Although poorly understood, ERU is the most common cause of blindness in horses. It is thought to be an auto-immune disease. In its classic form, the body's defenses attack the internal structures of the front chamber of the eye (the uvea, which is the colored iris) causing the ciliary muscles to spasm and constrict the pupil.

Why the immune system gets confused and attacks the eye is not completely understood. The bacterial infection Leptospirosis may be involved. In some cases, it results from an underlying eye injury and it can appear after other illnesses. In most cases the cause is never determined, however there may be a genetic component.

Regardless of the cause, a cycle of inflammation and damage begins, including adhesions of the iris to the lens (posterior synechia). "Active" periods of pain and inflammation (that usually last two to three weeks) are interspersed with periods of relative "calm."

ERU can end in the destruction of the eye, including detachment of the retina and blindness. It is notoriously difficult to treat. It tends to recur after treatment ends, and often makes surprise re-appearances later. This condition is much more common in the Appaloosa breed. It is important to consider this condition in any case of stubborn, repeated or puzzling eye inflammation.