ASD

The Rocky Mountain horse breed carries a genetic defect called Anterior Segment Dysgenesis (ASD.)  The defect affects the eyes of the horse and is linked to those horses Rocky Mountain breeders term “chocolate” or “red chocolate.” Chocolate and Red Chocolate colors come from the silver dapple gene (Z.) The Rocky Mountain horse, along with other breeds, carries the silver dapple gene. Because of the way the gene is carried, horses with a chestnut based horse color can also have the defect.

Most horses that carry the genetic defect—what is normally coined “cysted”—have no issues with vision. These horses have only one copy of the gene, and represent the vast majority of horses affected by the defect. Dr. David Ramsey and Dr. Susan Ewart published the first study about ASD in 1999.  Ramsey wrote about the defect: "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."

Those horses with two copies of the gene have what we have coined “ASD.” Normally, these horses have functional vision.  Only a small handful has vision problems.  About this syndrome, Ramsey said: "A syndrome of multiple developmental eye abnormalities was seen less frequently [than cysts].  Again, horses born with multiple eye anomalies had normal functional vision with rare exception...."