ing for bi­o­log­i­cal mark­ers (such as spe­cific cell pro­teins) that are as­so­ci­ated with ma­lig­nancy. For now, though, changes such as rapid growth or the ap­pear­ance of new melanoma clus­ters serve as red flags.

By fall it was clear to Alyssa that Goose’s growth was get­ting big­ger and she ar­ranged an exam by an equine vet­eri­nar­ian. The vet con­firmed that it was a melanoma and found sev­eral more tu­mors, a few tiny ones higher on the pe­nis and one un­der Goose’s tail. She ad­vised treat­ment.

Treat­ment Op­tions

“If melanomas are few in num­ber, in ar­eas that are not caus­ing prob­lems and are not grow­ing, they may not be treated im­me­di­ately,” Dr. By­ron says. “How­ever, melanomas that are grow­ing or are in an area that may cause prob­lems as they get big­ger should be treated quickly.” Main treat­ment op­tions are removal and chemo­ther­apy.

Removal. Re­mov­ing melanomas— through surgery, with laser treat­ment or with cryother­apy (freez­ing)—is the surest way to re­solve these tu­mors, at least while they are small. The larger and more in­va­sive a tu­mor is, the trick­ier it can be to re­move. Once a melanoma be­gins to in­fil­trate sur­round­ing tis­sues, get­ting all of it out may be nearly im­pos­si­ble. That’s wor­ry­ing be­cause in­com­plete removal can trig­ger in­creased growth or spread.

Chemo­ther­apy. Cis­platin and other chemo­ther­apy drugs can be in­jected di­rectly into melanomas to shrink the tu­mors. Chemo­ther­apy is often ad­min­is­tered along with surgery, but in­jec­tions of cis­platin beads have com­pletely re­solved the tu­mors in some cases. The treat­ment doesn’t cause the de­bil­i­tat­ing side ef­fects that peo­ple un­der­go­ing sys­temic chemo-

ther­apy often suf­fer.

Some ex­per­i­men­tal treat­ments, not yet widely avail­able, show prom­ise. They can in­clude:

Ther­a­peu­tic vac­ci­na­tion. A ther­a­peu­tic vac­cine (ONCEPT ® from Me­rial/ Boehringer In­gel­heim) trains the horse’s im­mune sys­tem to tar­get an en­zyme

The larger and more in­va­sive a tu­mor is, the trick­ier it can be to re­move. Once a melanoma be­gins to in­fil­trate sur­round­ing tis­sues, get­ting all of it out may be nearly im­pos­si­ble.

con­cen­trated in melanoma cells so the sys­tem will at­tack and kill those cells. “The vac­cine was de­vel­oped for use in dogs with melanoma, but some horses have re­sponded well to the treat­ment,” Dr. By­ron says. The horse gets an ini­tial series of four shots, fol­lowed by boost­ers ev­ery six months. The cost ranges from $400 to $600 per shot, and only vet­eri­nary on­col­o­gists or in­ter­nal-medicine spe­cial­ists can ob­tain the vac­cine. Use in horses is off-la­bel for now although a ver­sion for horses may be avail­able in the fu­ture. Re­search on an equine vac­cine is un­der way at the Univer­sity of Flor­ida.

High-fre­quency ir­re­versible elec- tro­po­ra­tion (H-FIRE). “Elec­tro­po­ra­tion ther­apy may also be a use­ful ther­apy. Re­search is cur­rently be­ing con­ducted on this treat­ment,” Dr. By­ron says. H-FIRE, a ver­sion be­ing in­ves­ti­gated at VMCVM, zaps tu­mor cells with in­tense bursts of elec­tric­ity. Each burst lasts only about 100 mi­crosec­onds and is de­liv­ered di­rectly into the tu­mor through tiny elec­trode nee­dles. The mi­crobursts de­stroy tu­mor cells by cre­at­ing mi­cro­scopic holes in their cell mem­branes, caus­ing them to “leak” their con­tents and die. The treat­ment may be used along with chemo­ther­apy. It’s done with the horse stand­ing and se­dated, us­ing a lo­cal anes­thetic, and is said to be no more painful than an in­sect bite.

