The most common reasons for circular areas of hairloss and scabs in dogs are skin infection due to bacterial infection (most common), demodex mites or fungal (ringworm) infection. When faced with a case with these symptoms, I first perform skin scrapings to evaluate for infection and parasites, +/- ringworm culture. If the scrapings show bacteria, then I treat with a minimum 3 week course of oral antibiotics plus antibacterial shampoos 1-2 times weekly. If infection resolves then recurs, then there has to be an underlying cause. The most common underlying causes are allergies (food allergy or pollen/dust allergies) and hormonal diseases (low thyroid or Cushing's disease). If symptoms begin later in life, food allergy and hormonal diseases are most common (pollen and dust allergies usually start 6mo - 5 yrs of age). So in older dogs I start with full labwork and thyroid profile. If the thyroid levels are low then that is treated with lifelong medication. If liver enzymes are elevated and there are other consistent clinical signs of Cushing's disease (excess adrenal steroid production), then I test for Cushing's. If all labwork is normal, then I investigate possible food allergy with a hypoallergenic diet trial, consisting of one protein and one carbohydrate that the dog has never had before, and no treats, table scraps, rawhides, milkbones, chewable supplements etc. for 8 weeks. Natural Balance makes a fish and sweet potato diet which you can try. Blood allergy testing for food is not accurate in dogs. If symptoms persist or recur despite the strict hypoallergenic diet, then I consider allergy testing for pollen/dust allergies and desensitization injections. Since you have gone to numerous veterinarians with no real answer, I recommend consulting with a veterinary dermatologist--you can find one in your area at www.acvd.org.
Kimberly Coyner, DVM DACVD