Thank you for the excellent photo of an epidermal collarette. These are roughly circular skin lesions most often indicative of a Staph infection of the skin (pyoderma). They start out as pustules and then spread out in a circular manner. The center quiets down and often hyperpigments (as shown) but the rim can stay actively infected and inflamed. This is addressed with a prescription systemic antibiotic such as one in the cephalosporin class (cephalexin, e.g.) dosed for 3-4 weeks and 1 week past clinical signs. Bathing twice weekly in a chlorhexidine- or benzoyl peroxide-containing shampoo should hasten remission. These shampoos can be found over the counter in your local feed/agricultural merchant store, online, or at Bandha's vet hospital.
Please respond with further questions or concerns if you wish.
Sunlight and chronic allergic dermatitis might trigger some cases of pemphigus. I'm not aware of pyoderma being a trigger, however.
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Thank you for your kind accept. I appreciate it.