The most common cause of blindness in horses is equine recurrent uveitis, also called ERU. ERU is inflammation of structures inside the eye that seems to come and go, and in the past was called moon blindness because it seemed to occur and disappear with different phases of the moon. Of course, this was shown not to be the case and it's actually an immune mediated disease reacting to various antigens, one of the which may be the bacterial organism Leptospira. Appaloosas are the number one breed affected in the United States, although warmbloods are also commonly affected but the disease can occur in any breed. Most horses will show pain in the eye by squinting and having excessive tearing with a blue to white appearance to the cornea.
Now it is important to stress there are many equine eye diseases that can cause these symptoms such as corneal ulceration, and these two diseases require totally different treatment. Glaucoma can also appear to be similar and again requires completely different treatment. For this reason, never place an eye ointment in your horse's eye without having your vet first examine the eye. Topical treatment for uveitis consists of cortisone. which cannot be used if there is a corneal ulcer as a fungal infection can develop that can lead to blindness. With uveitis, the eye should be dilated with atropine, which should not be done if glaucoma is involved. As far as products at the feed store that indicate they can be used in the horse's eye, please do not use them as many may cause damage to the eye, and time spent treating with these products allows the disease to progress; this is valuable time that should be used treating the horse as your vet suggests.