 |
About the Anterior Segment Dysgenesis (ASD) in Rocky Mountain Horses |
Anterior segment 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. Evaluation by a board-certified ophthalmologist is recommended due to the small size and subtle nature of some of the ophthalmic lesions. They may be missed unless a critical ophthalmic examination is performed. Integral to a thorough evaluation is specialized diagnostic equipment, including a slit-lamp biomicroscope and indirect ophthalmoscope. The eyes must be examined both before and after pupillary dilation. Rosebud River Ranch is serious about this condition and has all of their breeding horses and foals evaluated by myself, Dr. Victoria Grevan1. I am a board-certified veterinary ophthalmologist who has been working closely with Rosebud River Ranch for several years. The most common finding in the ASD condition of Rocky Mountain Horses is uveal cysts. Peripheral retinal lesions are the second most common abnormalities with this condition. Horses with either cysts alone or cysts and peripheral retinal lesions are considered mildly affected and heterozygous for the condition. This means that they carry one defective gene that codes for this condition (Aa). Two affected genes are required for full manifestation of the syndrome (AA). If a horse has two defective genes for the syndrome (homozygous; AA), multiple ocular anomalies will be evident and may include a large, protruding cornea (cornea globosa or “pop-eye”), pupillary abnormalities (small pupil, abnormally-shaped pupil, and a pupil that fails to dilate), and/or cataract formation. The horses with the full syndrome of multiple ocular anomalies are relatively uncommon amongst Rocky Mountain Horses; 14% incidence in one study.2 It is important to remember that this is a non-progressive condition and that horses are not blind with ASD. In a publication that reviewed ophthalmic evaluation of 514 Rocky Mountain Horses2, 48% showed some sign of ASD. However, functional vision did not appear to be impaired in any of the horses. All homozygous affected foals (AA) that have cornea globosa will initially have subnormal vision. But their eyes compensate as the foal matures and vision is considered normal by the time the eyes have matured (8-12 months of age). A rare manifestation of ASD involves progressive cataract formation from partial lens dislocation. These horses will have some vision deficit but this is only about 2% of Rocky Mountain Horses.2 The vast majority of ASD horses will have normal, functional vision. Since the gene for ASD is so widespread in the Rocky Mountain Horse population, it is currently accepted that horses with cysts can still be bred. Ideally, a horse with cysts should only be bred to a clinically normal horse. If bred to another horse with cysts, there is a 25% chance that their foal will get two defective genes and exhibit the severe form of the disease with the full spectrum of lesions. In summary, the most common ocular abnormality with ASD is ciliary cysts. These are not painful, they do not limit vision, and are of no consequence to the eye or to the horse. References: 1) Ramsey DT, Ewart SL, Render JA, et al. Congenital ocular anomalies of Rocky Mountain Horses. Vet Ophthalmol 1999; 2: 47-59. |
|