Both of my horses have developed allergic reactions to flies at my current barn, and it happens to be right near wetland.
I tried using a fly sheet, but it only made the welts worse, and my gelding’s welts will actually have puss and scabs if they are allowed to develop, so that made it harder because flies were then more attracted to the poor guy. My mare gets sweet itch on her belly and both horses are VERY itchy in the summer months. My mare will actually lie on the ground and rub her belly on the dirt to try to scratch it. They both will lose hair constantly if they are allowed to go into a full-blown allergic reaction state as well.
There are things that I have found to help, though. Hydroxizone (it is prescribed by my vet, an antihistamine) helps alleviate the allergic reactions and the itchiness that my horses have. I was told that it was either going to be Hydroxizone or steroids, and that Hydroxizone doesn’t work on all horses, but it works on both of mine. I also keep Benadryl lotion/gel handy, and use a combo of Neosporin and Benadryl to take care of welts and the reactions on-site. It helps ward off any infections and relieve itching at the same time. I use Swat in hard-to spray areas and to circle open wounds so that flies are less likely to bug the areas.
For my mare specifically, I keep a silver oxide paste on hand, kind of like Desitin but more powerful, to help alleviate her rash. I also found another topical treatment with anti-fly ingredients that can safely coat her belly and sensitive areas (it’s made for horses, I can’t remember the name of it now). I also use fly spray, and spray it on a towel first, then rub it on the body to that fly spray isn’t wasted and is more evenly applied.
But with my gelding’s welts, the best preventative is the Hydroxizone, because it thwarts the allergic reaction that causes the many welts. I wanted to try feed throughs, but they seem quite expensive to gamble on when I can just use preventative care and have relatively happy horses.