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Equine Recurrent Uveitis
(Chronic Intraocular Inflammation)

One of the most common eye problems in horses is chronic uveitis, or intraocular inflammation. Old terms for this disease include Moon Blindness, and Equine Periodic Ophthalmia. These terms allude to the waxing and waning clinical course of this disease-- it comes and goes, and was thought to occur in conjunction with the phases of the moon (which is not the case). The technical term for this disease is Equine Recurrent Uveitis (ERU).

ERU can be devastating, and over the course of months to years, cause blindness in one or both eyes of affected horses. In some cases, the uveitis causes secondary glaucoma (an increase in the intraocular pressure of the eye) to develop, which is both uncomfortable and blinding. If glaucoma develops, it is much more likely that the eye will eventually require surgical removal (enucleation).

Clinical Signs

Horses with ERU can exhibit many different clinical signs. The most common ones are: increased tearing, squinting, AND/OR redness and/or swelling of the conjunctiva, AND/OR cloudiness. Other clinical signs which can be present: misshapen pupil, small pupil (miosis), a "mass" inside the eye of fibrin (fibrin is essentially a blood clot without the blood, and is a creamy color), blindness (but this may not be apparent if the other eye is still visual), cataract formation, and/or bleeding inside the eye. If the horse has been rubbing the eye, there may also be present a corneal ulcer secondary to rubbing. In eyes with chronic ERU and secondary glaucoma, the eye is often cloudy, and the cloudiness might wax and wane. Additionally, eyes with long-standing glaucoma are usually enlarged, and they are blind. It is important to know that even if the horse does not act painful with glaucoma, the eye is uncomfortable and gives the horse a "headache" type of discomfort that is usually not obvious to the owner.


The causes of ERU are not nailed down, unfortunately. ERU is a non-specific, immune-mediated disease that results in repeat episodes of attacks of inflammation of the eye or eyes. The current theory is that the body develops antibodies to certain infectious organisms, such as the organisms causing leptospirosis (a bacterial infection) and onchocerciasis (a parasitic infection). Many other types of organisms can trigger this antibody reaction, however. These antibodies are produced by a special white blood cell called a T-lymphocyte. and the antibodies may mistakenly attack tissues in the eye, creating inflammation. The antibodies can continue to attack the eyes months and years after the organism has left the body, either constantly, or intermittently. With each new ERU attack, the eyes are more and more painful and undergo more damage. It is a complicated theory, however, because not all horses with antibodies to some of these organisms will develop ERU. Genetics play a role too, as Appaloosas with antibodies to Leptospira are 8 times more likely than other breeds to develop ERU and 4 times more likely to become blind as a result of the ERU. Many other infectious diseases including viral diseases can also trigger ERU.


A thorough ophthalmic examination is essential in the diagnosis of ERU. Your general practitioner veterinarian may recommend referral of your horse to a veterinary ophthalmologist for further evaluation. It is important to measure the intraocular pressure in these patients (this is called "tonometry") if possible, but most general practitioners do not have the very expensive equipment needed to perform tonometry in horses. Sometimes blood tests will be recommended, particularly testing for antibody titers to Leptospira, especially the serovars (this means different strains of the bacteria) pomona, bratislava, and autumnalis in North America. The titer results can be difficult to interpret. Although it is not a practical thing to do because general anesthesia is required, submitting both a sample of blood and of intraocular fluid (aqueous humor) and comparing the two titer results is a better diagnostic test method for Leptospirosis-caused ERU, than just measuring the titer from a blood sample.


Overall, the prognosis is fair to poor for keeping the eye visual and controlling the uveitis, especially in Appaloosas. Treatment is usually lifetime and can take much time and money, but it prolongs comfort and vision in most cases. It is important to understand that if the eye is blind and painful, it should be removed (enucleated) by your veterinarian. Some veterinary ophthalmologists offer surgical treatment of ERU if the eye is still visual, in which a portion of the vitreous humor (the gel-like substance in the back chamber of the eye) is surgically removed. This is called core vitrectomy.

Medical treatment is by far most commonly prescribed, and can include topical corticosteroid ointment or drops, subconjunctival steroid injections, topical and/or systemic nonsteroidal anti-inflammatory medications, topical dilating drops, and/or systemic antibiotics if an active leptospirosis infection is suspected (but this treatment is controversial). It is important to realize that medical treatment is usually lifetime, even if the uveitis is controlled. If medication is discontinued or decreased without input from your veterinarian, the horse will likely have a repeat attack, and the attack will probably be a severe one. Each attack of uveitis is cumulative; that is, the damage from each attack adds on to the damage from all previous attacks, and brings the eye that much closer to blindness and potential removal. Consistent treatment is important!

Homeopathic treatment has been described, in which a substance called MSM powder has been used. There is no clinical research evidence that it helps, but it apparently does no harm when given.

A very promising combination of medical and surgical treatment is also available, where a small implant is surgically placed inside the eye. This implant is like a small pump, and it delivers a drug called cyclosporine to the inside of the eye. The cyclosporine helps to stop the T lymphocytes from producing the "bad" antibodies that attack the eye, and the implant is usually effective for 5 years. This procedure is performed at the Veterinary Teaching Hospital, Department of Clinical Sciences, at North Carolina State University. For horses to be candidates for this treatment, the following criteria must be met:  1) The horse must have no other generalized illnesses,  2) The eye must have good vision, have good retinal function, and be of relatively normal size,  3) The ERU must be able to be controlled well with medication, and  4) The eye must not have cataract formation.


ERU is an unsoundness, and if the horse is ridden, careful consideration should be given to the hazard of riding a horse if the animal's vision is compromised.

It is important to recognize that lifetime treatment is needed in most horses with ERU, and that if the eye(s) are not treated as recommended by your veterinarian or veterinary ophthalmologist, or if the animal is not reexamined as frequently as recommended by your doctor(s), then the likelihood of blindness occurring is much higher, in addition to an increased likelihood of glaucoma developing. Therefore, the chance of the eye requiring enucleation is heightened if the eye is not treated or examined as recommended.


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