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Hello, I'm Dr Gary. I've been practicing veterinary medicine since 2007. I look forward to helping with your questions/ concerns.
This could be one of a couple things. I think the most likely is a skin infection- usually a combination of bacteria and yeast. These can be hairless and irritated/ red. They will also typically be itchy.
It's also possible that this is due to skin mites. A skin scraping would be ideal to rule out mites. If they are present, we use anti-parasitics to treat it.
Lastly, it could just be allergies to something. It's often times difficult to know exactly what they're allergic to. Common allergens are- food, plastic bowls, fleas, grasses/ pollens. You could try some Benadryl at home to see if it helps. The dose in cats is 1-2 mg per lb body weight every 6-8 hours, as needed.
I hope this helps, let me know if you have any other questions.