My horse has unfortunate reactions to the gnats they’ve got down here in Georgia. I used Anihist for him last year because it didn’t require a prescription but you can’t find it anymore this year. I figure they’re probably re-evaluating the prescription thing.
Anyway, you can get Trihist (with a vet’s prescription) and that’s really helped my horse. The first summer he was here, he was all eaten up on his chest and under his belly. This is our third summer and while he’s still itchier than usual, he’s not scratching himself bloody nor does he have welts.
Other than that, I just use fly spray liberally.
Lastly, I know somebody who ended up moving her horse to a barn that stalls during the day and has fans partly because of her reactions to bugs. That apparently helped a lot too.