Sweet Itch?

My old mare is itching her face raw. The flies torment her way more than her teammate. So much so that she will stand in the shade all day rather than go out and graze. I’m currently bringing her into the barn during the day and tying her up with hay in front of her. I don’t think she appreciates this setup, but at least she’s eating (she is too skinny, and hasn’t been gaining this summer. My pasture is pretty poor still, but her younger teammate has gotten quite fat on it). Today while I was looking at her, I started wondering if she is having an immuno-response to something. The midges? The flies? Undetected lice or mange? The Nutrena Boost I have her on? The boost is a ration balancer with added fat from rice bran. Or is it possible that having her on the boost is making her more attractive to flies? She also gets soaked oats and has free choice access to a variety of minerals.

Any tips for identifying the underlying cause of extreme itchiness? Also, what are recommendations for giving her some relief?

Lizzy