The ASD (anterior segment dysgensis) eye problem is an ophthalmic abnormality caused by a dominant gene found in some Rocky  Mountain Horses as well as Mountain Pleasure Horses and Kentucky Mountain Saddle Horses. ASD is NOT a degenerative eye disease, it is an inherited (genetic) defect in the formation and development of the eye, early in embryonic development.  Of all the horses with ASD almost all of them have acceptable vision which will not worsen with time. 
One of the most popular colors among the mountain horses, the chocolate with white or flaxen mane and tail is  related to the "silver dapple gene". Dr. David Ramsey's (DVM) research at Michigan State University, has shown a pattern that indicates the ASD condition is probably 
linked to the color combination of chocolate with white or flaxen mane and tail. Exactly where this link occurs, of how it occurs, is currently not known  for sure. It is best to treat the "suspicious" cases as if they were carriers until a genetic test is available 
Research underway hopes to develop a genetic (DNA) test that will diagnos ASD horses (AA), horses with cysts (Aa) and  normal horses (aa)  in a simple and inexpensive test. Until a genetic test  is developed,  phenotype (physical appearances) of the horse, and skilled, thorough eye examinations are the only means to identify ASD horses. 
The horses with ASD (anterior segment dysgensis) have problems in the anterior (front) part of the eye which did not develop normally. In some cases abnormalities found in the anterior part of the eye, also have abnormal development in the posterior (back) part of the eye. This genetic (inherited) condition (ASD) is identified in the studies by the symbol "AA" . Horses with less severe involvement, but who have eye cysts and are carriers of the condition are labeled "Aa". "Normal" eyes are identified as small "aa". 

The "aa"  horse exhibit no sign of eye abnormality. However, they could be carriers of the genetic 
defect. Analysis from the pedigree data collected indicate that this abnormality has  a semidominant mode of inheritance, but the trait is incompletely penetrant (is not evident upon examination) approximately 12% of the time. 

Horses heterozygous for the trait (Aa) have cysts of the ciliary body and peripheral retina, and 
retinal dysplasia (abnormally formed retina). 

Horses homozygous for the trait (AA) have multiple abnormalities of the eye. The signs of ASD positive horses include many, or most of the following conditions; 

  • cysts in the ciliary body of the eye (a structure behind the iris or the eye); 
  • abnormally small eyes (microphthalmia); 
  • pop-eyed or bulging eyeballs (megalocornea); 
  • abnormally shaped pupils (dyscoria); 
  • nuclear cataracts (the central portion of the eye lens is "clouded"); iris 
  • abnormalities (colored area around pupil); 
  • subluxation of the lens (partial displacement of the lens from its normal position); 
  • abnormal development in the retina of the eye (retinal dysplasia); 
  • granula iridica (normal iris tissue present at the top part of the iris and pupil that may encircle the pupil); 
  • small pupils that fail to dilate or dilate incompletely even when a pupil dilating drug is given to the eye; 
  • abnormal drainage of the eye; 
  • wide eyelid openings. 
Some ASD "AA" horses will suffer no visual impairment at all; some will be born blind or go blind later in life. Of the horses examined by Dr. Ramsey, approximately 14% were ASD A veterinary ophthalmologist needs to check all breeding stock, to determine the animals status PRIOR to breeding. 
Breeding Offspring
Normal (aa) to normal (aa) 100% normal (aa)
Normal (aa) to cyst (Aa) 50% normal (aa); 50% cyst (Aa)
Cyst (Aa) to cyst (Aa) 25% normal (aa); 50% cyst (Aa); 25% ASD (AA)
ASD (AA) to normal (aa) 100% cyst (Aa)
ASD (AA) to cyst (Aa) 50% ASD (AA); 50% cyst (Aa)
ASD (AA) to ASD (AA) 100% ASD (AA)
The current findings are supported by a "grant" to Dr. Ramsey, he has established a research fund at Michigan State University to support RMH research. That grant money is now depleted and monies must come from other  sources. All donations will be used to develop a genetic test to identify the genetic status of RMHs. All donations are fully tax-deductible. You can make a tax deductible donation to: 
Rocky Mountain Horse Research Fund 
c/o Dr. David Ramsey 
D-208 Veterinary Medical Center 
College of Veterinary Medicine 
Michigan State University 
 East Lansing, MI 48824-1314 
The University will send you a letter of receipt of your donation for your tax records. Also note: the research is under the auspices of Michigan State University and is independent of RMHA control. 
The Rocky Mountain Horse Association (RMHA) have published stringent guidelines for breeding. Moreover, they have given all ASD information available to the entire membership. 
This abnormality is not restricted to the Rocky Mountain Horse. David T. Ramsey, DVM, has examined found  registered as either Kentucky Saddle Horses (KSH) or registered as Mountain Pleasure Horses (MPH) with eye abnormalities the same as those of Rocky Mountain Horses. Analysis of pedigrees from affected horses revealed similar lineage and ancestry compared with the RMH. 

Board of Directors of the Kentucky Mountain Saddle Horse Association met with Dr. Gus Cothran at the University of Kentucky Animal Pathology Lab to establish their guidlines for determining the extent to which the ASD condition affects the Kentucky Mountain Saddle Horse. The board believes that there is a much higher percentage of normal horses within their breed because the KMSHA  accepts horses from many genetic lines and the books remain open for mares to be registered with the association. It has taken the Kentucky Mountain Saddle Horse Association longer in achieving  separate breed status because they continue to accept horses into the registry from other breeding lines, and cannot become a breed until they close the books. However they feel  the Kentucky Mountain Saddle Horse and will be less susceptible to other genetic problems caused from breeding too closely within specific horses lines.