My horse also gets HUGE welts. My vet said it was an allergic reaction to the bugs and suggested a fly sheet, and also could use Benadryl and just multiply the dose to equal your horse’s weight – though I haven’t actually tried that.
My horse got them mostly underneath his belly so I purchased a fly sheet that wrapped underneath him and also had a neck cover. It helped a lot and he didn’t get any bites like that after.
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