Melanoma in Dogs: Types, Causes and What It Looks Like

Melanoma in dogs is a cancer that starts in melanocytes, the pigment-producing cells found in skin, hair follicles, the lining of the mouth, the nail bed and the eye. If you have found a new dark lump, a swollen toe, a mass on the gum or a changing spot inside your dog's eye, book a veterinary exam rather than watching it for a few weeks, and if your dog is bleeding from the mouth, drooling heavily, refusing food or suddenly unwell, treat that as an emergency visit.

Any new lump, growth or pigmented spot on your dog should be examined by a veterinarian, and only a needle sample or a biopsy can tell you what it actually is. Appearance alone, yours or anyone else's, cannot separate a harmless pigmented mole from an aggressive tumour.

Yes, melanoma is cancer, but behaviour depends on where it grows

Melanoma is a true cancer: an uncontrolled growth of melanocytes. What makes it confusing for owners is that the same cell type produces tumours with completely different personalities depending on where they appear.

Melanocytic tumours that arise in haired skin are frequently benign. Pathologists often call these melanocytomas. They tend to be well-defined, slow growing and, once removed with clean margins, usually stay gone.

Melanocytic tumours that arise in the mouth, on the lips, in the nail bed or inside the eye behave very differently. Oral melanoma in particular is recognised as the most commonly diagnosed malignant tumour of the canine mouth, and it has a strong tendency to invade local tissue and to spread to regional lymph nodes and the lungs. The same diagnosis on a dog's back and on a dog's gum are not the same conversation.

This is why vets stage the disease rather than simply naming it. Location, how the cells look under the microscope and whether tumour cells have already travelled all shape what happens next.

The main types and where they appear

Can a dog have melanoma in more than one place? Yes. Dogs develop melanocytic tumours at several distinct sites, and each one presents differently.

Type	Where it appears	Typical behaviour	What owners usually notice
Oral and mucosal melanoma	Gums, lips, tongue, hard palate, inside the cheek	Commonly malignant, locally invasive, prone to spread	Bad breath, drooling, blood in the water bowl, a dark or pink mass on the gum, dropping food, a loose tooth
Cutaneous melanoma	Haired skin anywhere on the body	Often benign, though malignant forms occur	A firm, usually dark, raised lump or a dome-shaped nodule
Digital and subungual melanoma	Toe, nail bed, paw pad	Frequently malignant, often diagnosed late	Limping, a swollen toe, a cracked or lost nail, bleeding around the nail, a lump between the toes
Ocular melanoma	Iris, uveal tract, limbus, conjunctiva	Varies widely; some grow slowly, others spread	A dark spot on the iris that enlarges, a change in eye colour, a cloudy or painful eye, a visibly different pupil

The toe form is the one most often missed, because it is mistaken for a nail bed infection or a torn nail. A toe lesion that does not resolve the way an infection should deserves a second look and usually a biopsy.

Melanoma in dogs and cats is not quite the same picture either. Cats develop melanoma far less often overall, and when they do it is most commonly in the eye, typically as a slowly spreading pigmented change on the iris. Oral melanoma, the signature canine form, is uncommon in cats.

What melanoma on a dog actually looks like

There is no single look. Pigment is helpful when it is there, but a meaningful share of canine melanomas are amelanotic, meaning they carry little or no visible pigment and can appear pink, red or flesh coloured. That is one reason visual identification fails so often.

Features worth getting checked promptly include:

• A black, brown, grey or blue-black lump that is firm and raised
• A pigmented or pink mass on the gum, lip or palate, sometimes ulcerated or bleeding
• A spot that has changed in size, shape, colour or texture over weeks
• A lump that is ulcerated, weeping, or that your dog licks or chews at
• A swollen toe, a deformed or missing nail, or lameness with no injury history
• A dark patch on the iris that is spreading or thickening
• Facial swelling, jaw pain, or reluctance to let you handle the mouth

Size and speed matter more than colour. A small, stable, pigmented bump that has not changed in a year is a different situation from a nodule that doubled in a month, and your vet will want a timeline as much as a photograph.

How common it is, what causes it, and what does not

Melanocytic tumours are among the tumour types veterinarians diagnose regularly in dogs, and oral melanoma holds the top position among malignant mouth tumours in this species. Reliable, current incidence percentages vary between published surveys and registries, so it is more honest to say it is a well-recognised and far from rare cancer than to attach a number to it.

