My Cat Has Mysterious Crusty Patches on His Skin

CatChannel veterinary expert, Arnold Plotnick, DVM, discusses possible causes and cures.

Q: My Siamese-mix, 12-year-old male cat has developed a skin problem which our veterinarian can’t seem to identify. It started as a crusty patch on his back near his tail and for the last four weeks has steadily advanced up his back and spread to some small patches on his head and neck. Most of his hair falls out where these patches develop. However, as the patches advance, the skin left behind smoothes out, and only the leading edge of the patch is crusty. He has been treated with antibiotics and anti-fungal drugs with no apparent success. The condition doesn’t seem to bother him much, although he will occasionally scratch open an area. Can you help us?

A: It is always difficult to comment on dermatological cases without actually seeing or examining the cat in question. I’m not certain what is going on with your cat. The lack of response to antibiotics seems to rule out a bacterial infection.

I’m not sure if the skin was checked for ringworm, a skin fungus, but I doubt that ringworm is the culprit here because this would be an uncommon presentation for ringworm — 12-year-old indoor cat with his lack of response to antifungal drugs.

A skin scraping should be performed to rule out skin parasites such as demodex. Ultimately, I think a biopsy should be obtained. It would be important to biopsy an active lesion, ideally obtaining a specimen that contained the leading edge of the lesion. A biopsy might reveal either an autoimmune disorder such as pemphigus, or perhaps an allergic dermatitis.

A hypoallergenic diet trial can be conducted to see if food allergy is responsible for the signs you’re describing. If the biopsy shows an autoimmune disorder or an allergic dermatitis, anti-inflammatory drugs such as corticosteroids might be effective in resolving your cat’s skin problem.

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