Examining Eye Issues in Silver Dapple Horses

Examining Eye Issues in Silver Dapple Horses

Many horses with the silver dapple coat mutant gene suffer from multiple congenital ocular anomalies (MCOA) and other eye issues.

Photo: Dennis E. Brooks, DVM, PhD, Dipl. ACVO

Uniquely colored equine coats can be both eye-catching and eye disorder-causing. Take horses with the silver-coat mutant gene, for instance: Many suffer from multiple congenital ocular anomalies (MCOA), or eye issues that can cause, among other things, a bulbous bulging of the eye. Recently, a group of French scientists took a deeper look into affected horses' eyes, and their looking glass of choice was the ultrasound machine.

The silver dapple coat—which isn’t technically a “dapple,” but rather a dilution caused by a genetic mutation that produces a dark body color with a flaxen mane and tail—is common in certain breeds, including the Rocky Mountain Horse and the Icelandic horse. One breed with a very strong silver gene representation is the French Comtois draft horse, which has been selectively bred for centuries to favor coat color, said Jean-Luc Cadoré, DV, PhD, Dipl. ECVIM, professor of internal medicine at VetAgro Sup at the University of Lyon in France.

In the past few years, researchers have pinpointed the gene--PMEL17--responsible for both the silver coloring and the eye disorders. In their study, Cadoré and his colleagues investigated the eyes of Comtois horses and Rocky Mountain Horses carrying at least one copy of the allele and compared their results to horses of the same breeds without the PMEL17 gene. They evaluated the eyes of 85 horses (including 10 control horses without the mutation) using both a direct ophthalmoscope (a hand-held ophthalmoscope that human ophthalmologists use in clinical settings) and ultrasound.

They found that ultrasound was a practical, fast, and easy-to-use tool for investigating the possible disorders caused by a PMEL17 mutation, Cadoré said. In fact, for many pathologies—including ciliary cysts (fluid-filled cysts of the eye's ciliary body) —ultrasound was far more powerful than direct ophthalmoscopy in finding instances of disease.

Ultrasonography also allowed researchers to detect retinal detachment, which they couldn't identify with direct ophthalmoscopy. While retinal detachment was rare—occurring in only five of the 75 test horses—its existence is significant, he said, as it can be detrimental to a horse's vision.

Further, ultrasound allows the veterinarian to detect abnormalities even when cataracts, miosis (pupillary constriction), and other disorders that could interfere with direct ophthalmoscopy are present, he said.

“Ultrasound allows early and complete detection of all possible lesions,” Cadoré told The Horse. “What it does not do is indicate any kind of treatment for these lesions at this time.”

Cadoré said the study results also confirmed the genetic connection between the silver coat and the eye disorders. All 75 horses with the mutation in the study had at least one eye disorder, compared to none of the 10 control horses of the same two breeds, he said.

Horses that were heterozygous—meaning they had only one copy of the PMEL17 allele—tended to have cysts, whereas those that were homozygous—having two copies of the allele—showed signs of MCOA, he said. Knowing this will be critical in making future breeding choices, he added.

Future studies will focus on how these abnormalities affect the horse’s health and vision, he said.

The study, "Ultrasonographic features of PMEL17 (Silver) mutant gene-associated multiple congenital ocular anomalies (MCOA) in Comtois and Rocky Mountain horses," was published in Veterinary Ophthalmology

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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