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Malignant Melanomas in Cats

Posted February 03, 2015 in Cat Diseases & Conditions A-Z

Calico cat

Melanomas or tumors arising from pigment producing cells can be benign or malignant. In cats, melanomas are found most often on the head (especially ears and eyes), neck and lower legs. Luckily, malignant melanomas are relatively rare in cats1. For instance, melanomas, in general, comprise less than 3% of skin tumors with approximately 42% to 68% of those being malignant2. And malignant melanomas account for less than 1% of oral tumors in cats, according to Tufts University.

Why worry about malignant melanomas then? 
Unfortunately, in their malignant form, melanomas tend to be very destructive locally, to re-grow even after surgical removal and to metastasize or spread to other locations in the body. In cats melanomas involving the eyes are more common than those in the mouth or in other locations on the skin; and both ocular and oral melanomas tend to be more malignant than skin melanomas3.

This is where things can be especially tricky in cats because many cats develop benign pigmented changes in these same locations. These changes look a lot like freckles. You might know cats (especially orange, calico or sometimes silver cats) that develop dark spots on their lips, gums, eyelids and/or nose. This condition, called lentigo, can start showing up on cats as young as a year of age, but is completely benign and does not turn into cancer4.

This is not necessarily the case when pigment changes occur on a cat’s iris. In that location pigment changes can progress from the occasional freckle to a more generalized and enlarging, but still benign, melanosis that may eventually undergo a malignant transformation5.

So you should always be concerned with any new growths (pigmented or not since melanomas are not always brown) or color changes that occur on your cat. You may also discover tumors because of clinical symptoms associated with their location. For instance, tumors in the mouth may result in bleeding, drooling, bad breath, or difficultly eating/swallowing, while intraocular tumors can cause abnormal pupil size, pain, and ultimately vision problems.

How do you know for sure your cat has malignant melanoma?
As always, anything that concerns you should be checked by your veterinarian. In the case of melanomas of the iris, careful examination and/or ultrasound can aid in distinguishing between benign pigment changes and actual tumors. But to know for sure, your veterinarian will have to send a tissue sample to the laboratory in order to get a definitive diagnosis either by a needle aspirate or surgical biopsy.  If the diagnosis is malignant melanoma your veterinarian will want to run other tests (blood works, XRays, ultrasounds, aspirates) to establish your cat’s general overall health (to make sure she doesn’t have any other illnesses) and to stage the cancer (establish as best as possible if it exists in any other locations). This information is important in order to give you a clearer picture of your cat’s individual condition and prognosis.

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Mike has more than 35 years of experience in companion animal veterinary practice and is a valued member of IDEXX’s Pet Health Network team since 2013.