“Much more study is needed to determine why Appaloosa horses have a different type of disease and why they tend to go blind more frequently than other horses.”
Equine Recurrent Uveitis In The Appaloosa Horse
This page was contributed by Brian C. Gilger, DVM, MS, Diplomate of the American College of Veterinary Ophthalmologists. Dr. Gilger is Professor of Ophthalmology at the College of Veterinary Medicine, North Carolina State University, in Raleigh, North Carolina. We are very grateful to Dr. Gilger for providing this information.
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 higher. ERU is characterized, most commonly, by episodes of intraocular inflammation followed by variable lengths of time of lack of inflammation, or quiescence. In many horses, the bouts of inflammation continue until blindness occurs. Appaloosa horses have been shown to be very susceptible to ERU and in most studies they have the highest prevalence (up to 25%) of any breeds. Also, the Appaloosa horse has a different form of ERU – their disease is most typically the “insidious” type of uveitis.
Clinical Signs Of ERU
Three main clinical syndromes are observed in ERU: the “classic,” “posterior,” and “insidious” 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. 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 cataracts, intraocular adhesions, and phthisis bulbi.
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.
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 the Appaloosa and may be why this disease is so difficult to recognize and treat in this breed. Very commonly, the first thing the horse owner notices is blindness, since the inflammatory episodes are not recognized.
Appearance Of The Eyes
Typical clinical signs of active ERU in the Appaloosa include mild squinting (photophobia, blepharospasm), ocular cloudiness (corneal edema, aqueous flare, hypopyon), and constricted pupil (miosis). Clinical signs of long lasting, chronic ERU include a dark iris, cloudy eye (corneal edema, cataract), small pupils (miosis) and blindness. It is important to remember that keratitis (white to yellow corneal infiltrate) is NEVER a primary clinical sign of ERU.
ERU In The Appaloosa
Because of the insidious onset of uveitis in the Appaloosa, many experts believe that they have a different disease than standard ERU. Much more study is needed to determine why Appaloosa horses have a different type of disease and why they tend to go blind more frequently than other horses.
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