Has the hair been cut away, or fallen out? It is unusual for there be no reddening of the skin. What has your vet suggested? Have you given any medicated baths? The dandruff at the borderline made me think of cheyletiela. Commonly called Walking Dandruff, you will see it moving if you transfer combings to a sheet of paper, and examine under a magnifying lens. This mite is usually carried by rabbits, but can be transferred to puppies. It can cause itching on human skin too - usually the forearms.
