Crisis Time: Equine Pemphigus Dermatitis

Welts/bumps covering her neck, under belly and some on the legs. 

Oh my. Yes, it's been a bit of crisis time around the farm. In addition to everything else we have going on, our newest mare was recently diagnosed with Pemphigus Dermatitus. It is an auto-immune disease that seems to come out of nowhere. Within 1-2 weeks of bringing Krystall to the farm, she started getting what seemed to be a rash or hives. After another week with the condition getting worse, it was time to call out the vet.

Another week later with no results from the first vet's diagnosis, we went to the Idaho Equine Hospital for further evaluations. Further evaluation$...blood work, skin biopsies, hair folical exams and a team of specialists working on her ca$e. Can you $ee what's frustrating me? ;) In addition to this, she was also diagnosed with 3 bacterial infections. Yes, THREE.

This was where it started, with hair coming off by the handful on her back leg

I'm not at all convinced that this is a correct diagnosis, so I'm asking for help. 

The only things that have changed for her since coming to our farm, is new hay, which she wasn't thrilled with, new grass pastures (to her), she was also de-wormed. Her prior owner said she's never even had a runny nose in her 11 years (she has owned her since a weanling), so this is as much a mystery to her as well.

All those black spots are the hair folicals waiting to come off with hair attached

We've had the county extension agent over and walked the pastures to see if there was something she got into, but she said, no there wasn't anything poisonous for horses. Not sure though if that would mean she couldn't be allergic to a specific grass or weed ...Or in the new hay? Mind you, we also have Elsie Rose, who just came in as well, with none of these issues and was in the paddock next to her, sharing a water trough, until we quarantined Krystall.

Those aren't shadows, they are the lumps/welts/ whatever you want to call them, all over her neck, shoulder and chest

She has been in quarantine for 5 days now and we've changed her hay over to a simple grass hay with 3 cups of LMF Super Supplement.

She is now getting 250 mg of Prenisolone once a day (noonish) for the dermatisis, and 8,000 mg of Sulfamethoprim twice a day (morning and evening, 16,000 mg total) for the bacterial infections.

She started here about 250-300 pounds overweight. Her weight now looks normal at around 900/925 lbs, but she was about 950, 10 days ago when we went to the vet, so she has been losing weight. We have upped her hay so she always has it in front of her now to keep her from loosing too much weight as can happen with the meds.

Anyone have any thoughts?