Equine Vision: Part 3

By Lauren Lamb

Equine recurrent uveitis (ERU), also known as Moon Blindness, is a chronic, painful, inflammatory disease of the horse’s eye. The term Moon Blindness was first coined during the 1600s when this disease was initially documented. At that time the cause of the disease was thought to be related to the phases of the moon. Even with today’s technology and current therapies, ERU is one of the most difficult equine ophthalmic diseases to treat and the most common cause of blindness in the horse.

Clinical signs of ERU included squinting, tearing, swollen eyelids, congested vessels in the sclera (white part of the eye around the clear cornea), conjunctivitis (swelling of the pink tissue on the underneath side of the upper and lower eye lids) and constriction of the pupil and light sensitivity. The cornea may also become cloudy. The lens may become dislocated or develop a cataract in horses with several re-occurring episodes.

Additionally, the retina may become detached or the optic nerve damaged leading to blindness. When the eye is examined, rarely is there any fluorescence stain that is picked up by the cornea. Some eyes will develop a corneal ulcer secondary to ERU. Glaucoma, which is rarely seen in horses, can develop secondary to ERU.

The cause of ERU is still unknown. Hereditary, housing in a damp stall or marshy pastures, feeding bad grain or a bacterial infection of the eye are all theories for the cause of ERU in horses. Recently, equine ophthalmologists hypothesize that the cause of ERU is multi-factorial. The key factor is the ability of a virus or bacteria to penetrate the blood-ocular barrier. The blood-ocular barrier prevents large proteins and cells that are circulating in the horse’s blood stream from entering into the eye.

To learn more about ERU, read Equine Vision in OKFR!