Skin issues can be extremely irritating to patient and owner. Sorry your little guy is affected by this.
There are two or three things I think about in cases like this. The first is an infection with yeast organisms. These lesions are often black, have really thickened and greasy skin, may have a little sweetish, yeasty odor to them, and are very itchy. Yeast infections are generally considered to be secondary issues to some other problem - like allergies or Cushings disease or some other problem. So without controlling the secondary issues, the yeast infections are often recurrent or difficult to eradicate. OTC things you may try for this include gyne-lotrimin antifungal cremes and Selsen Blue shampoos. Make sure to leave the lather on for 10-15 minutes before rinsing for maximum effect. Unfortunately, these often require oral antifungal drugs to get rid of the bugs, and prescription shampoos work better.
The second thing I would consider would be chronic, recurrent bacterial skin infections. These are also often secondary issues. But if you don't control them, even if you address the primary issue, the dog remains uncomfortable. So if this is the case, he needs antibiotics - sometimes several weeks worth.
The third thing would be sarcoptic mange. This is certainly less common, but the mites that cause this are difficult to find on scrapes, so negative scrapes do not rule out sarcoptic mange. In cases that I suspect it, I will treat for it even if I don't find the mites.
In this breed, I would certainly recommend ruling out Cushings disease as an underlying cause of chronic infectious skin diseases - especially skin infections. This is done by blood work which your vet can discuss with you.
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