hello - I have just realised that my original thread in chat has gone poof a long time ago.
I brought home a guineapig in April who had such severe mites that she itched herself raw. the vets gave her .....for kittens to kill the mites as the usual mite stuff didnt work.
the ......worked in that the mites went, but she was left with a horrific fungal infection which caused severe itching and again open sores (due to itching/biting)
I fashioned a body glove out of a child's sock for her, and she wore it daily, I cleaned her wounds, bathed her and applied cream from gorgeous guineas (who were very helpful to me in teling me what I needed for her)
My pig was also incredibly thin and drinkign vast amounts per day. I took away her water bottle and mixed her food with water so she HAD to eat if she were to drink. Her weight shot up and within a month or two she was fat again and had to go up a sock size.
She was living indoors in a cat carrier to stop her moving too much and burning off the calories, and to keep her warm.
After a three months or so, and when she was fat enough she moved into her own hutch outdoors and got her water bottle back. Se also moved to sock with just 2 holes instead of 4, so she was wearing a sort-of t0shirt 
I can now report that she has been sockless for three whole days :) And although we are not out of the woods yet (one patch on her back still itchy, although she manages t itch it "sensibly" with no skin loss) she is soooo much better :)
the sad news is that her sister who she used to live with rejected her throughout and is now actively fighting her, so I guess she will have to live with us
Sadly all of the constant washing/creaming and socking has left her quite mistrustful of people and she is nowhere near as friendly as she used to be, but we hope this will change.