I'm a Thorobred, and my farrier finally determined why I'm sore on my right front foot.  He took these pictures and sent them to another farrier (Dave Fraser by email) who noticed that I have interbulbular dermatitis.  That's right.  I have thrush.  Notice the crack in the center back part of my frog.  When I apply weight to this foot, the crack in the heel of my frog opens up and exposes the underlying sensitive area of my frog to the air causing me pain.  The farrier treated it with iodine crystals to widen up the crack a little so that it could heal from the inside out.  I immediately felt better, and the thrush went away by the next shoeing.