Equine recurrent uveitis, also called "moon blindness" or "periodic ophthalmia", is an inflammatory disease of the eye. It is characterized by repeated inflammatory episodes of the iris, ciliary body, and choroid, collectively known as the uveal tract. It is a leading cause of blindness in the horse. ERU is an immune disease mounted in response to infectious agents or the eye's own components- there are many suggested agents which may stimulate the chronic state of the disease. Clinical signs include squinting, pain, and tearing. The treatment of ERU typically involves the reduction of inflammation with NSAIDS and corticosteroids, as well pupil dilators (myadriatics/cycloplegics). New therapies in ERU have shown great promise in treating this difficult disease.

Causes of ERU

The exact cause of equine recurrent uveitis is unknown. A number of events can cause the inflammatory response which characterizes ERU. Proposed causes include bacteria, viruses, parasites, and even trauma.

Horses with ERU have repeated bouts of acute inflammation, and many develop chronic, secondary side effects. During the period of acute inflammation, it is common to have reddened, swollen, painful, cloudy, and tearing eyes. The horse often squints, and is sensitive to light. The fluid within the eye can appear cloudy. The cornea can take on a blue-white appearance and small blood vessels may appear on the corneal surface. The pupil constricts as a result of increased inflammation in the eye.

In addition, horses with long term ERU can develop secondary side effects. Adhesions between the iris and lens, called posterior synechia, occur as a result of pupil constriction. Scarring and fibrosis of the cornea and iris can occur. Cataracts and retinal degeneration can develop, leading to blindness.

Your veterinarian will diagnose ERU upon the presentation of clinical signs and history of recurrent ocular inflammation. A blood test, serology, can be done to detect antibodies to Leptospira or other infectious agents. However, a positive diagnosis only suggests a possible cause, not a confirmatory diagnosis of ERU.

The primary goal in the treatment of ERU is the reduction of inflammation in the eye and the prevention of vision loss. Treatment consists of topical and systemic therapies. If an infectious cause is identified, treatment of the cause is initiated as well. Two new therapies, pars plana vitrectomy and Cyclosporine-A implantation, are available.

Non-steroidal anti-inflammatory drugs (NSAIDS) and corticosteroids are commonly used to reduce inflammation and pain in ERU. NSAIDS such as Banamine and Phenylbutazone (Bute) can be administered systemically to decrease the inflammatory response. Topical corticosteroids such as

Dexamethasone can also be used to reduce inflammation, but they must be used with great care. They cannot be used in cases with corneal ulceration, as they inhibit the healing of the ulcer, and may predispose the eye to significant infection.

Often, topical pupil-dilating agents, called mydriatics-cycloplegics, are used in the treatment of ERU. Dilation of the pupil decreases pain, and reduces the contact area between the lens and iris, reducing the possibility adhesion formation, called posterior synechia. 1% atropine ointment is commonly used as the mydriatic agent.

On some occasions, a subpalpebral lavage tube may be placed. This is a small tube that is placed through the eyelid so that it rest just above the eye. This allows for frequent treatment of the eye with topical medications, without even touching the eye.

Cyclosporine A implantation is a new treatment which has potential in the treatment and prevention of further outbreaks of ERU. Cyclosporine A is an immunosuppressant drug that decreases T-lymphocyte activity. The Cyclosporine implant is placed into the eye in a surgical procedure done under general anesthesia. Horses that have undergone implantation have shown significant, long term improvement, with decreases in the severity and number of inflammatory episodes.

Pars plana vitrectomy is a surgical technique which has shown promise in the reduction of recurrence of inflammatory episodes of ERU. The vitreous, or fluid in the eye, is removed to improve clarity and remove inflammatory cells from the eye. Removal of the vitreous is proposed to reduce the immunologic memory of the eye, and as a result reduce the inflammatory reaction.

Equine recurrent uveitis is a chronic inflammatory condition of the eye which often necessitates significant treatment, and can ultimately lead to blindness. By working closely with your veterinarian and/or veterinary ophthalmologist, ERU can be well managed to reduce the frequency and severity of episodes.