Melanoma is a cancer of the melanocytes, the pigment-producing cells of the body. The most common site for melanoma tumors to occur in the dog is in the mouth. Melanoma is a very aggressive disease and tumors are often very large, frequently invading the surrounding bones of the oral cavity before they are even detected by an owner or veterinarian.
Oral melanomas also have a high chance of metastasizing (spreading) to other parts of the body. The most common locations for melanoma to spread are lymph nodes within the head and neck, and the lungs. Certain breeds are more likely to develop melanoma tumors than others, including poodles, dachshunds, Scottish terriers, and golden retrievers.
The size of the tumor is important in considering the overall prognosis for canine oral melanomas. Veterinary medicine has adopted the World Health Organization’s staging system, where Stage I disease is represented by a tumor less than 2 cm in diameter, Stage II is represented by tumors 2-4 cm in diameter, and Stage III tumors are 4 cm or larger, or are any type of tumor with local lymph node involvement. Stage IV disease includes any tumor with evidence of distant spread.
The primary treatment for oral melanoma in dogs is surgical removal of the tumor. However, since the majority of tumors invade the boney structures of the jaw, even with very aggressive surgical measures, complete resection (removal) can be difficult.
The average survival times for dogs with oral melanoma can vary, but with surgery alone, survival times are generally reported as:
Stage I: approximately one year
Stage II: approximately 6 months
Stage III: approximately 3 months
Stage IV: approximately 1 month
When the tumor cannot be completely removed and/or it has spread to local lymph nodes of the head and neck (but not beyond), radiation therapy becomes important in the treatment of this disease. Remission rates with radiation therapy alone are up to 70% in some studies. However, recurrence of disease or more distant spread can occur following this type of therapy and survival times are often only in the range of 5-7 months.
For cases of oral melanoma with spread to distant sites like the lungs, historically, veterinary oncologists relied on chemotherapy as a form of treatment. Unfortunately, melanoma seems to be inherently resistant to chemotherapeutic drugs, and response rates and durations are disappointing. Studies do not indicate a survival benefit to adding chemotherapy to aggressive surgery and/or radiation therapy plans.
Recent technological advancements have allowed the development of a DNA-based vaccine as a treatment option for canine oral melanoma. This form of treatment is called immunotherapy and is based upon the concept of using the body’s own immune system to control the growth of, or potentially even eradicate, tumor cells.
The melanoma vaccine works in a similar way to the other vaccinations administered to protect your dog against various infectious diseases. Conventional vaccines typically contain a small amount of a weakened disease-causing organism, modified so that when it is injected into a dog it will not cause disease but will generate an immune response effective in killing the actual active form of the organism, should exposure occur in the future.
The melanoma vaccine contains the human DNA sequence encoding a specific protein only found within melanocytes called tyrosinase. Tyrosinase is an enzyme crucial to the melanocyte’s ability to produce melanin (pigment), and also to the survival of the melanocyte itself. Once injected into the dog, the human DNA segment is processed so the dog’s body actually generates small amounts of the human tyrosinase protein. Just like the weakened disease-causing organism in a conventional vaccination, the human tyrosinase protein is recognized by the dog’s immune system as foreign. Subsequently, the dog’s immune system will generate a response towards the human tyrosinase protein designed to destroy it.
The human tyrosinase protein is similar enough in structure to the dog’s own natural tyrosinase protein, so this very same immune response will be effective in attacking the tyrosinase that is present its own melanoma cells. The end result is destruction of the tyrosinase in the cancerous melanoma cells, and ultimately, the inability of the tumor cells to survive.
The melanoma vaccine is currently only available through veterinary oncology specialists. The vaccine is initially administered every two weeks for a total of four doses; booster vaccinations are administered every six months for the remainder of the dog’s life.
The melanoma vaccine is not a replacement for existing conventional therapies, rather, it is best used in conjunction with other treatment modalities such as surgery, radiation therapy, and chemotherapy. Side effects are very uncommon. Most importantly, the life expectancy of dogs with oral melanoma that would have typically only survived a few weeks to months has been extended to well over a year or more.
The canine melanoma vaccine represents an exciting new technological advancement within the field of veterinary medicine. Not only can we see benefits for our canine patients, but information from results of studies with dogs treated with this vaccine are being used to help generate new treatments for people with melanoma, reminding us once again of the unyielding power and limitless potential of the human-animal bond.
Dr. Joanne Intile