Equine Recurrent Uveitis(ERU) is also called moon blindness, periodic ophthalmia, or iridiocyclitis. Many clients I speak to are familiar with the term moon blindness, but they don’t really understand the disease, what causes it, and how we treat it. It is a complex disease with potentially serious consequences. It affects all ages and breeds of horses, but certain breeds, Appaloosas for example, are more commonly affected than others.
The term uveitis means inflammation of the uveal tract. The uveal tract is defined as the iris(the colored part of the eye), the choroid, and the ciliary body. Together the components of the uveal tract maintain a steady intraocular pressure and supply the eye with blood and nutrients. When the uveal tract becomes inflamed, pressure inside the globe is altered, the pupil constricts, and the flow of blood and nutrients to the eye is disrupted. As you can imagine this is a serious condition that needs prompt medical intervention. Common clinical signs of uveitis include pain, sensitivity to light, corneal edema(bluish tinge to the eye), redness or watery eyes. If your horse has a blue iris normally, his eye may change colors to yellow or green. This happens because the inflammation inside the eye changes the refractive index of the light entering the globe. Severe cases can have fibrin clots or adhesions in the front or back(behind the lens) part of the eye. One or both eyes may be affected to varying degrees. In some cases clinical signs are severe and obvious, in others they are insidious. In these insidious cases I think it is helpful to stand in front of a horse and look at the direction and symmetry of their eyelashes. Eyelash asymmetry especially in an ERU case can indicate mild ocular pain because a horse may be holding his eyelids barely shut compared to the contralateral eye.
The causes of uveitis can be organized into three categories: traumatic, systemic, and immune mediated(ERU). Basically any trauma to the eye has the potential of resulting in a reflex inflammatory response within the eye. Systemic causes would be a bacterial, viral, or parasitic disease that makes the horse systemically ill, and subsequently develops uveitis. The third category, equine recurrent uveitis, is different from the previously mentioned two categories because it occurs repeatedly. Regardless of the initial cause, the “flare ups” are due to over activation of the horse’s immune system in response to active immune cells such as T-lymphocytes.
The goal of treatment is to reduce the inflammation as quickly as possible. Typically atropine is used topically to dilate the eye. Non-steroidal anti-inflammatory medications are typically given either intravenously or by mouth, and sometimes are given topically. Steroids are given topically and commonly intravenously, intramuscularly, or by mouth to aid in anti- inflammatory efforts. In my opinion, steroid absorption through the cornea is poor especially in an inflammatory state. I frequently administer systemic steroids for a few days in severe cases. In some cases tests for specific causative agents, Leptospirosis or Streptococcus for example may be run as well. If an infectious cause of the uveitis is diagnosed, then appropriate systemic treatment for the infectious agent is indicated. In my experience, duration of treatment is quite variable from case to case. Also, it is not always the most severely affected eyes that require the longest treatment. Each case is different as each individual’s immune system is responding in a different way to an inciting cause that we often can never diagnose. In addition, in ERU the interior structures of the eye become damaged and don’t function as well as they should making successful treatment more and more difficult. In some individuals, cyclosporin(an immunosuppressive agent) is implanted intraocularly.
Predicting when or if a horse will relapse is next to impossible. Some horses have one case of uveitis and never have another case. In most of these individuals, we never diagnose what “agitated” their immune system to cause the episode. Other horses who have ERU have periodic “flare up” of uveitis a few to several times per year. These cases can be very frustrating and as with every case of uveitis it is imperative to recognize that every moment there is intraocular inflammation, damage is being done to the interior of the eye that can result in permanent damage. Potential complications from ERU include cataract, lens luxation, glaucoma and blindness.
Dr. Jennifer M. Smallwood is an equine veterinarian in the Bluegrass Region of Kentucky, specializing in the areas of reproduction, sports medicine, acupuncture, lameness, and track practice. She also travels routinely to other states to provide acupuncture services. Dr. Smallwood received her Bachelor’s Degree from Texas A&M University where she received cum laude honors in the field of Animal Science. She attended the Texas A&M College of Veterinary Medicine, graduating in 2003.