I live in New York, so not really in the winter.. I have given him dust bath in wood ash and diatomaceous soil,. I have even brushed him then given him the dust bath.Some of it reminds me of mites.
I think I would bathe him in baby shampoo or Tail and Mane, or is Mane and Tail, horse shampoo to see what he looks like afterwards.
That might play right into what Ken asked.