CHAPTER 138 Equine Recurrent Uveitis
Equine recurrent uveitis (ERU), also known as moon blindness, iridocyclitis, and periodic ophthalmia, is a syndrome that is the most common cause of blindness in horses and may have a prevalence rate in the United States of 2% or greater. The disease is characterized by episodes of intraocular inflammation followed by variable periods of lack of inflammation or quiescence. In many horses, the bouts of inflammation continue until blindness results. Fortunately, recent advances in the treatment of horses with ERU have led to successful management of this disease. This chapter discusses some important facts about ERU and treatment options.
Three main clinical syndromes are observed in ERU: classic, insidious, and posterior. “Classic” ERU is the most common and is characterized by active inflammatory episodes in the eye followed by periods of minimal ocular inflammation. The acute, active phase of ERU predominantly involves inflammation of the iris, ciliary body, and choroid, with concurrent involvement of the cornea, anterior chamber, lens, retina, and vitreous. The signs of active, acute uveitis can recede and the disease enters a quiescent or chronic phase. After variable periods, 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.
In the “insidious” type of ERU, however, the inflammation never completely resolves, and a low-grade inflammatory response continues that leads to progression of chronic clinical signs of ERU. Frequently these horses do not demonstrate overt ocular discomfort, and owners may not recognize signs 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 is associated with clinical signs affecting only the vitreous and retina, with little or no anterior signs of uveitis. In this syndrome, vitreal opacities, retinal inflammation, and retinal degeneration are seen.
Typical clinical signs of active ERU include photophobia, blepharospasm, corneal edema, aqueous flare, hypopyon, miosis, vitreous haze, and chorioretinitis (Figure 138-1). Clinical signs of chronic ERU include corneal edema, iris fibrosis and hyperpigmentation, posterior synechia, corpora nigra degeneration (smooth edges), miosis, cataract formation, vitreous degeneration and discoloration, and peripapillary retinal degeneration (Figure 138-2).
Figure 138-1 Eye of a horse with signs of active equine recurrent uveitis including photophobia, blepharospasm, corneal edema, aqueous flare, hypopyon, miosis, vitreous haze, and chorioretinitis.
Figure 138-2 Eye of a horse with signs of chronic equine recurrent uveitis. Typical signs include corneal edema, iris fibrosis and hyperpigmentation, posterior synechiae, corpora nigra degeneration (smooth edges), miosis, cataract formation, vitreous degeneration and discoloration, and peripapillary retinal degeneration.