One of my girls had this a couple of months ago - just around the neck - but there was no bare skin, just short fleece. It wasn't until a week or so ago that I noticed all four of them itching like mad. Prior to that, despite observing them all daily, there was no sign of anything - not even when I looked in their wool. As soon as I noticed them itching, I gave them all a good dose of Crovect.