Very well said Wallaby. I also have a horse, 23 yr old mare, that has been battling ERU for several years. It appears her trigger is allergy based, so spring and fall are seasons I hold my breath and pray!
She has just come out of an episode, in her only sighted eye, and is now almost sightless. She has no night vision, but seems to navigate fairly well in daylight. Only time will tell as to what will be.
I've done extensive research in hopes to prolong the time between flare-ups, but there are as many treatments as there are "causes" , as Wallaby said, it is very individual. With T, we are struggling to build up her immune system. My vet believes it is auto-immune based disease, and is always "bubbling" behind her eye, waiting for a trigger.
It is very painful for the horse. It is imperative that the eye dilates before it is "frozen" in a constricted state. Time is of the essence with a flare-up.
With all the searching I've done, I haven't come across the beta carotene suggestion. I will look into it.
Best of luck to you and your young horse Coptordoctor. It is a long journey.
Be the kind of woman that when your feet hit the floor each morning the devil says, "Oh crap, she's up!".