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Equine Recurrent Uveitis
by Richard Sylvester, DVM • www.richsylvester.com;
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Equine recurrent uveitis (ERU) (also known as moon blindness, iridocyclitis and periodic ophthalmia) is a major ophthalmic disease of the horse and is the most common cause of blindness in this species. 1-4 This immune-mediated, uveitis (inflammation of the entire eye) has approximately an 8 to 25% prevalence rate in horses in the United States.1,5 Fortunately, recent advances in the treatment of horses with ERU have led to the successful management of this disease. This article discusses some important facts about ERU, its causes, and treatment options for the affected horse.
Clinical Signs of ERU
ERU is characterized by episodes of inflammation that involve every structure in the eye. This inflammation develops weeks to months after an initial uveitis episode subsides;1-4,7 however, not every case of initial equine uveitis will develop into ERU. Horses can develop ERU at any age, but the peak time of the initial uveitis episode is 4-6 years, a time when most horses are at or nearing their prime performance years.5
Three main clinical syndromes are observed in ERU, the “classic,” “insidious,” and “posterior” type of ERU. “Classic” ERU is most common and is characterized by active inflammatory episodes in the eye followed by periods of minimal inflammation. The acute, active phase of ERU involves the entire eye. Following treatment with nonspecific anti-inflammatory medications such as corticosteroids, the signs of active, acute uveitis can recede and the disease enters a quiescent or chronic phase. After variable periods of time, the quiescent phase is generally followed by further and increasingly severe episodes of uveitis. It is the recurrent, progressive nature of the disease that is responsible for development of cataract, intraocular adhesions, and phthisis bulbi (scarred eye).1-4,7 In the “insidious” type of ERU, however, the inflammation never completely resolves and a low grade inflammatory response continues that leads to chronic clinical signs of ERU. Frequently, these horses do not demonstrate obvious discomfort and owners of these horses may not recognize the presence of disease until a cataract forms or the eye becomes blind. This type of uveitis is most commonly seen in Appaloosa and draft breed horses. The posterior type of ERU has clinical signs existing entirely in the vitreous and retina (the back part of the eye), with little or no anterior signs of uveitis. The inflammation and resulting permanent damage can only be detected by examining the back part of the eye with an ophthalmoscope or other specialized equipment. This is the least common type of uveitis and is most commonly seen in Europe.
Typical clinical signs of active ERU are similar to signs of uveitis in other species: light sensitivity, squinting, and “milky” color of the cornea (the front of the eye). Ultimately, even with aggressive treatment, many horses develop a chronically painful eye and blindness as a result of secondary cataract, synechia (intraocular adhesions), scarring, glaucoma, and development of phthisis bulbi (scarred, white cornea).1-4,7 When your veterinarian examines the inside of the eye with a special lens or ophthalmoscope s/he will see signs of inflammation involving most or all of the internal parts of the eye.
Organisms/Infectious Agents Associated with Initial Uveitis (and Possibly ERU)
Several organisms have been associated with the initiation of equine uveitis. In some instances, but not all, the uveitis associated with these systemic infections may develop into immune-medicated uveitis, or ERU. One of the most commonly associated systemic diseases associated with uveitis is leptospirosis.11-14 Leptospirosis is a disease of horses that involves their whole body, including the eye. Having this disease sensitizes some horses’ immune systems in such a way that months or years later if the horse is exposed to the causative bacteria (leptospira) again the immune system reacts very severely. It’s this severe immune reaction that causes the inflammation which causes permanent damage to the eye. Ironically, it’s the horse’s own immune system, not the bacteria, which is blinding him. Onchocerciasis (a parasite of warm climates that can burrow in the eye) is another systemic disease associated with equine uveitis. This disease is much less common now with the widespread use of ivermectin; however, it is still a common initiator of uveitis. Other systemic infectious causes of uveitis include Streptococcus equi infection, brucellosis, toxoplasmosis, equine herpes virus (EHV-1, 2), equine viral arteritis, parainfluenza type 3, and generalized septicemia, endotoxemia, neoplasia, tooth root abscess, or trauma.
ERU is a non-specific immune-mediated condition that results in recurrent or persistent inflammatory episodes in the eye. To diagnose the syndrome of ERU, you must differentiate it from non-ERU uveitis. As mentioned above, there is a long list of infectious and non-infectious agents responsible for causing acute uveitis in the horse. Although any of these causes of uveitis may allow horses to develop ERU, not all of these acute uveitis cases will develop into ERU.
