Uveitis and Equine Recurrent Uveitis

Take Home Message  

Equine recurrent uveitis (ERU) is a major ophthalmic disease of the horse and is the most  common cause of blindness in horses.^1-4 This immune-mediated, pan-uveitis has approximately a  2 to 25% prevalence rate, depending on the breed.^1,5 Fortunately, recent advances in the  treatment of horses with ERU have led to the successful management of this disease.  

Introduction  

The equine industry in the United States has an estimated annual worth of 112 billion dollars and  provides approximately 1.4 million full time jobs across the country.⁶ Because ERU has a  prevalence rate of approximately 2 to 25% across horse breeds in the United States, the impact of  this disease on the equine industry could be as high as a billion dollars a year. ERU causes these  large economic losses in the equine industry because it disrupts training, decreases performance,  and disqualifies horses from competition (due to medication use, etc). Furthermore, horses with  ERU have decreased value as a result of vision deficits or blindness. Finally, treatment,  veterinary care, and personnel costs add to the economic impact of the disease. The purpose of  this lecture is to review important facts about ERU, its causes, and new treatment options for the  affected horse. 

Clinical Signs of ERU  

ERU is characterized by episodes of intraocular inflammation that develop weeks to months after  an initial uveitis episode subsides;^1-4,7 however, not every case of initial equine uveitis will  develop into ERU (see below in diagnosis). 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 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. 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 progression to chronic clinical signs of ERU.  Frequently, these horses do not demonstrate overt ocular 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, with little or no anterior  signs of uveitis. In this syndrome, there are vitreal opacities and retinal inflammation and  degeneration. This is the least common type of uveitis, which was seen almost exclusively in  Europe, but is becoming more common in the USA.  

Typical clinical signs of active ERU are similar to signs of uveitis in other species: photophobia,  blepharospasm, corneal edema, aqueous flare, hypopyon, miosis, vitreous haze, and  chorioretinitis (Fig. 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 (Fig. 2). Either type of ERU (“classic” or “insidious”) can have either  predominantly anterior (cornea, iris, lens, and ciliary body inflammation) or posterior (ciliary  body, vitreous, and chorioretinal inflammation) segment involvement.^1,2,4,8-14 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.^1-4,7 
 

Infectious Agents Associated with Initial Uveitis (and Possibly ERU)

Several organisms have been associated with the intiation 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 Roberts demonstrated that ERU can develop after primary infection (and acute uveitis) of leptospirosis; however, ERU typically did not develop until 1 year after the systemic infection. Therefore, measuring titers in cases of documented ERU is generally not beneficial for management of the condition unless there is a herd or barn outbreak of the uveitis. Onchocerciasis 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.  [...]