I've never dealt with it before, so don't have any advice I could offer. I knew a guy once who's horse had it, but it only took the one eye. They had the vet medicate the horse. It can still be ridden, but is a bit shy on that side due to the impaired vision. There is a great horse vet over by antler hill on the QE2 (south of RD) if that helps at all... In the meantime consider buying a guardian type fly mask to keep the sun and bugs out of his/her eyes. Here are some links I found.
http://www.igs.net/~vkirkwoodhp/eru.htm
http://www.merckvetmanual.com/mvm/in...m/bc/30300.htm
http://www.horsegroomingsupplies.com...ess-79973.html
http://www.horsegroomingsupplies.com...ml#post1035450
http://www.ksl.com/?nid=148&sid=263170