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Uveitis in Horses Anatomy of the Eye

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Uveitis in Horses Anatomy of the Eye ANIMAL EYE CLINIC INFORMATION SERIES! www.seattleaec.com Uveitis in Horses Anatomy of the eye The Uvea Uveitis is the leading cause of blindness in horses. The word “uveitis” means inflammation inside the eye. The uvea is the middle layer of the eye. It includes the iris, the ciliary body, and the choroid. Inflammation of these structures can result from many causes, and can result in damage to structures inside the eye, potentially leading to vision loss and discomfort. ! PAGE 1 ANIMAL EYE CLINIC INFORMATION SERIES! www.seattleaec.com WHAT IS UVEITIS? Uveitis is inflammation of the inner middle layer of the eye. Inflammation can result from many causes, and basi- cally means swelling, redness, oozing. Just as a finger will swell, turn red, and become painful if struck by a hammer or becomes infected, the uvea can swell, become red, and leak protein, fluid, or white blood cells when in- flamed. Many horses develop a long term recurrent uveitis- Equine Re- current Uveitis (ERU), also called “Moon Blindness”. WHAT ARE THE CAUSES OF UVEITIS IN HORSES? Uveitis can result from trauma (blunt or penetrating), various types of infection within the eye, certain types of cancer, or due to autoimmunity. ! Trauma: Just as a finger swells and turns red when struck by a hammer, the layers of the eye will swell & become inflamed when subjected to trauma. This can be a blunt in- jury, or some type of penetrating injury. Typically the inflammation subsides on its own over time, but permanent damage to the eye can result. ! Infection: Inflammation is one way the body fights infection. It allows the body to send antibodies, white blood cells, and other disease-fighting molecules to the site of infec- tion. Uveitis can result from infections inside the eye (bacterial or fungal), or on the surface of the eye (cornea). ! Cancer: Various types of cancer can result in uveitis. Solitary tumors inside an eye will cause inflammation as they grow within the normal tissues. These can be primary tu- mors- i.e. started in the eye, or metastatic tumors- i.e. spread to the eye from some other place in the body. Cancer elsewhere in the body can also cause the immune system to at- tack the eye inappropriately- so called paraneoplastic uveitis. ! Autoimmunity: This is by far the most common type of uveitis in horses. In this situation the body’s immune system has become sensitized to uveal tissue- it regards these normal intraocular structures as a dangerous foreign invader and attacks the uvea as if it ! PAGE 2 ANIMAL EYE CLINIC INFORMATION SERIES! www.seattleaec.com were a bacteria or virus. The cause of this sudden change is unknown. This is similar to other autoimmune conditions like Lupus, Rheumatoid arthritis, etc... Equine recurrent uveitis, as the name implies, is a disease in which the eye(s) becomes in- flamed time and time again. The majority of cases are believed to be auto-immune in na- ture. Something- trauma, dis- ease, genetics, sensitizes the immune system so that it now thinks that the uveal tissues are foreign, and must be elimi- nated. From time to time the body attacks the eyes, trying to rid itself of the persisting “for- eign” presence, as if the uvea were a bacteria or viral invader that needed to be cured. In some cases there can actually be a persistent infection. A bacterial infection- leptospiro- sis, has been found to be a cause of ERU. Leptospirosis can be carried by wild rodents, rac- coons, and other animals, and can be shed in their urine. Horses in areas frequented by in- fected wild animals can become infected as well. This usually results in a brief mild illness, but the bacteria can get into the vitreous- the transparent gel that fills the back of the eye. In this location the bacteria can “hide” from the immune system. From time to time the body realizes that the bacteria are there and attacks the eye. This is not a common cause in the Pacific Northwest region of the United States. In parts of Europe it has been found to be the leading cause of equine uveitis. Based upon the specific historical and ophthalmic findings in your horse, your doctor may suggest various tests to help determine the specific type of uveitis. WHAT HAPPENS WHEN THE UVEA IS INFLAMED? In most cases active uveitis is uncomfort- able. Horses are light-sensitive, the pupil will often become very small (constricted), the eye may become very cloudy or change color. In the short term vision can be af- fected by increased murkiness of the in- ! PAGE 3 ANIMAL EYE CLINIC INFORMATION SERIES! www.seattleaec.com traocular fluids. Inflammation can lead to permanent damage to structures within the eye as well. Damage to the lens can lead to cataract formation. Damage to the drain inside the eye can lead to increasing pressure as fluid is unable to exit normally (glaucoma). Fluid can leak from the back wall of the eye separating the retina from the wall (retinal detach- ment). Every time the inflammation is active, additional damage is done, so keep- ing uveitis controlled is very important if vision and comfort are to be maintained. HOW IS UVEITIS TREATED? When a specific cause is discovered, treatment includes specific therapy directed at that cause. In addition to these specific therapies we would employ antiinflammatory medica- tions. These might include corticosteroids like prednisone and dexamethasone (drops and/ or tablets depending on severity and part of the eye involved) and nonsteroidal antiinflam- matory medications (NSAIDs). Both treat inflammation, but steroids have the additional benefit of suppressing the immune system’s reactivity. Cyclosporine is an antirejection drug which can also reduce the frequency and severity of episodes. In certain cases we can im- plant a slow release cyclosporine implant into the eye to provide continual treatment. This surgery can often be performed as a standing procedure, and the implants are very well tol- erated. Often we will treat with drops to dilate the pupil, as much of the discomfort associ- ated with uveitis results from spasm of the muscles that constrict it. Dilation therefore im- proves comfort. Autoimmune uveitis usually requires lifelong treatment to maintain con- trol, although medications are usually decreased over time to reduce potential side effects. Your ophthalmologist will discuss the treatment options based upon your animal’s individ- ual situation. WILL MY HORSE LOSE VI- SION COMPLETELY? This is possible- even with all available treatment. Your doctor will discuss the prognosis for your horse based upon the stage of disease and treatments se- lected. It is important to keep in mind that many blind horses enjoy a good quality of life as long as comfort is main- tained in a protected and loving envi- ronment. ! PAGE 4.
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