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The above does not contain all the abnormalities that can be displayed by a horse with ASD, but it contains the most common of the abnormalities.
Dr. David Ramsey, an ophthalmologist from the College of Veterinary Medicine at Michigan State University, researched a large section of Mountain Horses and found an alarmingly high quantity of chocolate Mountain Horses were afflicted with ASD, or were carriers of it. So far the studies have shown this problem is fairly isolated in that it's a problem displayed almost entirely with the chocolate Mountain Horses. Other colored Mountain Horses can be carriers for ASD or have full-blown ASD, but it isn't near as likely.
Though the exact genetic details of ASD are still under investigation, the cause for this has been shown to be inbreeding. This is also the reason the problem is predominately a chocolate based one; inbreeding was extensively used to produce the chocolate color with the flaxen mane and tail. Those who placed the almighty dollar over the quality and stability of this breed have caused this problem.
Does this mean Mountain Horses are unsound or unsafe to purchase? Not at all! The majority of Mountain Horses are not carriers or ASD-afflicted. This problem is pretty much exclusive to excessively inbred lines and the chocolate color. And if you are considering a horse with ASD, you only need to know how ASD works to make a safe purchase. ASD in many cases will not get worse as the horse ages. Chances are the vision the horse has now is the same vision the horse will have all his life. If one of the symptoms is progressive cataracts your horse's vision will get worse over time, but luckily progressive cataracts is not one of the common symptoms of ASD. If your horse has ASD but seems to be suffering no loss of vision, chances are you have little to worry about.
If you are looking to purchase a horse with full-blown ASD, even though vision usually doesn't get worse over time it is vital you determine what symptoms of ASD the horse has to ensure vision will not be impaired further over time. If you are purchasing a stallion or mare and believe there is even the slightest chance you may wish to breed your horse in the future, do NOT buy a horse with ASD. To do so will only spread the problem - it needs to be stopped, and only responsible breeding practices can achieve this.
The above information dealt with horses that have full-blown ASD; it did not consist of carriers of ASD. Carriers are a much safer bet to deal with for several reasons. A carrier's symptoms will be cysts along the ciliary body of the eye. Carriers show no signs of eye abnormalities, nor do they show a loss of vision. It is important to ensure your veterinarian knows exactly what to look for when checking your horse's eyes for ASD or cysts, because many vets across the nation have misdiagnosed a horse as 100% clean when these horses were not. ASD is easy to diagnose, in many cases you can see the symptoms of ASD by just looking at your horse's eyes. Carriers are not so easy to diagnose, because the eye IS normal and the cysts along the ciliary body can be missed if the vet isn't careful or isn't aware they should check that area of the eye out closely. The cysts are flat in nature (they aren't truly cysts, but have been classified as such), so a vet may miss these cysts if he looks at them from the side. We suggest if you wish to examine your horse for ASD or ciliary cysts you use an ophthalmologist rather than a veterinarian. The added experience, knowledge of the eye and professional equipment an ophthalmologist possesses is important.
If you are looking for a riding horse, it doesn't really matter if the horse is a carrier for ASD. His vision will not be affected since his eyes are healthy. If you believe you may breed your horse in the future, you can, with careful breeding, ensure the offspring of your horse does not contract full-blown ASD. You cannot prevent your foal from possibly becoming a carrier, but you can protect the foal from ASD. Though it was always my preference to deal in stallions and mares that were both clean of ASD and not carriers of it, if you fall in love with a carrier and simply must breed the horse, it is possible to do so somewhat responsibly.
If you are interested in the breeding aspect of Mountain Horses, the information contained in ASD & Mountain Horses: Breeding Concerns will be particularly important to you.