Un­der­stand­ing Equine me­lanoma

Dressage Today - - Content - By Jody Hall­strom, DVM

When most peo­ple hear the term “me­lanoma,” they think of skin can­cer caused by ex­ces­sive sun ex­po­sure. in horses, how­ever, melanomas are as­so­ci­ated with coat color, specif­i­cally the gray coat color. Up to 80 per­cent of gray horses over the age of 15 will have at least one vis­i­ble me­lanoma. The gene that is re­spon­si­ble for the gray color is a dom­i­nant gene that causes pro­gres­sive de­pig­men­ta­tion of the hair. Th­ese horses can be born any color—black, bay, chest­nut, even pinto, but the col­ored hairs are re­placed with gray hairs as the horse ages. The first signs of gray hairs are found around the eyes or muz­zle in foals and can be seen the day they are born. young horses with mixed gray and col­ored hair may be con­fused with roans. a true roan main­tains the orig­i­nal base color on the head and legs and will not con­tinue to lighten as he ages. an aged gray horse might be called white, but true white horses have pink skin. a gray horse will have black skin and dark eyes.

how does this re­late to melanomas? melanin is a dark pig­ment pro­duced in the skin by cells called melanocytes. it is thought that a dis­tur­bance in melanin me­tab­o­lism as­so­ci­ated with pro­gres­sive gray­ing of the coat results in an over­pro­duc­tion and ac­cu­mu­la­tion of pig­ment within the cells. in time, th­ese cells un­dergo a neo­plas­tic trans­for­ma­tion and be­gin to form tu­mors. Th­ese tu­mors typ­i­cally present as firm, dome-shaped, black masses in the hair­less areas of the body. They are com­monly lo­cated on the un­der­side of the tail, around the anus or vulva, in the sheath or on the ud­der. They can also be found in the cor­ners of the lips, around the eyes or in the throat­latch region. While most melanomas cause no clin­i­cal signs, some can be prob­lem­atic due to size or lo­ca­tion. Tu­mors in the lip cor­ners can make a horse re­sis­tant to the bit. large masses in the throat­latch can make neck flex­ion dif­fi­cult. larger tu­mors can ul­cer­ate and at­tract flies and some will dis­charge a black, tarry sub­stance. mas­sive ac­cu­mu­la­tions of tu­mors around the anus can oc­cur, which may in­ter­fere with defe­ca­tion.

The ma­jor­ity of melanomas ex­hibit slow growth for years with no metas­ta­sis Hspread­ing through­out the bodyI. This is why melanomas are gen­er­ally con­sid­ered to be be­nign in horses. Some melanomas that have been static for years may sud­denly grow and metas­ta­size. a smaller per­cent­age of melanomas will ex­hibit rapid growth and metas­ta­sis from the on­set. melanomas found in non-gray horses are gen­er­ally con­sid­ered to be highly ma­lig­nant. metas­ta­sis can oc­cur in re­gional lymph nodes near the orig­i­nal tu­mor or in body cav­i­ties or in­ter­nal or­gans Hlungs, liver or spleenI. melanomas have also been found in the mus­cles, spine, heart and gut­tural pouches. many horses show no clin­i­cal signs of in­ter­nal metas­ta­sis, but lame­ness, colic or neu­ro­logic symp­toms can oc­cur.

any dark mass on a gray horse is pre­sumed to be a me­lanoma and most are di­ag­nosed by ap­pear­ance alone. a biopsy will con­firm the di­ag­no­sis and may in­di­cate how ma­lig­nant that par­tic­u­lar tu­mor is, al­though it is not nec­es­sar­ily pre­dic­tive of tu­mor be­hav­ior or metas­ta­sis. in­ter­nal tu­mors may be found with ul­tra­sound or rec­tal pal­pa­tion.

While there is cur­rently noth­ing we can do to pre­vent me­lanoma de­vel­op­ment, there are sev­eral treat­ment op­tions avail­able. Tra­di­tion­ally, melanomas have been left alone due to their slow growth and lack of pain or clin­i­cal signs, but early treat­ment may slow or stop pro­gres­sion.

Small tu­mors are eas­ily re­moved sur­gi­cally. De­pend­ing on the lo­ca­tion of the mass and the tem­per­a­ment of the horse, they can of­ten be re­moved with stand­ing se­da­tion rather than gen­eral anes­the­sia. Sur­gi­cal lasers have been very suc­cess­ful at re­mov­ing melanomas, even when the tu­mors are a bit larger. com­plete ex­ci­sion will be cu­ra­tive for an in­di­vid­ual tu­mor, but new tu­mors may arise later. very large tu­mors or those in dif­fi­cult lo­ca­tions may re­quire ex­ten­sive surgery and of­ten can­not be com­pletely ex­cised.

The an­ti­his­tamine cime­ti­dine has im­munomod­u­la­tory ef­fects that have been re­ported to re­duce the size and num­ber of tu­mors. long-term ther­apy

(at least three months) is re­quired and clin­i­cal ef­fi­cacy has not been re­peat­able in ev­ery study. In­tra­tu­moral in­jec­tion of chemother­a­peu­tic agents, such as cis­platin, can be ef­fec­tive for small masses, but there are health and safety con­cerns for the horse, owner and treat­ing vet­eri­nar­ian when us­ing th­ese drugs.

Re­cently, a vac­cine was de­vel­oped for treat­ment of me­lanoma in dogs. Early stud­ies are show­ing prom­ise for its use in horses. This vac­cine cre­ates an im­mune re­sponse against the me­lanoma cells, caus­ing the body to fight the tu­mors. The vac­cine is ad­min­is­tered four times at two-week in­ter­vals. If the horse re­sponds, the vac­cine is then given ev­ery six months.

While it is a safe bet that if you have a gray horse you will deal with melanomas, the good news is that most are not life-threat­en­ing. Proac­tive early in­ter­ven­tion may slow the pro­gres­sion of the disease and new treat­ments are on the hori­zon.

A gray mare with her darker foal show­ing the ĂRSě SKINS OH IRaXKNI

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