I'm in the heart of sweet itch country and because of midges: tiny mosquito like insects with bites like alligators! We even have midge forecasts here! Kid you not! Just like a weather forecast!
Some of the worst sweet itch horses I've ever seen though have been in the USA and poor sops have had skin like rhinosuarus hide! Mainly because of poor pest and parasite control and such as bots etc.
Because basically it's a total over-exagerated immune response to the likes of that, then it's vital to tackle the root cause and quickly and effectively as possible.
The key as far as I'm concerned is to protect as far as you can from what is causing the problem and that's why I'm slightly manic about fly repellent.
Those who may know me in real life or from other forums know I 'market' my own fly repellent recipe for a donation to leukaemia research. I first 'discovered' it on the quest for something that works in an area where there's masses of water and midges AND horse and deer fly.
The following is about as good an online information resource as possible for sweet itch: The National Sweet Itch Centre and the Boett Blanket - sweet itch rugs