Silver Dapple Color and Eye Abnormalities Connection Found

Horses come in a fantastic array of colors from black to gray, from bays to chestnuts, and with or without varying amounts of white. The glorious spotted coats of Appaloosas are a common sight for western Canadians, as are golden palominos.

On the other hand, silver dapple is a color that's still on the unusual side. The color occurs in several breeds--including the gaited Rocky Mountain Horse, in which the typical, smoky chocolate coat with flaxen mane and tail is often seen and highly admired.

Veterinarians aren't usually concerned with the coat color of their equine patients, but when a particular color or pattern is associated with disease conditions or abnormalities that can affect horse health, it becomes an issue.

Such is the case with congenital stationary night blindness (CSNB) in Appaloosas. A few years ago, a research team led by veterinary ophthalmologists at the Western College of Veterinary Medicine (WCVM) confirmed that CSNB is linked with horses that are homozygous for the leopard spotting gene.  

Members of that same team, headed by veterinary ophthalmologist Bruce Grahn, DVM, Dipl. ABVP, ACVO, have also been investigating eye anomalies in purebred and crossbred Rocky Mountain and Kentucky Mountain Horses.

The study's findings, which were published in the July 2008 issue of the Canadian Veterinary Journal, support the long-standing theory that these anomalies appear to be associated with color--specifically, silver dapple. However, the WCVM study's results also question two previous conclusions about the mode of inheritance and the precise nature of these anomalies.

Besides Grahn, the research team included Chantale Pinard, DVM, MSc, Dipl. ACVO, of the Faculté de médecine vétérinaire at the Université de Montréal and WCVM veterinary ophthalmologist Lynne Sandmeyer, DVM, DVSc, Dipl. ACVO. The genetic aspects of the study were conducted by George Forsyth, MSc, PhD, of WCVM's Department of Veterinary Biomedical Sciences, Rebecca Bellone, PhD, of the University of Tampa, and Sheila Archer, a Saskatchewan-based phenotype researcher.

The Heather Ryan and L. David Dubé Veterinary Health Research Fund, which was created in 2006 to support multi-year equine health research projects at the WCVM, provided funding for the two-year research study.

Does color indicate a problem?

Unlike the eyes of Appaloosas suffering from CSNB, which appear completely normal on ophthalmic examination, the eyes of Rocky Mountain Horses show a variety of obvious lesions. While veterinary ophthalmologists have identified defects of the iris, cornea, retina and eyelids, fluid-filled cysts of the ciliary body (a muscular ring located in the front part of the eye) is the most common condition.


Rocky Mountain horse eye

Focal areas of retinal degeneration related to temporal ciliary cysts (fluid-filled cysts of the ciliary body). These cysts are the most common lesions in Rocky and Kentucky Mountain horses.

In most cases, affected horses don't suffer from significant visual impairment but rupture of these cysts occasionally leads to retinal detachment and affected vision. While this condition is congenital (present at birth) and inherited, it isn't progressive.

The association between the eye anomalies in Rocky Mountain Horses and the silver dapple color is undeniable. But the exact nature of the association--whether or not the same gene is responsible for both the eye abnormalities and the horse breed's color--has yet to be determined.

Tracking the expression of the dilution gene that produces silver dapple is somewhat difficult since the gene affects only eumelanin (black pigment) and not pheomelanin (red pigment). In other words, chestnut horses (whose coats contain no black pigment) might carry the silver dapple mutation, yet they will look no different from chestnut horses that do not carry the gene. Meanwhile, a silver bay whose black points have been diluted but has a red body coat might look very much like a flaxen-maned chestnut horse at first glance.

These kinds of situations require close study and analysis, points out Grahn. "We have two coat color experts on the team, but coat color genetics is still an imprecise science."

Question of inheritance

The WCVM-based research study includes horses that come from two herds--living on opposite sides of Canada--that are linebred within their own ranks but unrelated to each other. One herd includes 97 purebred and crossbred Rocky Mountain Horses, while the second herd consists of 37 Kentucky Mountain Horses. The incidence of ocular anomalies within this population of horses is close to 50%, consistent with the findings of earlier research done elsewhere involving these two breeds.

During the project, veterinary ophthalmologists examined the eyes of purebred or crossbred Rocky and Kentucky Mountain Horses as well as the eyes of horses that were unrelated in breeding. The specialists used a transilluminator, a biomicroscope, and an indirect ophthalmoscope to examine the horses' eyes after their pupils were dilated.

Next, the team constructed a pedigree with related horses to investigate the mode of inheritance of the multiple eye anomalies and their relationship to coat color. Coat color experts assessed all of the horses, and photographs of all animals involved in the study were archived.

After a detailed pedigree analysis, the research team was able to confirm that the mode of inheritance of the ocular anomalies in Rocky Mountain and Kentucky Mountain Horses is an incomplete penetrance of a dominant inherited trait.

"Our conclusion is based on the fact that when mares with no relation to Rocky or Kentucky Mountain Horses were bred to affected Rocky Mountain stallions, there was a range of outcomes. Some offspring had complete, multiple ocular anomalies, some had temporal ciliary cysts, while other foals were completely normal," explained Grahn. "These findings are inconsistent with the codominant mode of inheritance."

For many years, veterinarians and horse owners have described the collection of eye anomalies associated with silver dapple color as anterior segment dysgenesis (ASD), because of the apparent similarities to anterior segment lesions that are well-documented in other species--including humans. Grahn said it's certainly possible that Rocky Mountain horse eye anomalies develop partially as ASD. However, after close examination of the affected horses involved in this study, he and his colleagues found none of the lens-related diseases or abnormalities that are usually associated with ASD.

The WCVM research team also observed that the corneas in affected horses weren't significantly different in shape from the corneas in non-affected horses. This observation concurs with what other researchers have previously found in earlier studies.

Work still needs to be done to determine whether the gene controlling silver dapple color, which has now been identified, is also responsible for the ocular anomalies. While many researchers believe this will turn out to be the case, Grahn is skeptical.

"We don't think it's the same gene, but another gene close by on the same chromosome. We have a black horse with the condition that isn't considered silver dapple, although some people think this classification must be a mistake. I don't agree. We also have a silver dapple horse that's clear (of ocular abnormalities) and there may be others. We need to extend the pedigree."

With most of the ophthalmic research completed, Grahn says the investigation of ocular anomalies in Rocky and Kentucky Mountain Horses is now in the hands of the geneticists on the research team: "The area where the gene resides is known, and we're getting close."--Roberta Pattison


Roberta Pattison is a freelance writer who is a regular contributor to the national publication, Dogs in Canada. Recently retired from grain farming, she still lives on her farm near Delisle, Saskatchewan.


Reprinted with permission of Horse Health Lines, publication for the Western College of Veterinary Medicine's Equine Health Research Fund. Visit for more information.

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