If you are seeing scabs it could well be mites as Louise suggested which will need treatment with Ivermec/Xeno 450. Fungal can cause scabbing too however, an identification guide which may help you is here:
http://www.gorgeousguineas.com/indentification.html however Ivermec will NOT be effective to fight fungal, this will need a special shampoo.
If you are still in doubt, try emailing Chrissie at Gorgeous Guineas, she is very helpful.
http://www.gorgeousguineas.com/contact.html
If you are seeing white wriggling things, this is likely to be lice. I've just posted about a lice treatment that I personally use in another post, so I'll paste it here below as well

It is possible that pigs can have lice and mites/fungal at the same time. If you are using a lice shampoo, you will need to wait a time after the application on Ivermec if it is a spot-on treatment as you won't want to wash it away.
The best and most gentle lice treatment I have used is the Gorgeous Guineas Lice and Easy shampoo, see
www.gorgeousguineas.com you leave it on for a few mins, and then repeat the treatment a week or so later (can't remember exactly how long off the top of my head). One of our fosters came in with lice and it cleared them very efficiently and they've never come back
As for where the lice come from and changing your hay supplier - it could have been a dodgy batch or something like that, always make sure the hay is the best quality you can afford if you can. It needs to be nice and fresh smelling, not mouldy or dusty. I use a type regularly and I got a very bad one off batch which was never repeated (as yet) so it does happen sometimes!