My mare had hives for three weeks and then they grew huge (think golf ball size) before sinking in the middle and turning into large rings (1-3" in diameter). She had probably 15 on each side from neck to rump. I panicked and thought she had ringworm. The vet came out and said he had just left a friend's barn about 1/4 mile away whose mare had the exact same rings after having had hives.
The vet treated my friend's mare with steroids and mine with Banamine because of IR issues. Both mares have been hive/ring free for about ten days. Today my friend called to say her mare has them again!
We use different hay suppliers, different feeds. She blankets, I do not. The only similarity we can come up with is that we both mowed our fields later than normal and they are shorter than usual going into winter. Perhaps our horses are eating something they normally do not when the grass is longer. I hope not, as we will have to keep our mares stalled or muzzled until spring. We also each have a gelding on the same fields who have had no reaction.
I am curious if this has been an issue for anyone else in the area this fall.