I havn't had very much experience with it but from what my friend has dealt with it's usually progressive and eventually causes blindness in the eye.
HOWEVER, there are "fly masks" that are made for uveitis.. they look like bug eye mask. My friend's horse had to wear the mask pretty much 24/7 while outdoors unless you take it off during night time hours.
Good luck! Sorry this happened to you and your horse.
~ Starline Stables ~