Equine Recurrent Uveitis (ERU) is an immune-mediated inflammatory condition of the eye and is the most common cause of blindness in horses. It may affect one or both eyes. You may have heard other names for this disease, such as Moon Blindness, Iridocyclitis, and Periodic Ophthalmia. While the exact causes are still under heavy research, bacterial, viral, parasitic, and protozoal infections, and trauma have been linked to this disease. In reality, most of the cases never have a definitive cause identified. In any circumstance, it is known that the initial bout of inflammation leads to leakages of substances from within the eye that are usually kept separate from the rest of the body. The horse’s overactive immune system therefore sees these substances as “foreign” and attacks them, often causing visible damage to the eye. It is important to understand that ERU is not one disease, but rather a grouping of ill-defined repeated incidences of inflammation within the eye.
Some horses show classic bouts of acute, painful inflammation followed by variable periods of comfort. Horses that are experiencing an acute ERU flare-up often appear very painful. Swelling around the eye, excessive tearing, squinting, a blue or whitish look to the cornea, blood or pus in the eye, a small pupil, and a change in the color of the iris may be seen. These horses may also be depressed, not maintain their order in the herd hierarchy, and be sensitive to light. Other horses, especially Appaloosas, are considered to fall into the “insidious” category, where a low level of inflammation is present but the horse does not appear to be painful to you. These cases are particularly difficult for owners because a veterinarian may notice a significant amount of irreversible damage to the eye(s) on a routine health examination and the owner has been unaware of the changes. Both scenarios can cause chronic changes that can include an irregular shape to the pupil due to adhesions within the eye, mineral deposits on the cornea, cataracts, movement of the lens, glaucoma, scarring of the retina, or retinal detachment. In fact, ERU is considered the most common cause of equine glaucoma, which can cause degeneration of the optic nerve and prevent transmittal of visual information to the brain (i.e. cause an irreversible loss of vision).
If your horse is diagnosed with ERU, a treatment plan will be implemented. Likely, some form of maintenance therapy will be needed even when your horse appears comfortable, and this is JUST as important as treating acute attacks. In additional to topical medications, oral aspirin is often given as a maintenance drug to help reduce inflammation; you can find equine aspirin over-the-counter at tack stores and online. Topical steroids are still needed to help quiet the inflammation you cannot see, but may be present.
The prognosis for a horse that has ERU is always guarded. While there is no way to prevent a horse from developing this devastating disease, careful evaluation of your horse’s eyes by your veterinarian at a pre-purchase examination or routine health care visit can alert him/her to the need for medical intervention. By working together, we can try to maintain comfortable, visual eyes as long as possible.