Clearing up Misconceptions about A.S.D.
By Dr. David Ramsey, DVM
I read with interest the article entitled Check Out Those Genes published in the most recent edition of Horse Illustrated (Breckenridge T. Check Out Those Genes. Horse Illustrated 1999;23(3):76-84). This article discusses several known heritable (genetic) diseases affecting horses, and suggests that other less common equine diseases may also have a heritable basis. Coat color genetics are also discussed as are certain diseases that may be closely linked to specific color genes. The article was well written but several statements in print are inaccurate or incorrect.
In the original article, the author states:
Another rare, genetically linked condition for which a reliable test will probably soon be developed is anterior segment dysgenesis. Most common among Rocky Mountain Horses, the ASD defect results in horses that develop painful and vision-limiting cysts in their eyes, or may rarely be born with no eyes.
The abbreviation ASD refers to Anterior Segment Dysgenesis, a colloquial term that includes an extensive set of developmental conditions that affect the anterior segment (front part) of the eye. There is a wide spectrum of conditions that may result from ASD from a malformed eye that is blind at birth, to conditions that do not affect vision or the health of the eye. Contrary to the published report in Horse Illustrated, the Rocky Mountain Horse eye is virtually unaffected. With the exception of two blind horses from a population of 2500 Rocky Mountain Horses examined to date, vision is rarely affected and the health of the eye is virtually normal in Rocky Mountain Horses with ASD.
In 1996, Michigan State University began a clinical investigative and molecular genetic study of ASD. Aims of our research were to 1) describe the clinical findings in a large cross section of related horses, 2) to determine if heritability was a risk factor for the disease, and 3) to collect DNA from affected and normal horses for future comparisons of pedigree and DNA marker or candidate gene linkage analysis.
The most common abnormal trait that was documented was cysts that arise from the ciliary body (inside the eye). These cysts are never painful, do not limit vision, and are therefore of no consequence to the eye or the horse. A syndrome of multiple developmental eye abnormalities was seen less frequently. Again, horses born with multiple eye anomalies had normal functional vision with rare exception, were never painful, and were never born with no eyes.
Results of our research indicate that the trait has a semidominant mode of inheritance, meaning that if a developing embryo has one abnormal copy of the gene, the foal will be born with very minor anterior segment abnormalities (cysts, peripheral retinal folds). The importance of a horse being born with cysts is of no consequence to the horse. The importance of cysts in a horse is only related to production of offspring produced by mating a horse with cysts. A foal born from the breeding of a stallion and a mare that both have cysts (both carry one abnormal copy) has a 25% chance of inheriting two abnormal copies of the gene. When two abnormal copies are inherited, the foal will be born with multiple developmental abnormalities inside the eye. Whether born with one or two abnormal copies of the gene, the abnormalities are not progressive, meaning an affected horse will not develop more heritable eye abnormalities with age.
Today there are nearly 6000 Rocky Mountain Horses that are registered by the Rocky Mountain Horse Association. I have now personally examined over 1500 Rocky Mountain horses, and collectively over 2500 Rocky Mountain Horses have been examined by veterinary ophthalmologists in the United States and Canada. Approximately 5% of Rocky Mountain Horses that I have examined have two abnormal copies of ASD. Only a very small population of affected horses (approximately 30 horses) have problems with vision. All but two horses that I have examined have had normal functional vision. Only two horses were blind. The percentage of blind Rocky Mountain Horses in the general population of Rocky Mountain Horses is actually far less than the percentage of blind Thoroughbreds, Quarterhorses, Saddlebreds, Appaloosas, etc., in each respective population that I have examined.
The author also mentioned that a rarely seen part of the ASD trait is foals ...born with no eyes. Anophthalmia is the medical term describing a developmental defect characterized by complete absence of the eyes. Anophthalmia has never been described as part of the ASD trait. This condition is exceedingly rare in all animal species. In fact, anophthalmia has never been reported in the horse. Certainly, anophthalmia is not one of the described anomalies of ASD in horses.1-6
ASD is not unique to the Rocky Mountain Horse it occurs in all breeds I have examined that carry the Silver Dapple gene most of those that have a chocolate color coat with or without dappling. This includes the Shetland Pony, Miniature breeds, Rocky Mountain, Kentucky Mountain Saddle, Mountain Pleasure, Morgan, Bashkir-Curly, Naraganssett Pacer, and Haflinger. The author's statement that the disease is most prevalent in the Rocky Mountain Horse breed is probably not accurate. The disease is probably just as prevalent in some of the other aforementioned breeds. It has only been studied extensively in Rocky Mountain Horses because the breed Association recognized it and was proactive to determine whether the abnormality was a severe defect.
Because of exceptional popularity and financial considerations, it is virtually unheard of when a breed Association recognizes a potential trait that may alter the breed's future and subsequently financial worth. It is even less common when a breed Association or Registry acknowledges their breed has a potentially heritable problem decides to become proactive, publicly inviting outside investigators to study the suspected disease. A classic example of an equine breed Association that failed to recognize and address a serious (sometimes life-threatening) disease until widespread dissemination of the trait had occurred throughout the breed (HYPP), was mentioned in the previous article. I commend the Rocky Mountain Horse Association for recognizing a minor abnormality in their breed, for taking the initiative to determine its cause, and for their interest and participation in this collaborative study. In doing so, they have established guidelines that select against propagation of undesirable traits within the breed while promoting genetic diversity within the breed. The Rocky Mountain Horse Association should serve as the model against which other breed Associations and Registries that acknowledge a potentially heritable abnormality should be compared.
David T. Ramsey, DVM
Diplomate, American College of Veterinary Ophthalmologists
1. Ramsey DT, Ewart SE, Render JA, Latimer CA, Cook CS, Schott HC. Anterior Segment Dysgenesis in Rocky Mountain Horses. Proc Am Coll Vet Ophthalmol 1996;27:46-48.
2. Ramsey DT, Ewart SE. Clinical and Genetic Aspects of Anterior Segment Dysgenesis in Rocky Mountain Horses. Proc Rocky Mountain Horse Association 1997.
3. Ramsey DT, Ewart SE, Render JA, Cook CS, Latimer CA. Congenital Ocular Abnormalities of Rocky Mountain Horses. Vet Ophthalmol 1999, in press.
4. Ramsey DT, Hauptman JG, Petersen-Jones, SM. Comparison of Corneal Thickness, Intraocular Pressure, and Corneal Diameter in Rocky Mountain Horses With Cornea Globosa and With Normal Corneas. Am J Vet Res 1999, in press.
5. Ewart SE, Ramsey, DT. The Horse Homolog of Congential Aniridia Conforms to Semidominant Inheritance. Genomics, 1999, in review.
6. Ramsey DT. Ocular Disorders Presumed to be Inherited in Purebred Horses. American College of Veterinary Ophthalmologists Genetics Committee 1998 and/or Data from the Equine Eye Registration Foundation All Breeds Report, College of Veterinary Medicine, Purdue University, 1998.
7. Website: http://www.naturalgait.com
8. Website: http://rmhforum.com
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This is a reprint of an article in the Early Summer 1999 issue of Natural Gait News.