I have a horse here that has/had Pemphigus. The hypothesis is that was triggered by getting a multitude of vaccines all at once (he was in Ontario, and they vaccinated for eight things in one day) OR by something in the hay.
I believe they treated with steroids. Once he was well enough, his owner sent him to me in Alberta, with hopes a less humid area and more relaxed environment would help him. When he came his big abscess lumps were gone, but he was still extremely itchy and missing hair on his groin, neck, head and arm pits.
He was still on steroids when he came, but we weaned him off them, and two years later he has not relapsed. He did have a lot of foot abscesses in the first 6 months of coming here. We did space out his vaccines the first year, but this year we did it all at once, but we donāt vaccinate for Rabies or Potomac here, either way, he didnāt react at all and has shown no signs of relapse.
Obviously not everyone can mail their horse to a new climate, but there may be hope. It is tough when your horse has a rare disease.