Anterior Segment Dysgenesis, more simply known as ASD, is an inherited, genetic condition that most often occurs in horses that are dark or chocolate brown in color and have a white or cream colored mane and tail. Typically horses with ASD are part of the Rocky Mountain Horse breed, which is the gaited horses bred and developed in the Rocky Mountain area of the United States.
ASD is present at birth and does not become progressively worse with age; rather it remains constant throughout the life of the horse. Screening by a veterinary ophthalmologist when the foal is approximately four months old can confirm the presence or absence of the condition, and foals can then be cleared for breeding stock if the problem is not present at that age. Horses cannot develop ASD as they mature so it is relatively easy to keep affected foals from being produced if owners are diligent with testing before breeding. Many breeders will still breed mildly affected Rocky Mountain horses with ASD but only to other horses that show no clinical signs of the condition and are tested to be free from ASD. Since the full presentation of the eye condition is only noted when both mares and stallions show the condition, it is safe to use these mildly affected horses in breeding programs with appropriate non-affected breeding lines. Breeding a mildly affected horse with a horse that has tested clear will, at very worse, produce a mildly affected foal which will have normal vision by the time it is about a year old.
The condition is characterized by lesions or cysts , usually appearing to be filled with fluid, on the retina and ciliary body of the eye. Horses with cysts will only have one gene for the condition and are considered to be mildly affected. Typically with the cysts there will also be some mild to moderate lesions of the retina and typically the condition will be bilateral, or affecting both eyes in roughly the same ways.
Horses that carry a double gene for ASD will have more significant eye problems including a "pop eyed" appearance where the cornea protrudes from the eye. The pupil may be very small, oddly shaped or may fail to dilate and contract when exposed to different types of lighting. This condition is much more significant as it can affect the horses ability to see in particular light conditions and can pose a safety hazard for both the horse and any rider that may be on the horse. Most horses that have significant problems either through pupil malformation or rarely a cataract problem will total less than 2% of the foals born in any given season. Typically horses carrying both genes for ASD are less than 14% of the total population of Rocky Mountain Horses although about 48% of the total population of the breed shows some signs of ASD.
ASD is usually not problematic for the horse nor does it cause blindness or vision problems. Usually foals that have ASD adjust to the condition and are excellent riding and performance horses when mature. Most horses have fully adjusted to the condition and are considered to have compensated to normal vision by the time they are one year old.