We have our horses on a product called Equisano. It is an all organic blend of Mushrooms. We used to have the hives issue all of the time. We saw this Doctor speak about mushrooms and his product. We will never take our horses off of it. It's awesome and very affordable!
It could be a food allergy. After months of trying many supplements and herbs mixed in with daily pellets and beet pulp, it turned out my mare was allergic to the beet pulp. Once I stopped the beet pulp, all the hives went away. A veterinary allergy blood test found the problem.