Equine Melanoma: What Options Have We?

Several advances in equine medicine and surgery provide promise for horses with melanoma.
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Several advances in equine medicine and surgery provide promise for horses with melanoma.

Whether we care to admit it, virtually 100% of grey horses either have, or eventually will have melanoma. The white hair and black skin which define a grey horse prime it to get this form of cancer by optimizing ultraviolet light penetration into the skin. Melanoma tumors are encapsulated masses made up of cancerous pigment producing cells. They can range in size from a small pea to a baseball and are most commonly located:

Melanoma3

· In the perianal/ rectal area

· On or under the tail

· In the croup area

· In the throatlatch region just below the ears and extending down the curvature

of the jaw (mandible)

· In the commissures of the lips

· In the sheath

Make no mistake about it- if you see a melanoma on the surface of your horse’s skin, there are going to me more internally. In other words - there is never just one. Also keep in mind that benign, dormant melanomas that are not disrupting body functions do not necessarily need to be addressed. You and your veterinarian must make the determination for your horse based on individual circumstances. In general, if melanomas are disrupting eating, defecation, urination or are inhibiting performance, treatment should be considered.

For decades equine veterinarians had very few options to treat melanomas. But as our horse population continues to survive longer due to advances in veterinary medicine and better horse husbandry by owners, the push to find options for horses stricken for melanoma has led to some promising treatments. They include:

1. Many veterinarians are setting up routine minor surgeries to remove melanomas by traditional scalpel blade excision. This can be done with the horse standing under sedation using a local anesthetic. Often the veterinarian will take between 5 and 10 melanomas out of the lips, tail and perianal area. As time progresses, veterinarians and owners get a handle on the melanomas and remove them when they are small. This option is less useful for larger melanomas or for situations in which multiple melanomas have coalesced.

2. Laser removal is also gaining popularity since it cuts and cauterizes in one procedure, leaving the horse with a burn/ scab rather than a row of sutures that are put in place with traditional scalpel excision. Lasers are precise but require an operator who has training and experience in using them. They are often utilized for larger melanoma removal or for palliative excision of multiple clusters of tumors.

3. Melanoma vaccines are being developed and tested in several veterinary schools around the world. The vaccines are generally designed to prime the body to target and attack antigens (in this case a foreign protein) on the melanoma cell surface. To determine if a vaccine is available through a veterinary teaching hospital in your area, contact the large animal clinic of a school in your area and inquire.

4. Intralesional chemotherapy: Veterinarians now have the ability to inject or implant chemotherapeutic agents directly into the tumors. Cisplatin beads are one such example. Some chemotherapeutic agents are easy to obtain while others require veterinarians to obtain special authorization from health authorities. They do not result in systemic illness that is commonly experience by people taking chemotherapeutics.

5. Intralesional gene therapy: This involves injecting DNA segments that code for interleukins (molecules that promote an inflammatory response) directly into equine melanomas. While in early stages of development, clinical trials of gene therapy show very promising results in getting tumors to regress.

If you own a grey horse, perform routine thorough checks of the entire body by running your hands over it to check for firm nodules or bumps. If you find a suspicious mass, having your veterinarian take a look and developing a treatment plan early on is best. If your horse already has multiple melanomas, knowing where and how many there are can be useful in determining how aggressive the malignancy is. This may influence your decision on treatment(s).