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Equine Recurrent Uveitis or Moon Blindness in Horses

Reprinted from petcaretips.net



Equine recurrent uveitis (ERU) is the most common cause of blindness in the horse. ERU is also known as "Moon blindness. Which is due the recurring nature of the disease once thought to coincide with the phases of the moon. Uveitis is the inflammation of the vascular structures of the eye. These structures include the iris (the colored part of the eye surrounding the pupil), ciliary body (structures in the eye that help support the lens) and the choroid (the vascular structure in the back of the eye that supplies blood to the retina). ERU is thought to be an immune-mediated disease process that can be triggered by many different causes. Bacterial agents such as Leptospira as been most commonly associated with ERU, but some viral agents such as Influenza, Equine Herpesvirus and Equine Viral Arteritis have also been implicated as the initial cause of ERU. Blunt trauma or damage to the eye itself has also been shown to start the immune-mediated reaction that leads to ERU. Hypersensitivity to certain parasites such as strongyles, and Onchocera cervicalis are also known to cause ERU. The initial episode of the uveitis may go unnoticed by the owner and clear up on its own. Repeated episodes, due to the immune-mediated aspect of the disease, can lead to damage to other structures in the eye, such as the lens and cornea resulting in blindness. Clinical signs associated with ERU vary depending upon the severity and duration of the disease. Episodes that appear suddenly are usually painful and many horses become head shy. Excessive tearing from the eye, eyelid spasms (blepharospasm), and light sensitivity (photophobia) are often noticed. Within 1-3 days the affected eye will become cloudy and red from underlying inflammation. A thick yellowish discharge will also develop. Changes within the lens such as pigmentation and the development of cataracts can occur weeks to months after the uveitis begins. ERU is diagnosed by noting a history of recurring and chronic eye disease characterized by the previously mentioned signs and physical changes to the eye noted upon examination. As soon as signs are noticed, treatment for ERU should start immediately to prevent further damage. Treatment includes anti-inflammatory eye drops and drops that dilate the pupil. Systemic anti-inflammatory drugs, such as fluxin, phenylbutazone or ketoprofen are given to decrease inflammation and pain. Anti-inflammatory drugs are the most important aspect of therapy and may be necessary for several months. Dosage and frequency are dependent upon the severity of the disease. Discontinuing drug therapy too soon is often a cause for relapse. Stabling the horse in a dark stall or covering the eye with a hood can also be helpful in treating the pain associated with the disease. If a specific cause of the uveitis is known or suspected, it should be investigated and treated appropriately. Serological testing for Leptospira and other bacterial agents may help in confirming the cause, but a negative test result does not rule out ERU. Biopsies of the conjunctiva (the outer lining of the eye) can help diagnose the larval form of Onchocera cervicalis as the underlying cause. Clinical signs of the disease can come and go. Quiescent stages, the stages when the eye seems normal, may last from weeks to months to years before another episode occurs. Darkening of the iris commonly occurs with ERU as it becomes more chronic. Common ocular complications resulting from ERU include glaucoma and limited pupil movement. The iris attaching itself to the back of the lens or cornea causes limitation of pupil movement. Prompt symptomatic treatment is helpful in prolonging the integrity and vision in the affected eye. Unfortunately, each attack of ERU leads to more damage to the eye and eventually blindness develops, resulting in a poor long-term prognosis.

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