ERU, or equine recurrent uveitis — more commonly known as periodic ophthalmia or moonblindness — is a devastating disease of the eye. It is characterized by recurrent episodes of severe, painful inflammation that eventually leads to blindness related to cataract formation, adhesions in the eye and damage to the retina itself.
ERU cases start for various reasons, including injury to the eye, systemic infections with bacteria or viruses, and parasite infections in the eye. Leptospira infections are particularly common as a cause.
In addition to the inflammation that the infection causes, the immune system develops antibodies that can cross-react with the tissues of the eye. There is some debate among researchers regarding whether horses with ERU have persistent organisms in the eye or whether the disease can continue even after the infection is gone. In either case, the cross-reacting antibodies, called ”auto” (meaning self) antibodies, are truly a significant problem.
ERU symptoms occur in an on-again/off-again pattern with periods of severe inflammation alternating with quieter periods. The usual treatment was systemic anti-inflammatories combined with topical applications of drugs to the eye every few hours, preferably around the clock. The reason for the close and constant spacing of topical drugs is that they have to be absorbed through the cornea to do the horse any good, and the production of tears in the eye begins to wash them away shortly after they are applied.
Long-term use of NSAIDs (non-steroidal anti-inflammatory drugs) isn’t a desirable option because of the risk of side effects, and many owners find it extremely difficult to keep up with the multiple applications of topical drugs. The horses themselves are also usually resistant to having their painful eyes treated.
Injection of drugs under the thin membrane covering the white of the eye can result in better drug concentrations in the eye, but doesn’t work for all types of drugs and some veterinarians aren’t comfortable doing this procedure.
Surgical removal of the vitreous, a jelly-like sac that sits in the back of the eye behind the pupil, is sometimes used in an attempt to stop ERU attacks. It’s believed that removing this structure eliminates the T cells that are producing auto-antibodies and any resident organisms. It’s over 90% effective in stopping painful attacks. Unfortunately, many of the horses go blind anyway because as many as 45% develop cataracts postoperatively.
Now, a new technique that eliminates the need to treat the horse with systemic drugs and topicals on the eye is showing great promise. The results of a long-term study into the effectiveness of a device implanted into the eye that continuously delivers the drug cyclosporine A (CsA) was released in the June issue of Investigative Ophthalmology and Visual Science.
Dr. Brian Gilger, at North Carolina State University, coordinated the study. A small device holding the medication is implanted deep into the sclera (the white of the eye). CsA is a potent immunosuppressing drug, not something you want to be giving to the horse systemically but useful in blocking the damaging antibody reactions in the eye. The implantable device targets the horse’s eye tissues, greatly reducing the amount of drug needed and the chance of any side effects.
Painful flares were greatly reduced by this treatment. In a prior trial with implanting the device directly into the vitreous, there were also excellent results. Implantation of the device requires surgery under general anesthesia. The devices will deliver the drug for several years before needing to be replaced.
Horses that don’t already have extensive damage to the lens or retina are considered the best candidates for the treatment. It’s important to realize that this type of damage is irreversible and may progress even if the inflammatory attacks are controlled. When the horse’s eye is already extensively damaged and pain control/avoidance of further flares is the goal, vitrectomy could be a better long-term solution.
For information on obtaining treatment at North Carolina State University Veterinary School, or on centers that may be performing this procedure in an area closer to you, contact the Department of Clinical Sciences at North Carolina State at 919-513-6239. Dr. Gilger’s web page is www.cvm.ncsu.edu/docs/brian_gilger.html.
