I have a 22 year old TB mare that is currently covered in small hives and I can't figure out what is causing it. I noticed one hive on her shoulder over the weekend when I brought her up to the barn for the night and removed her blanket. Since then the quarter sized spots have been increasing - a few dozen on each side with some really tiny ones on her neck and haunches.
The weather has improved so she hasn't worn a blanket since the weekend. I'm in the desert so she is dry lotted, no weeds to snack on. Have been bringing the horses in at night this past week (normally out 24/7 with blankets if cold). She is on the same pine shavings that I've been using for years. Hay changed six weeks ago - a nice orchard grass with a tiny bit of alfalfa. No pesticides used on the field. Grain also changed six weeks ago to Strategy Healthy Edge. No recent vaccinations or deworming. There is no improvement when she is turned out.
The hives are fairly small. I found one with a crusty "head" to it. None of my other horses are having issues. All are on the same hay but only one other is on this grain. She does not seem to be in any distress - bright eyed, healthy appetite, etc. She does occasionally get hives in the summer if the bugs are bad but I've never seen them on her in the winter. Any ideas?
ETA: I'm also feeding the Purina Free Balance vitamin/mineral mix with the grain. Any chance too much of some mineral could cause hives?