I have a Herdwick ewe who started to lose her fleece in patches on her back in Nov - she didn't rub, or scratch. First thought it was Scab - asked for advice on here - thought it could be Lice... I took scabs and tufts of fleece for analysis at local lab - not scab or lice, eventually came back with 'wool rot' caused by wet warm weather, could be that if you have your ewes housed they have got warm -surprising how hot they are if you put your hand into their fleece next to their skin - but their fleece has still been damp/wet and has rotted off.
I put Sudocream on the scabs - which helped keep the rain off by being like a waterproof membrane, while keeping the scabs from getting crusty - also clipped her back shorter left about 1" of fleece so her skin could breath, in future I will clip her in the late summer as well - or at least take off her top coat over her back area so it doesn't happen again.