MCOA (previously called ASD) is a common congenital eye disease seen in Rocky Mountain Horses, less commonly in other gaited breeds. It has been reported in Icelandics, Shetlands, American Miniatures, Belgians, Morgans, Kentucky Mountain Saddle, and Mountain Pleasure horses. It affects about 40-50% of Rocky Mountain Horses. It is more common in horses with a chocolate coat color and a light mane and tail color. Affected horses generally have the "silver dilution" gene, which can be tested for.  www.vgl.ucdavis.edu/services/horse/silver.php

Affected horses are born with the condition and, generally, it does not worsen with time. If there are no signs of the condition by an examination at 4 months of age, the horse is considered to be free of the syndrome.

Generally, this condition does not cause blindness but severely afflicted horses might have cataracts and some reduction in sight.

Changes caused by ASD can often be seen in the front chamber of the eye but there are usually changes in the back of the eye too. The most common finding is uveal cysts, but other abnormalities often exist. These may include a a large, protruding cornea, small or abnormally shaped pupils, or cataracts. A smaller percentage of the population of these horses (10-20% of the population) has the full set of signs.

This condition is caused by an autosomal dominant gene. Horses can be free of the disease (aa), be carriers (show mild signs of disease) if they have one copy of the gene (Aa) or be homozygous for the condition (AA, and have the full set of signs).

In a publication that reviewed the ophthalmic evaluation of 514 Rocky Mountain Horses, 48% showed some sign of ASD. Attention should be paid to matings between individuals of this breed. Apparently, the breed association still supports the breeding of mildly affected horses. Ideally, a horse with signs of the condition should only be bred to an apparently normal horse. If bred to another horse with signs, there is a 25% chance that the resulting foal will get two defective genes and exhibit the severe form of the disease. From a purchase exam standpoint, if the changes are mild and since they are not likely to worsen, horses should function fine.