About Color (and ASD) Anterior Segment Dysgenesis
It will not be long once you have started to research Rocky Mountain Horses that the term ASD presents itself ... and for good reason. ASD is a hereditary eye disease that is linked to the Silver gene (Silver Dapple or Chocolate) which gives us the very popular Chocolate (Silver Dapple) colored horses with flax to white m/t. In cyst (or singular form 'Aa') there no effect with vision of the horse, it is when the gene is concentrated with two copies ('AA'), that eye deformities and sight issues can occur.
As outlined in the article by Dr. Victoria Jones, DVM~ "Anterior Degment Dysgenesis" (ASD) is a congenital syndrome of ocular anomalies that commonly afflicts Rocky Mountain Horses. Affected horses are born with the condition. It is not progressive. A screening eye examination for this condition can be performed any time after 4 months of age. If there are no signs of the condition by that age, the horse is considered to be free of the syndrome. It does not show up later in life. Blindness is not a feature of this disease. It is an inherited condition. Ophthalmic lesions are more prevalent in horses with a chocolate coat color and a white mane and tail color."
Does this mean Mountain Horses are unsound or unsafe to purchase? Not at all!
The ASD gene does exist without issue in many different breeds other than Mountain Horses including Morgans, Shetlands, Saddlebred, Quarter Horses etc. However, it is recongnized that ASD is more prevalent in the Rocky Mountain Horses than other breed as there have been trends to breed more specifically for the Chocolate (Silver Dapple) thus increasing the chances of producing an ASD offspring.
Geneticists from Michigan State University have determined, in one of the largest equine studies ever done, that the gene or genes affecting the physical development of the eye is associated with the the color gene that give us the Silver Dapple (chocolate). The ASD syndrome is a collection of differences in the structure of the front part of the eye.
Although ASD is a serious concern, responsible breeding can manage the risk of ASD; however, public demand can also negatively influence some breeding practices.
Normal, Cyst or ASD?
|aa (Normal)||These horses do not have ASD or "cysts" and their vision is normal.|
|These horses do not have ASD and their vision is normal. They carry one copy of the gene and have a cyst(s) in one or more eyes which does not interfere with their vision.|
These horses carry two copies of the gene and have ASD syndrome. A percentage will have their eyesight effected in some way.
|(aa) to (aa)||100% chance for a normal (aa) horse|
|(aa) to (Aa)||50% chance for a (aa) foal, 50% chance for a (Aa) carrier|
|(Aa) to (Aa)||25% chance for a (aa) foal, 50% chance for a (Aa) foal, 25% chance for a full blown ASD (AA) foal|
|(AA) to (aa)||100% chance for a (Aa) foal|
|(AA) to (Aa)||50% chance for a (AA) foal, 50% chance for a (Aa) foal|
|(AA) to (AA)||100% chance for a (AA) foal|
Currently there is no test available to detect ASD; however there is an eye exam that can be performed to identify cyst and can also confirm that horse does not have ASD. It should be noted that the eye exam is not 100% to determine normal (as it is a visual exam and cyst can be very difficult to see). In these instances, horses have been known to be silent carriers as they have produced cyst or ASD foals when bred.
A color DNA test is available through these two companies to determine if one or two copies of the Silver gene is present ... especially good for suspected Chestnut (red) based horses. Check out the other colors that can be tested for to add more insight into your horse. Color can be fun!