Multiple Congenital Ocular Anomalies (MCOA)

Multiple Congenital Ocular Abnormalities syndrome (MCOA) is an eye disorder associated with presence of the silver dilution mutation in PMEL17 gene. MCOA is characterized by defects that occur in the anterior and posterior segment of the eye. The severity of the syndrome is dosage related; it varies depending on the number of copies of the silver mutation present. Horses with 1 copy of silver (N/Z) have less severe signs (Cyst phenotype), typically presenting as translucent cysts (or fluid filled vesicles) that can be 1 centimeter in diameter. Horses with 2 copies of silver (Z/Z) have more severe signs (MCOA phenotype) with cysts and additional abnormalities such as enlargement of the cornea, abnormally formed iris and/or retina, among others, which impair vision and can cause blindness.

MCOA shows incomplete penetrance, which means that there is variation in presentation of MCOA among horses with N/Z and Z/Z genotypes. Research suggests that there may be a progressive change in vision of horses with the silver mutation. To avoid producing offspring with the severe MCOA associated with 2 copies of silver mutation, breeders should not mate two horses with silver mutation (N/Z or Z/Z, regardless of base color) to each other. Although the silver mutation has no effect on the coat color phenotype of chestnut/sorrel horses, it can cause ocular abnormalities if present. Chestnut horses from silver lineages should be tested to inform breeding decisions. Regardless of base color, it is advisable that horses with the silver mutation be examined by a veterinary ophthalmologist to determine the extent and severity of ocular condition.

Testing for the silver dilution/MCOA is important in assisting clinicians, owners, and breeders in identifying affected horses. Breeder can use results from the test as a tool for selection of mating pairs to avoid producing homozygous (Z/Z) individuals.