Canine Melanoma

Canine melanoma is cancer that arises from the cells in the skin that produce dark pigment. This page looks at the symptoms, causes, and conventional treatment of melanoma in dogs.

Melanoma is a form of malignant skin tumor that arises from melanocytes, the cells in the skin that produce dark pigment (browns and blacks). The cancer can occur anywhere in the body where there is dark skin coloration, such as the inside of the mouth, on the lips, on the toes, on the trunk and limbs, on the eyelids, and sometimes under the eyes.

(There is another form of skin tumor that is benign called melanocytomas which also arise from melanocytes.)

Canine melanomas often look rather knobby, are dark and fleshy, and feel solid to the touch. Rarely, you will find an unpigmented melanoma.

Most skin tumors found on a dog's haired skin are benign growths such as melanocytomas. However, melanomas found in the following sites are usually aggressive:

in the mouth (most cases of oral cancer in dogs are in the form of melanoma);
at the junction between the skin with hair and skin without hair (e.g. on the edge of the lip, or at the junction between the haired and non-haired skin on the edge of the nail bed).

Melanomas at these sites can metastasize (spread) to other sites in the body rather rapidly, usually to the lymph nodes, liver, adrenal glands, and lungs.

Canine melanoma usually occurs among middle-aged to older dogs (9 years or older). Male dogs tend to be more commonly affected.

Symptoms of Canine Melanoma

The sign of melanoma is generally a dark raised visible mass on the skin. Sometimes the masses look like infected, swollen sores.

Dogs with the oral form of melanoma show symptoms such as:

Excessive drooling
Bloody saliva
Bad breath
Difficulty chewing
Difficulty swallowing
Appetite loss
Weight loss
Facial swelling

As the cancer progresses, it will spread internally often to the lungs, causing the dog patient to cough (sometimes blood will be coughed up). Meanwhile, the tumor gets larger and sometimes becomes ulcerated and infected.

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Causes of Canine Melanoma

Unlike melanoma in people, sunlight does not increase the risk of occurrence of melanoma in dogs.

Instead, many veterinarians suggest that genetics is the biggest risk factor. Certain breeds of dogs are more susceptible to this form of tumors. They include:

Airedale Terrier
Boston Terrier
Boxer
Chihuahua
Chow Chow
Cocker Spaniel
Doberman
Golden Retriever
Irish Setter
Scotish Terrier
Springer Spaniel

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Diagnosis of Canine Melanoma

A definitive diagnosis can be made by biopsy of the tumor.

In addition, chest x-rays and x-rays of the local and surrounding area of the tumor will be taken to determine if the cancer has spread, and if so, to what extent.

Moreover, regional lymph nodes should also be examined for possible involvement. Fine-needle aspiration may be necessary to biopsy suspicious nodes to check for the presence of cancerous cells.

Conventional Treatment of Canine Melanoma

Surgery is the conventional treatment of choice for skin melanoma in dogs, as well as melanomas that occur in the toes.

For melanoma of the mouth, aggressive surgery removing not only the tumor but also at least 2 cm of normal tissue at all margins of the tumor is necessary. Surgery may involve removing part of the upper or lower jaw. Depending on the size of the tumor at the time of the operation, approximate survival times may range from 3 months (for very large tumors) to 18 months (for very small ones).

If surgery is not possible due to the location of the tumor, radiation can be used. While the tumor will shrink in 3 out of 4 dogs, this response is temporary - the tumor will almost always reappear.

Oral melanoma in dogs does not respond well to chemotherapy due to its rapid speed of metastasis. Only about one in 5 dogs will respond to chemotherapy. Common drugs used include cisplatin and piroxicam. They can achieve an approximate survival time of 4 months.

A gel that contains the drug cisplatin is sometimes injected into the site of the oral melanoma tumor, allowing the cisplatin to be released directly into the tumor itself. In a study, it has been shown that 14 out of 20 dogs responded, and in over half of those 20 dogs the tumor regressed completely and the dogs survived almost one year.

A melanoma vaccine is now available and can be purchased through the manufacturer (Merial). Note that this vaccine cannot cure melanoma, but can increase lifespan after surgery by boosting the dog's immune system.