Eczema Treatment Care

i have just sold by pony, a 14.1 black welsh section D mare, 14 yrs old. she has gone somewhere with cattle, and there are allot of flys, but her owner phoned up the other day to say she was really itchy and that she had rubbed a big patch by where her girth goes, we went to see and it looked like the girth had rubbed to advised she got a girth protector for her, but now she has told us that she has lines on her back, about 1 inch long that stand out along her back and they have turned to blisters, she doesn’t no what it is and neither do we, she is putting a fly sheet on her every day to try and stop her itching, she has always been a itchy horse but never this bad. i was wondering if it was something to do with the cattle, we are going over tonight to look and see if we can see anything in the Field she is in other than the cattle that could be causing her to itch so much, she was itching on a trailer that was in the Field but that has been fenced off now. does anyone have any other ideas what it could be, we are wondering if it is lice, Irritant Contact Dermatitis,Seborrhea or something like that, is there any supplements we could recommend too? as she is really itchy
thanks