Sweet itch will worsen/improve depending on the condition the horse is kept in.
My horse has severe sweet itch but can be managed with a Boett/sweet itch rug and a suitable feild. He has had sweet itch for 9 yrs and it hasn't got any better or worse in terms of his sensitivity to it...but has got better/worse depending on where he has been kept.
depends on the amount of midges also is it def sweetitch and not protein itch alot of ppl make this mistake, someone i no thought her horse had sweetitch and spent a fortune on him last yr this yr he is without the feed and is loads better and is without rug most of the time whereas b4 he rubbed so bad he was very sore