mare with a terrible reaction to bug bites

my pregnant mare seems to be having a reaction to bugs, she has rubbed out her beautiful tail and mane. Her belly is raw and she just looks uncomforable,she is constantly rubbing herself on fences, trees and the ground. I have been trying to hose her down with just water paying special attention to her belly and not using any product(which i think also lends to this itchyness)I have even made a marigold tea rinse that I pour over her to help ward off the bugs but nothing seems to help after a few days she is back to rubbing again. Is there anything that you can recommend to help relieve this. Someone suggested that i do a flush of her system to get out the toxins built up from the bug bites but i am a little hesitant due to her pregnancy. please help my itchy mare!

This sounds like sweet itch.
   
There are lots of products on the market to stop itching but the best advice is to get a fly rug for her and use plenty of fly repellent and keep her stabled either side of dawn and dusk.
   
There are sweet itch creams that work the most effectives being Heel To Hoof and the NAF creams. Follow the directions on the tub.