Uveitis means inﬂammation of the iris and deeper structures inside the eye. Equine recurrent uveitis (ERU) is the most common cause of defective vision or blindness in the horse. ERU is also known as “periodic ophthalmia” and “moon blindness”. It is characterized by multiple episodes of “active” uveitis alternating with periods when the eye is “quiet”, or not inﬂamed. One or both eyes may be involved in the disease. Chronic ERU may result in partial or complete loss of vision. Signs of “active” ERU include tearing, squinting, sensitivity to light, and a cloudy or red eye. The active episodes of ERU cause permanent changes in the eye, including scarring of parts of the eye and possibly cataracts or retinal detachments. These permanent changes can be seen by a veterinary ophthalmologist even during the “quiet” stages of the disease. During the “quiet” stages, the eye is not painful and inﬂammation is minimal. ERU is caused by an overly active immune response of the eye to many different sources. The source may be bacterial (such as Leptospira, Brucella, Salmonella, or Streptococcus), viral, parasitic (such as Onchocerca, Strongylus, or Toxoplasma), or fungal. Trauma to the eye or certain diseases of the body can also be sources of ERU. It is important to remember that the source only initiates ERU. The original source may be treated or may go away, but the horse’s own immune system may perpetuate the inﬂammation and disease in the eye. Relapses of inﬂammation may continue to occur although the original source of inﬂammation is no longer there. Identifying the original source of inﬂammation may prove to be very difficult, and in many cases, the causative agent or original source of inﬂammation is never identified. A genetic predisposition to ERU as some breeds, such as Appaloosas, which have a higher incidence of the disease, may exist.
Treatment of ERU alleviates the pain associated with the “active” stages of the disease and decreases inﬂammation in the eye while also attempting to preserve vision. Medications should be prescribed by your veterinarian as soon as signs of ERU are noted. For each “active” stage of the disease, it may be necessary to continue treatment of the eye for several weeks to several months, depending on the severity of inﬂammation. Although most treatment involves a combination of topical eye drops/ointment and oral or injectable medications, some patients may be candidates for a surgical implant device that slowly releases cyclosporine, an immunomodulatory medication.
Relapses of inﬂammation of the eye require reinstatement of topical and systemic treatment by your veterinarian. It is important to contact your veterinarian or veterinary ophthalmologist if you feel your horse is having a recurrence of ERU. Signs of ERU can be similar to other conditions such as glaucoma or corneal ulceration and some of the medications used to treat ERU may be contraindicated for these conditions. Thus, it is important that you consult your veterinarian or veterinary ophthalmologist prior to restarting medications for ERU. It is also important that you contact your veterinary ophthalmologist prior to stopping or decreasing the frequency of any prescribed medications. It is helpful to keep the horse in a darkened stall with limited exercise during treatment periods. If treatment fails and the eye becomes blind and painful, the eye may need to be removed to eliminate discomfort.
The prognosis for vision in horses affected with ERU can be poor. Prompt treatment may slow or prevent the occurrence of blindness.