There could be something out in the field that he laid in causing the hive like condition. I had a mare that got rain rot or some crud one summer. It was awful to the point where she lost a lot of hair… Once she got better, I pretty much stopped bathing or hosing her off after riding. I’d curry her really well or on really hot days sponge her off with water or water & ventrolin. She never had any issues after I stopped hosing her off so much. I’ve done pretty much the same thing with horses since that mare.
I agree that there could be something else going on, is it rain rot or maybe an allergic reaction to something horse may have laid down in the fields - some weed or sting? Vet might also be able to give you some oral meds to help. Way back there was a good homeopathic which cleared up rain rot really well but you just don’t see those homeopathics so much any more. Banixx or MTG would be a good standby for future needs.