Are they scabs in the photo or is it more like a growth, sorry I can't really tell
If they are scabs then I would lean towards perhaps what we call greasy heal, one of my old horse used to get it and it would take ages to heal, it looked very similar, I had to change her diet and dress her legs in Rosemary Cream to soften the scabs then wash them in a fungicidal wash.
Did the vet take a swab at all? It sound like he wasn't very committed
That is what I was wondering as obviously they have very different treatment regimes! Does anyone happen to (as you do) have any literature on the likelihood of growth formation in different areas of the horse's body?
I do think though a vet (possibly a different one) would be a better use of time than taking another picture - we can only tell so much.