Hello, 

After reviewing the photos, the pattern of wool loss with the major loss on the sides is highly indicative self biting to relieve some sort of itch.  The most common source of itching in winter is lice.  This is a very common condition.  It can be hard to treat this in full fleeced sheep but not impossible either.  It would not be advised to shear if in cold housing until mid March either.   I suggest considering a pyrethrin based treatment plan which can be administered directly on the skin in a relatively small area by parting the fleece.   Be sure to consult a veterinarian on this treatment plan.   

The sheep appeared to be in good body condition and nothing I could gather from the photos suggests a nutrient deficiency.   Ewes that are lactating in winter do occasionally lose wool also due to a combination of cold and nutritional stress.  The wool loss in those instances are along the top line and these appear to be on the sides of the sheep and furthermore, these ewes do not appear to be in lactation. 

Best regards, 

Richard Ehrhardt