Equine Recurrent Uveitis (ERU)

Noelle La Croix, DVM, Dip. ACVO

Understanding Equine Recurrent Uveitis

The complicated anatomy of the eye can be roughly divided into 3 layers: an outer fibrous layer (cornea and sclera), a middle vascular layer (choroid and iris), and an inner neurological layer (retina).  Uveitis is defined as inflammation within the middle layer commonly called the uvea (Latin for ‘grape’). The name probably derives from early anatomists that would remove this small bluish-purple (grape-like) tissue from the human eye.

Inflammation is the body’s response to foreign invaders (bacteria, fungi, and viruses). Many tissues of the eye are generally considered immunoprivileged.  A special blood ocular barrier prevents small peptides (antigens), derived from foreign invaders, from freely flowing into or out of the eye. Some foreign antigens (particularly if present in great numbers) can overcome the blood ocular barrier and intensify an ocular immune response resulting in uveitis. There are three primary etiologies of uveitis: infectious, neoplastic, and autoimmune/idiopathic.  
The appropriate treatment for uveitis will depend upon its underlying etiology, a dampening of immune responses causing collateral damage, and the alleviation of any associated pain.

How is ERU diagnosed?

In suspected cases of equine uveitis, an initial evaluation should include a physical examination, an ocular examination, and basic serology (complete blood count with differential, and blood chemistry).  Horses with suspected uveitis should also be tested for leptospirosis as this agent has been associated with equine recurrent uveitis (ERU).  Any other underlying causes for inflammation (e.g. an intraocular tumor) will also need to be addressed for full resolution of some cases of equine uveitis. If an underlying etiology cannot be determined, the uveitis is assumed to be autoimmune-mediated or idiopathic.

How is ERU treated?

Medical therapy for equine uveitis includes pain management and the dampening of any immune response that may permanently damage the eye. 

In cases of equine uveitis, it is important to consider if the occurrence was unique and self-limiting, or if it is part of a progressive recurring syndrome known as ERU. Appaloosa horses are especially prone to the development of ERU.

A loss of ocular immune privilege may be the root cause of ERU in which the blood ocular barrier has permanently broken down allowing continuous exposure of the eye to foreign antigens, immune cells, and/or inflammatory cytokines derived from the bloodstream. In cases of ERU, white blood cells entering from the bloodstream form “follicles” within the eye creating a chronic state of inflammation.  

There are three subclasses of ERU that have been described as classic, posterior, and insidious. Classic and posterior ERU present as recurrent bouts of uveitis associated with pain that are typically appreciated by horse owners.  There is a predispostion for both classic and posterior uveitis in warmblood horses, and for classic ERU in Icelandic horses. In cases of insidious ERU, owner’s do not typically appreciate their horse’s pain, but rather a color change of the horse’s eye. Insidious ERU presents low-grade inflammation with a gradual loss of function and/or necrosis of the eye. There is a predisposition for insidious ERU in the Appaloosa, Knabstrupper, and draft horse.

Recurrent equine uveitis that arises every 3 to 4 months, and requires longer durations of treatment with each occurrence, can result in debilitating ocular complications including glaucoma and blindness. There are treatments shown to dampen these recurrent immune responses including intravitreal injections of steroids and/or gentamicin sulfate, as well as supracoroidal injections of steroids. Surgical options for ERU include suprachoroidal implantation of sustained release cyclosporine, and a dual-port pars plana vitrectomy. However, conservative medical management is the first line of therapy following an initial diagnosis of ERU.  Horses diagnosed with ERU require diligent lifelong care to prevent blinding and painful complications. Care of horses with ERU is optimally facilitated by the guidance of a veterinary ophthalmologist.

Figure 1:  Typical signs of classic ERU in the right eye of a warmblood horse with epiphora and blepharospasm.

Figure 2:  Insidious ERU in the right eye of an Appaloosa horse.

Figure 3:  Phthisis bulbi in the right eye of an Appaloosa horse with insidious ERU.