HHL.FALL08.EYE.ANOMALIES.pdf

The COAT

COLOUR

Connection

H orses come in a fantastic array of colours from black

to white, from bays to greys, and with or without

varying amounts of white. The glorious, spotted coats

of Appaloosas are a common sight for western Canadians and

so are golden palominos — thanks to Roy Rogers and Trigger.

On the other hand, silver dapple is a colour that’s still on the

unusual side. The colour 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 colour of their equine

patients, but when a particular colour 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 (see the Summer 2008 issue of Horse Health Lines for the latest update, www.ehrf.usask.ca ).

Members of that same team, headed by veterinary ophthalmologist Dr. Bruce Grahn, 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

colour — 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 Dr. Chantale Pinard of the Faculté de médecine

vétérinaire at the Université de Montréal and WCVM veterinary ophthalmologist Dr. Lynne Sandmeyer.

The genetic aspects of the study were conducted by Dr. George Forsyth of WCVM’s Department of

Veterinary Biomedical Sciences, Dr. Rebecca Bellone 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.

"Y2OBERTA0ATTISON

Findings from a new,

WCVM-based study

conﬁrm that coat colour

is linked to the ocular

anomalies that affect one

of out every two Rocky

and Kentucky Mountain

Horses in Canada and

the United States. The

College’s researchers also

question two long-standing

conclusions about the

mode of inheritance and

the precise nature of the

eye abnormalities found in

these two breeds.

Does Colour Indicate a 0roblem?

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

12 (ORSE(EALTH,INESs!UTUMN

cysts of the ciliary body (a muscular ring located in the front part of the eye)

is the most common condition.

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 colour 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 colour — has yet to be determined.

Tracking the expression of the dilution gene that produces silver dapple is

somewhat difﬁcult since the gene only affects eumelanin (black pigment) and

not pheomelanin (red pigment). In other words, chestnut horses (whose coats

contain no black pigment) may 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 may

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 colour experts on the team, but coat colour genetics

is still an imprecise science.”

“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,” explains 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 colour as anterior

segment dysgenesis (ASD), because of the apparent similarities to anterior

segment lesions that are well-documented in other species — including

humans. Grahn says 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 colour, 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 abnormalies] 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 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.

1UESTIONOFInheritance

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 per cent, 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 colour. Coat colour 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.

Previous page: Large temporal ciliary cysts — ﬂuid-ﬁlled

cysts of the ciliary body — are the most common lesions

in Rocky and Kentucky Mountain horses. Above (left):

This image of a Rocky Mountain Horse’s eye shows retinal

degeneration secondary to previous cysts. Above (right):

This third image shows focal areas of retinal degeneration

related to previous cysts. Photos: Dr. Bruce Grahn, WCVM.

Western College of Veterinar y Medicine

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