Okay, this may be a little off the beam but I will give it a go. Has this horse been power packed in the last year? I would put out a Sulphur block for her, if she has some minor internal infection, she will go to that block. The background reasoning is that inflammatory responses to insects are related in part to allergies to the insects, (although of course you have to deal with the insects themselves), which points sometimes to diet or imbalances in the horses system. . For the bug part, I used a product called Belly Balm down the centerline of the belly and sometimes also used the clear Swat, and/or Zinc oxide to simply cover it in the case of the crevices of the body like between the legs, in and around the gelding sheath, or the mare’s udder, if there was a big problem with bugs, the Belly Balm works like a CHARM for the belly line and it stays on for at least a day sometimes 2-3. https://www.valleyvet.com/ct_detail…b-735d461107a6
I also used various kinds of mineral supplements, one from a product that is made by Dynamite which is a dolomite clay I think, (some itchy horses really like that and I fed that by hand), just offered as a treat, if they need it, they will lick it. Dynamite also has a volcanic mineral mix with which some horses really thrive. I was taking care of a horse which had a horrendous sweet itch problem, lumps all over and the itchiest skin I have ever seen, she was mauling herself by biting her skin because it hurt so badly, I thought she might also have had a skin infection of some kind so I bathed her in Eqyss fungal shampoo. In about a month of the whole program, offering the different minerals (Sulphur block, Cobalt block for B-12 insufficiencies, white and red minerals blocks, the PowerPAC, the vet just used Quest for that), and the two baths, she was almost wholly healed from the lumps and itching except for the worst area which was from the bite abrasions.
On the question of the sweet itch, I was working from the thought that I was dealing with Neck Threadworms, and was working from a protocol developed by a member of the COTH, there is a sticky or favorites archive with that title, take a peak at that if you can find it. I also had gotten a briefing on an article from a woman who had researched the subject at the University of Washington’s veterinary sciences archives. She came across a research paper from some veterinarians who had done a study of horses in I think it was Finland?, where they tested horses in the wild who had sweet itch and those who did not. They found that the horses with sweet itch there were low on Copper, so the red salt block takes care of that if that is a deficiency. The horses will go to the salt and the minerals they need to make up for the regional lack.