Every year at the end of spring/beginning of summer my horse breaks out in hives and his eyes would swell to the point where he couldn't open them, from something in his pasture, but since he's on 80 acres, we can't specify exactly what. However, up until recently, he was pasture boarded so we didn't have the option of putting him in a stall. Some may bash me for this, but we used to give him benedryl. We would give him 8 tablets (recommended number by the vet) and it helped tremendously, his hives would go away and he would be better in a matter of days.
Just throwing it out there, because it did work very well for me.
He's also allergic to all fly sprays *rolls eyes*.
But good luck!
~Kait & Mark~