Equine uveitis

Introduction

Equine uveitis, especially recurrent uveitis, is the most common cause of blindness in horses, and understanding the pathogenesis, diagnosis, and treatment of these uveitis diseases is essential for any veterinarian.

Uveitis is inflammation of the uveal tract of the eye. This inflammation may involve primarily the iris and ciliary body (anterior uveitis) or the choroid and retina (posterior uveitis), but most commonly all anatomic areas of the uveal tract are inflamed (pan-uveitis). In the horse, the inflammation can be acute in onset, chronic, or recurrent.^1–4 Further, recurrent uveitis can be active or quiescent (no inflammation). It is important that the equine clinician differentiates between acute uveitis, which has many underlying causes, and equine recurrent uveitis (ERU), which is an immune-mediated disorder.

Therefore, when the clinician examines a horse with suspected uveitis, they answer three important questions to best diagnose and manage the case:

1. Does the horse have signs of active or quiescent uveitis?
2. Is the uveitis acute in onset, chronic (unresolved), or recurrent?
3. Is the uveitis primary (endogenous) or secondary to an ocular or systemic abnormality?

Common signs of active uveitis include photophobia, blepharospasm, diffuse corneal edema, aqueous flare and cells, hypopyon, miosis, iris hyperemia, synechia, vitreous haze and cells, and chorioretinitis. Note that corneal opacities of any color, other than diffuse edema, are not common in primary uveitis (including ERU); instead, when corneal opacities are present, primary corneal disease should be considered.

Chronic uveitis may develop after a few days of unrelenting severe inflammation or following multiple recurrent episodes of uveitis. Common signs of chronic uveitis include corneal edema, keratic precipitates, iris fibrosis and hyperpigmentation, posterior synechia, corpora nigra degeneration (smooth edges), miosis, cataract formation, vitreous degeneration and discoloration, and peripapillary retinal degeneration. End-stage uveitis is an advanced stage of chronic uveitis when the eye has developed phthisis bulbi, a small and shrunken eye.

Equine Recurrent uveitis is characterized by episodes of intraocular inflammation that develop weeks to months after an initial uveitis episode subsides; however, not every case of initial equine uveitis will develop into ERU.

Three main clinical syndromes are observed in ERU: the “classic,” “insidious,” and “posterior” types of ERU.¹ “Classic” ERU is 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 of time, the quiescent phase is generally followed by further and increasingly severe episodes of uveitis. It is the recurrent, progressive nature that causes cataract, intraocular adhesions, and phthisis bulbi (end-stage eye). In the “insidious” type of ERU, however, the inflammation never completely resolves, and a low-grade inflammatory response continues that leads to progression to chronic clinical signs of ERU. 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, with little or no anterior signs of uveitis. In this syndrome, there are vitreal opacities and retinal inflammation and degeneration.

Medical Therapy for Uveitis - Because vision loss is a common long-term manifestation of chronic uveitis, initial therapy must be aggressive. In acute cases, treatment in the form of systemic and local therapy consisting of antibiotics, corticosteroids, and non-steroidal anti-inflammatory drugs is used, many times simultaneously. 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 uveitis (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 the recurrence of disease (such as in ERU).

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