In May, I took my horse to a training program for three weeks, and in that time she developed a bump under her jaw (possibly strangles) and she started losing hair. She was in a stall the whole time and had little to no interaction with other horses. When she got back home she was put in a round pen for isolation and is still there. She never acted sick, so I never had the vet out. The bump under her jaw burst and healed very quickly. The hair loss also stopped spreading once I started giving her Viodine baths and the hair has been growing back for about a month an a half. The lady who owns the place where my horse is boarded at started talking to me today about putting her in with the other horses once the quarantine period for the strangles is over. She said that he is not worried about the strangles(if that's what she had), but she is worried about the hair loss, because she has never seen it before. And says that even though the hair has grown back, there is a possibility that my horse is a carrier and wants to have a vet out. However, she no longer has an infected area for the vet to take sample of, so I figured I would see if anyone on here knew what it was first. My best guess is that my saddle pad picked up something from the saddle rack it was on at the other facility which transferred it to her back. It mostly affected one side of her back, but then spread a little on the other side and back past where the saddle sits. She also got a couple spots on her neck.
This was the hair loss at its worst:
And this is how it looks now: