Equine Multiple Congenital Ocular Anomalies Syndrome

This syndrome describes a group of abnormalities in the horse’s cornea, iris, lens, and ciliary body.
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Equine multiple congenital ocular anomalies (MCOA) syndrome describes a group of developmental abnormalities of the cornea, iris, lens, and ciliary body in the front portion of a horse’s eye. The condition is generally nonpainful and nonprogressive and has a genetic basis, stemming from a gene mutation. Most mildly affected horses have normal vision.

Affected horses can be separated into Cyst and MCOA phenotypes (physical expressions of a genetic trait) depending on their types of ocular defects. Horses with the milder form (Cyst) appear normal and require rigorous ophthalmic examination for diagnosis. These horses are presumed to be heterozygous (having inherited a different allele from each parent) for the mutant gene and have uveal (the cellular layer of the eye that contains blood vessels, iris, and ciliary body) cysts. A smaller number of Cyst horses also have moderate retinal folding, which occurs when layers of the retina do not form together properly.

Horses with the more severe form of MCOA are homozygous (having inherited the same allele from both parents) for the mutant gene. They express the same clinical signs as horses with the Cyst phenotype plus iris thinning, iridocorneal angle (juxtaposition of the cornea and the iris) abnormalities, pupillary constriction, congenital cataracts, exaggerated corneal curvature, nuclear (middle of the lens) cataracts, and pupils that do not dilate in response to pupil-dilating drugs. However, an affected horse does not necessarily possess all these clinical signs.

MCOA is most common in silver-colored Rocky Mountain Horses (RMH) and has also been observed in related breeds such as Kentucky Mountain Saddle Horses and Mountain Pleasure Horses. RMH owners and breeders often desire silver-colored horses (also known as silver dapple, silver dilution, and chocolate); thus, these horses are used frequently for breeding. Approximately 55% of registered RMHs are silver/chocolate. Because most silver horses are heterozygous for the silver mutation, they are likely to carry the milder form of MCOA

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Written by:

Dennis E. Brooks, DVM, PhD, Dipl. ACVO, is a professor of ophthalmology at the University of Florida. He has lectured extensively, nationally and internationally, in comparative ophthalmology and glaucoma, and has more than 140 refereed publications. He is a recognized authority on canine glaucoma, and infectious keratitis, corneal transplantation, and glaucoma of horses.

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