Veterinarians get asked for random pet advice outside of the office daily, whether it be from friends, family members, or complete strangers on the street. It’s considered an occupational hazard, I guess, which is why some veterinarians might be reluctant to share their profession at parties.

Recently, I was approached by a family member who had a question about her cat’s eye. It seems that her cat “Tiger” had an eye that was changing colors. I explained what it could be and realized that it would make a great blog topic, as dogs and cats can both be affected by melanoma of the iris.

What is melanoma of the iris?

The iris is the colored part of the eye, and changes in the appearance of the iris should always be mentioned to your pet’s veterinarian. Melanoma is a type of cancer, and among other organs, it can affect your pet’s eye. Most commonly, melanoma arises from the iris, but it can also rarely arise from other parts of the eye. While melanomas are the most common ocular cancer in cats and dogs, thankfully they are still relatively uncommon.

Melanomas usually occur in only one eye, and in cats they are typically seen in older animals (between ten and twelve years old) with no breed or sex predilection. In dogs, melanoma can occur at any age (though seeing it in a very young dog would be rare), and Golden Retrievers and Labrador Retrievers are at highest risk.

In the early stages of iris melanoma, you will notice a color change in the iris. Generally, iris melanomas are darkly pigmented, so you’ll see a change to a brownish color. It can start out as a small freckle that grows over time, or you may notice that the entire iris looks darker than usual, especially when compared to the other eye.

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Diagnosing iris melanoma

It is very difficult to diagnose iris melanoma as sampling the tissue for biopsy is generally not rewarding. Veterinarians generally use the animal’s history as a clue and couple it with exam findings. It is important to differentiate iris melanoma from iris cysts or iris melanosis, both of which can look similar to iris melanoma.

Iris melanomas can be slow growing and slow to spread, which is good news for concerned owners and veterinarians. This gives us time to watch the color changing eye over time to see how the eye behaves, thus hopefully differentiating it from a cyst or melanosis. The tumor can spread, and is more likely to do so in cats, where it can be found in local lymph nodes, the liver, and the lungs. Only 4% of canine cases metastasize.

Once iris melanoma has advanced, blindness and glaucoma can occur. If a diagnosis of iris melanoma is probable, enucleation (or removal of the affected eye) is generally recommended in cats. The goal would be to remove the eye before the tumor spreads to other organs. Often, knowing when to remove the eye is a grey zone, as definitive diagnosis is difficult, but sparing your pet’s life over his eye will be your vet’s priority.

Dogs come off a little luckier than cats when it comes to iris melanoma. As I mentioned, their tumors are less likely to metastasize, and specialists may be able to manage their tumors locally with laser ablation or surgical removal of the tumor rather than enucleation.

Although iris melanomas are generally slow growing, iris color change should never be ignored. Mention it to your vet so that she can follow the eye’s progress over time. If you notice a painful or large eye, consider that an emergency and try to get to the vet as soon as possible.

May 16, 2013
Pet Health

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