They are irregular. I have yet to determine any true pattern, though it does seem rather more reactive to rifts and the arcane energies of the Fade than it has been in the past. When we returned from fetching those Fade-touched animals - there was some tenderness for a few days following that. But I hesitate to definitively say it is a matter of exposure. The strain in the limb might have easily been due to the exertion of the work.