I have a 15 year old rescue palomino quarter horse mare I've been taking care of since Nov. She has never missed a meal & has gained weight beautifully. She has been in the same dry lot during the day and stabled on straw in my barn at night. Monday she had a sore develop on the back of her forearm about the size of a half dollar. The next day she had one on her thigh. I first thought she was scraping on something so investigated the premises and found nothing. By that night she had a large one 6" round develop near her flank. They look like a dirty spot on her coat then start weeping. The next morning she had 5 more about 2-3" round on her thigh & gaskin. I took her to the vet & he has never seen anything like it. He shaved the hair off & thought it looked like something caustic did it but there is nothing in her pen that should be caustic. He gave me 1% silver sulfadiazine cream to put on it. They weep but have no puss, seem very tender to touch. The weird part is they are only appearing on one side of her body. The only thing I've done different is worm her 3 days before the break out & a new round bale of grass/alfalfa mix was added the day before. I'd been feeding small squares of the same mix. I've read about photosensitivity in light colored horses due to alfalfa & am wondering if this could possibly be the cause??? We haven't had much sun though & she still has a heavy coat on although she's been shedding like crazy since Dec. No new sores have appeared since Thursday. I have another horse that shares the same lot & so far she is fine. I'm just wondering if anyone has any ideas or has dealt with the same problem. Thanks for any input.