The cause in any individual dog is usually not identifiable. Unlike melanoma in people, ultraviolet exposure is not considered a meaningful driver in dogs, and research suggests the disease arises instead from accumulated genetic changes within melanocytes. Chronic irritation or trauma at a site has been raised as a possible contributor, particularly in the mouth and on the toes, but that connection is not firmly established. Age is the most consistent association: these tumours are diagnosed predominantly in middle-aged and older dogs.

Is melanoma in dogs hereditary? Not in the sense of a single inherited gene passed from parent to puppy. What is clear is that certain breeds are over-represented, which points to a genetic component. Heavily pigmented breeds and lines such as Chow Chows, Scottish Terriers, Cocker Spaniels, Golden Retrievers, Gordon Setters, poodles and schnauzers come up repeatedly in the literature on oral and digital melanoma, and dogs with dark mucous membranes or black coats are seen more often than lighter dogs. Breed predisposition is a reason for closer monitoring, not a prediction.

Is melanoma in dogs contagious? No. It cannot pass from dog to dog, from dog to cat, or from dog to human. Nothing you did caused it, and nothing about the tumour puts the rest of your household at risk. Results vary from dog to dog, and the only person who can interpret your dog's biopsy is the veterinarian holding it.

Grading, staging and what 'high grade' means

When a pathologist calls a melanoma high grade, they are describing how the cells look and behave under the microscope, not simply how big it is. Grading usually takes into account the mitotic index, which is how many cells are actively dividing, the degree of nuclear atypia, how far the tumour has pushed into surrounding tissue, whether there is ulceration, and sometimes proliferation markers. A high-grade tumour carries a higher risk of spreading and is generally approached more aggressively.

Staging is a separate question and asks where the disease already is. That typically means feeling and sampling the nearby lymph nodes, imaging the chest, and in some cases imaging the abdomen or the head. A small oral mass with clear lymph nodes and clean chest images is a different clinical problem from the same mass with an enlarged node, even though both share the same name on the report.

If you want a broader orientation on how canine illnesses are assessed and monitored over time, a general library of canine health conditions is a reasonable place to build context before your appointment.

Diagnosis and the treatment conversation

Diagnosis usually starts with a fine needle aspirate or a biopsy. Pigmented tumours can often be strongly suspected on an aspirate, while amelanotic ones frequently need histopathology plus immunohistochemistry, using markers such as Melan-A or PNL2, to confirm the cell of origin. Imaging follows to complete the staging picture.

Treatment is built around what conventional veterinary medicine does well. Surgery is the foundation for most accessible tumours, and for oral and digital disease it may mean removing part of the jaw or amputating a toe, procedures dogs tolerate better than most owners expect. Radiation therapy is used where surgical margins are not achievable. Medical oncology options, including a licensed canine melanoma vaccine used in some oral cases, may be discussed by a specialist based on the tumour's site, grade and stage. Pain control and nutritional support run alongside all of it.

Ask for a referral to a veterinary oncologist if one is available. Second opinions are normal in oncology, and a specialist can explain where your dog actually sits on the spectrum from benign skin nodule to high-grade malignancy. Talk to your vet about what each option asks of your dog and of you, including time, travel and quality of life.

Key Takeaways

• Melanoma in dogs is cancer of the pigment-producing melanocytes, and it is genuinely cancer, not a mole.
• Skin melanomas are often benign; oral, nail bed and some ocular melanomas are frequently malignant.
• Not all melanomas are dark. Amelanotic tumours can look pink or flesh coloured, so colour is not a screening tool.
• Sunlight is not considered a major cause in dogs. Age and breed-linked genetics matter more.
• It is not contagious, and it is not inherited in a simple parent-to-puppy pattern.
• Limping with a swollen toe, bad breath with gum bleeding, or a changing spot on the iris all warrant prompt examination.
• Grade describes how the cells behave; stage describes where the disease has reached. Both guide the plan.

For a wider overview of the warning signs that mean a same-day or after-hours visit rather than a routine appointment, see Dog Emergency Signs.

Your veterinarian owns the diagnosis and the treatment plan here. Sampling, grading, staging, surgery, radiation and pain management are their territory, and nothing you read online substitutes for a biopsy result and a clinical exam. Bring your photographs, your timeline and your questions, ask what the grade and stage actually mean for your dog, and make every decision with that team alongside you.