I had a horse with Uveitis in her left eye. It is also called moon blindness and it is a terrible disease. We fought trying to get the Uveitis under control, but in the end we had to have the eye removed.

Many people I rode with said I should put the horse down. I remember one man who I was riding with one day. His horse was out of control and a total a basket case. My horse was enjoying the day out on the trail and nice and calm. He asked me if I put that horse down, and I said "nope, I am riding her right now". He was totally blown away that a one eyed horse could be ridden on a trail.

Thanks for that information regarding Uveitis. But what are the causes of this kind of disease? I don't want our horses to have one eye only. And what are the early symptoms of a horse having ERU?

I used to ride a horse like that. He developed it at about age 15 in his left eye. The horse was the owner's first and currently teaching her daughter to ride so putting him to sleep wasn't an option. We had the eye out and he was just as good as ever. We had to be a bit more vocal and careful about approaching from that side or running him into things but he was fine. He ended up with it in his right as as well a couple years later but we got it under control. She used to take him out to local shows without an issue. He was a great "pirate" in fun shows for the costume class. The kids used to enjoy taking off his eye patch and seeing the scar and scaring themselves silly.

He presented with symptoms of walking into his stall door (we had dutch doors with the tops open most times) and bumping into his hay rack even though he had been in the same stall for over 5 years. He also shied away from things that approached on his left side and crossed to his right side, going from the blind side into his good side without warning. Petcaretips.net describes it as:
Episodes that appear suddenly are usually painful and many
horses become head shy. Excessive tearing from the eye, eyelid
spasms (blepharospasm), and light sensitivity (photophobia) are often
noticed. Within 1-3 days the affected eye will become cloudy and
red from underlying inflammation. A thick yellowish discharge
will also develop. Changes within the lens such as pigmentation
and the development of cataracts can occur weeks to months after
the uveitis begins. ERU is diagnosed by noting a history of
recurring and chronic eye disease characterized by the previously
mentioned signs and physical changes to the eye noted upon
examination.