What is ERU and How Does it Impact Horses?

Equine recurrent uveitis, or ERU, is the most common cause of blindness in horses worldwide. Here’s what to remember.
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Q. A horse at my barn was diagnosed with ERU. What is it?

A. Equine recurrent uveitis (ERU), also known as moon blindness, is a painful condition where repeated occurrences of inflammation in the eye can cause permanent damage. The inflammation could cause career- or even life-ending complications, explains Jacquelin Boggs, DVM, MS, Dipl. ACVIM, senior technical services veterinarian with Zoetis. It is the most common cause of blindness in horses worldwide.¹

Equine recurrent uveitis is characterized by repeated occurrences, or flares, over the course of the horse’s life. The painful flares can reoccur months or even years apart; however, the interval between flares often shortens over the horse’s lifetime. With ERU, the horse’s immune system inappropriately attacks normal or host proteins in the eye, causing damage that can lead to blindness.

The condition can affect one or both eyes. You might notice an affected eye appearing cloudy or discolored. Your horse might squint or tear excessively, and the pupil could be constricted. These are frequently the first signs, with more severe changes to the eye developing with subsequent flare-ups. There is no cure for ERU. Even with the best treatment, some horses that develop ERU don’t return to their previous level of performance

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