Small Animal Dermatology Q&A 02

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A circular crusted lesion was found on the abdomen of a dog. The owner reports the lesion was a small red bump yesterday, and today there is this crusted lesion.

Question	Answer		Article
What are these lesions called?	This lesion is called an epidermal collarette, and is representative of a superficial bacterial pyoderma. It results from the rupture of an intact pustule. After the pustule ruptures, a crust develops and spreads in a circular fashion creating a ‘collar of crust’. There may or may not be a ring of erythema at the margin. As the lesion heals, the center often becomes hyperpigmented.		Link to Article
What other skin disease(s) can this lesion be mistaken for clinically?	Epidermal collarettes are commonly misidentified as ‘ringworm lesions’ or dermatophytosis. Skin scrapings should be done to rule out demodicosis. If there are other dermatological signs consistent with dermatophytosis, a fungal culture should be performed. Pending fungal culture, the patient is best treated with a minimum of a 21 day course of antibiotics; treatment should continue for 1 week past clinical cure. If skin scrapings and fungal cultures are negative and the patient does not respond to appropriate antibiotic therapy, a skin biopsy is indicated to rule out other rare causes of these lesions (e.g. PF).		Link to Article
What is superficial spreading pyoderma?	Superficial spreading pyoderma is a bacterial pyoderma characterized by large epidermal collarettes with an erythematous, mild exudative/crusted leading edge. What is unique about this clinical form of superficial pyoderma is the lack of pustules; lesions are extensive and pustules are conspicuously absent. In the author’s experience this form of bacterial pyoderma is often seen in longhaired dogs, especially collie and Shetland sheepdogs, and lesions are common on the trunk.		[[|Link to Article]]