MCOA stands for Multiple Congenital Ocular Anomalies - it is a dominant disease
MCOA affects horses with the Silver coat color dilution. Horses that have two copies of the Silver gene are more sseverly affected and a wide range of eye defects are seen including underdevelopment of the iris, greatly enlarged eyes, small cysts in the eye and cataracts. Horses with only one Silver gene have less severe defects, usually just cysts..
Breeds that have been diagnosed with MCOA include the Rocky Mountain Horse (including the two related breeds Kentucky Mountain Saddle Horse and Mountain Pleasure Horse), Icelandics, Shetland Pony, Exmoor Pony, American Miniature, Belgian Draft, and Morgans.