Lots of horses suffer from this, I have some knowledge on it as a horse at our yard suffers from it badly on his bely.
The actual chemical from the flys that the horse is allergic to is the fly's saliva.
When the fly's bite the horses there saliva does to. It can be very horrid and there is no cure. But any Q's on how to help with it if you have this problem this is a discusion all about Allergic + Flys