Goose’s vet­eri­nar­ian rec­om­mended surgery, not­ing that if the main tu­mor grew much larger the horse might not be able to re­tract his pe­nis. And it was grow­ing—by De­cem­ber, when it was re­moved in a stand­ing op­er­a­tion at the barn, it was a raised lump the size of a sil­ver dol­lar. The vet was con­fi­dent that all of it was re­moved. Goose healed well, and a sec­ond surgery to re­move his smaller tu­mors was set for Fe­bru­ary.

But by then the other melanomas had grown and mul­ti­plied, even re­cur­ring at the site of the pre­vi­ous surgery. The vet

told Alyssa that on-farm surgery was no longer an op­tion and even surgery at a clinic was un­likely to be successful. Af­ter con­sult­ing with col­leagues, the vet sug­gested try­ing ONCEPT ® , the vac­cine.

Avail­abil­ity and cost were road­blocks for Alyssa, putting her in a bind that any­one whose horse has had se­ri­ous health prob­lems can sym­pa­thize with. Her vet was un­able to ob­tain the vac­cine, so she directed Alyssa to a clinic that could. Goose would need to ship to the clinic, about 100 miles away, for each of the four ini­tial shots—and the cost would be high, about $2,600 plus ad­min­is­tra­tion fees and trans­port costs. Al­ready fac­ing a pile of vet bills, Alyssa cast about for a closer and po­ten­tially less ex­pen­sive vac­cine source. Mean­while, she started Goose on cime­ti­dine in the hope that it might slow the growths. This anti-ul­cer med­i­ca­tion has been said to shrink tu­mor size, although hard sci­en­tific ev­i­dence is lack­ing.

What to Ex­pect

Dr. By­ron em­pha­sizes that the out­look for most gray horses with melanoma is ex­cel­lent, pro­vided that the tu­mors are treated early. But, he says, “These horses have a poor prog­no­sis if tu­mors are al­lowed to grow to a large size, are in ar­eas that are caus­ing other health prob­lems [such as trou­ble pass­ing manure] or are in ar­eas that can­not be treated [such as the gut­tural pouch or ab­domen].” If an ag­gres­sive melanoma metas­ta­sizes through the body to dis­tant sites, not much can be done for the horse.

“My horse’s melanoma case is not typ­i­cal,” says Alyssa, who blogs about her ex­pe­ri­ence at goose­back­rid­ As of this writ­ing, Goose’s story was still un­fold­ing. His tu­mors didn’t re­spond to cime­ti­dine (in fact, more popped up), but Alyssa had lo­cated a vet­eri­nary on­col­o­gist in her area with access to the ca­nine vac­cine. And she had steeled her­self to bear the cost. “It’s hard to just stand by for an in­jury or ill­ness when these horses are so much more to us than just a rid­ing part­ner,” she says. “I owe it to Goose to do what I can.”

RIGHT: Goose had a large mass re­moved from his pe­nis in De­cem­ber 2017, but more small clus­ters had reap­peared months later.

LEFT: Alyssa David­son’s geld­ing, Goose, was 13 years old when she no­ticed a melanoma on him. Nearly 80 per­cent of gray horses over the age of 15 have at least one of these tu­mors.

LEFT: Melanomas can vary in size and shape and often won’t pose prob­lems to a horse un­less they’re ag­gres­sively grow­ing or lo­cated in a sen­si­tive area where they can in­ter­fere with bod­ily func­tions, such as the rec­tum or gen­i­tals.

ABOVE: Most equine melanomas grow on the skin and are eas­ily seen though some can be hid­den on a horse’s mouth or un­der the tail.

Your vet­eri­nar­ian can take a biopsy of a sus­pi­cious lump to send to a lab and con­firm the di­ag­no­sis.

Many horses with melanomas have re­sponded well to ONCEPT ® , a ther­a­peu­tic vac­cine de­vel­oped for dogs. How­ever, the vac­cine is costly and must be ad­min­is­tered in sev­eral doses as well as boost­ers ev­ery six months.

Af­ter try­ing dif­fer­ent treat­ment op­tions with lit­tle im­prove­ment, Alyssa planned to use the new ONCEPT ® vac­cine as a way to con­trol Goose’s melanomas.

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