Practical and Stable Management Practices to Decrease ERU
Practices that decrease ocular injury or minimize the inflammatory stimuli may decrease or eliminate the development of recurrent episodes of uveitis in ERU. It may be possible to eliminate environmental triggers (e.g., allergens, antigens, etc) of the recurrent episodes of uveitis by changing the horses’ pasture, pasture mates, or stable, increasing insect and rodent control, decrease sun exposure, or changing bedding type. Trauma to the eye(s) can also be decreased by eliminating sharp edges, nails, and hooks in the stable, removing low tree branches in the pasture, lightening training and show schedule, minimizing trailering, and constant use of a quality fly mask. Finally, ensuring that the horse has proper hoof care, optimal vaccination and deworming schedule and proper diet may also minimize uveitis episodes.
Medical Therapy for ERU
Because vision loss is a common long-term manifestation of ERU, initial therapy must be aggressive. In acute cases, treatment in the form of systemic and local therapy consisting of antibiotics, corticosteroids and anti-inflammatory drugs is used, many times simultaneously. Initial therapy is instituted for at least two weeks, and should be tapered off over an additional two weeks after the resolution of clinical signs. Because the eye must be treated with multiple medications every few hours, usually a plastic tube (catheter) is surgically installed in the underside of the upper eyelid so that medications can be delivered to the eye more easily. Many horses respond well to intermittent topical and/or systemic therapy of their active episodes of ERU. Other horses, however, do not respond to traditional therapy and may experience frequent recurrences of uveitis.
Traditional treatments used for ERU (i.e., corticosteroids and non-steroidal anti-inflammatory medications) are aimed at reducing inflammation and minimizing permanent ocular damage at each active episode. They are not effective in preventing recurrence of disease.
A recently described surgical procedure is aimed at preventing the recurrence of uveitis and therefore providing long-term control of the disease: sustained release cyclosporine devices (CsA) implantation.
Recent studies in the U.S. and Italy have shown that ERU patients who had the strong immunosuppressant cyclosporine surgically implanted into the tissue of the upper eyeball had significantly fewer ERU “flare-ups”. Also, of the 171 eyes treated, only three required a repeat implant, and these didn’t need the repeat treatment for over four years. This is a very exciting option for horses that suffer flare-ups several times a year.
Long-Term Prognosis for ERU
In general, the prognosis for eye afflicted with ERU is poor. Most horses with the disease have multiple recurrent episodes that eventually lead to blindness. Diligent observation and treatment is required by the owner in many cases to maintain vision long-term. Some horses have progressively increase severity and frequency of their bouts of inflammation and these horses are ones that may benefit from surgical therapy.
References
[Adapted from “Equine Recurrent Uveitis: What’s New in Pathophysiology, Diagnosis, and Management?” presented to the American Association of Equine Practitioners, Focus on Ophthalmology Seminar in Raleigh, North Carolina, October 2, 2009, by Brian C. Gilger, DVM, MS, Diplomate ABT, Diplomate ACVO.]
1. Schwink KL. Equine Uveitis. Vet Clin NA: Equine Pract 1992;8:557-574.
2. Miller TR, Whitley RD. Uveitis in horses. Mod Vet Pract 1987:351-357.
3. Davidson MG. Anterior uveitis In: N. E. Robinson, ed. Current Therapy in Equine Medicine. Philadelphia: W.B. Saunders, 1992;593-594.
4. Abrams KL, Brooks DE. Equine recurrent uveitis: current concepts in diagnosis and treatment. Equine Pract 1990;12:27-35.
5. Nelson M. Equine recurrent uveitis, a report of 68 horses in the United States and Canada. ERU Network 1995.
11. Dwyer AE, Crockett RS, Kalsow CM. Association of leptospiral seroreactivity and breed with uveitis and blindness in horses: 372 cases (1986-1993). J Am Vet Med Assoc1995;207:1327-1331.
14. Sillerud CL, Bey RF, Ball M, et al. Serologic correlation of suspected leptospira interrogans serovar pomona-induced uveitis in a group of horses. J Am Vet Med Assoc 1987;191:1576-1578.
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