What Is (ASD) Anterior Segment Dysgenesis?
Kentucky Mountain Saddle Horse Association Position Concerning
Genetic Opthalmic Abnormalities
The Problem
Recently, it has come to the attention of the horse community that some
mountain horses have been found to be visually impaired. Through the
research of Dr. David Ramsey, a veterinary opthalmologist from the College
of Veterinary Medicine at Michigan State University, the genetic
abnormality has been defined as a faulty formation of the front part of
the eye (the anterior segment) and is termed Anterior Segment Dysgenesis
(ASD). Strong correlations were evident between coat and mane/tail color
and the eye abnormalities. The incidence of abnormalities is highest in
horses with a chocolate coat color. Horses with white mane/tail color are
at the greatest risk for ASD.
Brief Description of Opthalmic Abnormalities
There are basically three categories into which a horse's eye condition
may fall: normal, cyst, and ASD. First, horses may have normal (aa) eyes.
These horses exhibit no sign of eye abnormality. However, they could be
carriers of the genetic defect. Analysis from the pedigree data collected
indicate that this abnormality has a semidominant mode of inheritance, but
the trait is incompletely penetrant (is not evident upon examination)
approximately 12% of the time. Once a genetic test is developed and we are
not dependent upon physical examination of the eye, this percentage will
be reduced. Second, horses may have cysts (Aa) of the ciliary body and
peripheral retina and retinal dysplasia (abnormally formed retina).
Approximately 48% of the horses examined by Dr. Ramsey exhibited this
condition. Cysts do not affect vision and the condition does not worsen
over time. Finally, horses may have full blown Anterior Segment Dysgenesis
or ASD (AA). Upon examination, these horses have multiple abnormalities
of the eye that may include small pupils that fail to dialate or dilate
incompletely when a pupil dilating drug is administered to the eye, have
flat grandula iridica (normal iris tissue present at the top part of the
iris/pupil) that may encircle the pupil, abnormally shaped pupils
(dyscoria), abnormal drainage angle of the eye, cataracts, cysts, and
retinal displasia. A smaller group of AA horses also has enlarged corneas
("pop-eye"), craniofacial defects, wide eyelid openings, lens subluxation
(lenses that are not in their normal location), cataracts that are
progressive (catracts that get worse in time), and abnormally small eyes
(micropthalmia). Some ASD horses will suffer no visual impairment at all;
some will be born blind or go blind later in life. Of the horses examined
by Dr. Ramsey, approximately 14% were ASD.
Breeding Implications
Our challenge is to preserve the gentle temperament and smooth gait of the
Kentucky Mountain Saddle Horse. We want to be a responsible association
that keeps its membership informed by providing suggestions that will
allow our breeders to select appropriate dams and sires. According to the
information provided by Dr. Ramsey the possible genetic combinations
follow. The categories are: aa is a normal eyed horse, Aa is a cyst horse,
and AA is an ASD horse.
The Future
The Kentucky Mountain Saddle Horse Association is proud to say that we have
chosen to face this problem. The board has reason to believe that there is a
much higher percentage of normal horses within our breed because the KMSHA
accepts horses from many genetic lines and our books remain open for mares.
We feel that with strict control of certification, implementing blood typing
for proof of parentage by 1999, and with responsible breeding, we can very
soon eradicate this problem. We have always felt that a strong genetic pool
must also be a wide one. It has taken the Kentucky Mountain Saddle Horse
Association longer in achieving our goal to become a separate breed because
we have chosen to continue to accept horses into our registry from other
breeding lines. We have not closed our books and cannot become a breed until
we do. However, because of these past decisions, we feel we have a bright
future with the Kentucky Mountain Saddle Horse and will be less susceptible
to other genetic problems caused from breeding too closely within specific
horses lines.