Multiple Congenital Ocular Anomalies (MCOA)

Quick Summary

Multiple congenital ocular anomalies (MCOA) is an inherited eye disorder that is associated with the silver dilution and is characterized by ocular cysts, enlargement of the cornea, abnormally formed iris/retina, and additional abnormalities.

Phenotype: Multiple congenital ocular anomalies (MCOA) is associated with the silver dilution and is characterized by ocular cysts, enlargement of the cornea, abnormally formed iris/retina, and additional abnormalities.

Mode of Inheritance: Incomplete dominance

Alleles: N = Normal/Unaffected, Z = Multiple congenital ocular anomalies; Silver

Breeds appropriate for testing: Appaloosa, Arabian, Ardenne, Finnhorse, Gypsy Cob, Gypsy Vanner, Icelandic Horse/Pony, Kentucky Mountain Horse, Miniature Horse, Missouri Foxtrotter, Morgan Horse, Mountain Pleasure Horse, Norwegian Nordland, Paint Horse, Quarter Horse, Rocky Mountain Horse, Saddlebred, Shetland Pony, Swedish Warmblood, Welsh Pony, related breeds

Explanation of Results:

  • Horses with N/N genoytpe will not have multiple congenital ocular anomalies (MCOA) or the silver dilution, and cannot transmit this MCOA variant to their offspring.
  • Horses with Z/N genotype will likely develop a less severe form of multiple congenital ocular anomalies (MCOA), and will be silver dilute. They may transmit this MCOA variant to 50% of their offspring. Matings with N/N horses will result in a 50% chance of producing a foal that will likely develop a less severe form of MCOA.
  • Horses with a Z/Z genotype will likely develop a more severe form of multiple congenital ocular anomalies (MCOA) and will be silver dilute. These horses will pass down this MCOA variant to all of their offspring. Matings with any genotype will results in a foal that will likely develop some form of MCOA.

Chestnut horses with one or two copies of the MCOA variant will not be silver dilute but are still likely to develop ocular abnormalities/MCOA.

Price

$25 one test per animal

Additional Details

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.

 

The genetic test for the MCOA is the same as Silver Dilution for coat color.
Turnaround Time
2-6 business days
Type of Sample

Species

Results Reported As
Test Result Silver and Multiple Congenital Ocular Anomalies (MCOA)
N/N No copies of silver dilution detected.
N/Z

One copy of silver dilution detected. Horse will likely have less severe form of MCOA.

Z/Z

Two copies of silver dilution detected. Horse will likely have more severe form of MCOA.

References

Brunberg, E., Andersson, L., Cothran, G., Sandberg, K., Mikko, S., & Lindgren, G. (2006). A missense mutation in PMEL17 is associated with the Silver coat color in the horse. BMC Genetics, 7(46). doi: 10.1186/1471-2156-7-46

Andersson, L. S., Lyberg, K., Cothran, G., Ramsey, D., Juras, R., Mikko, S., Ekesten, B., Ewart, S., & Lindgren, G. (2011). Targeted analysis of four breeds narrows equine Multiple Congenital Ocular Anomalies locus to 208 kilobases. Mammalian Genome, 22(5-6), 353-360. doi: 10.1007/s00335-011-9325-7

Andersson, L. S., Wilbe, M., Viluma, A., Cothran, G., Ekesten, B., Ewart, S., & Lindgren, G. (2013). Equine Multiple Congenital Ocular Anomalies and Silver Coat Colour Result from the Pleiotropic Effects of Mutant PMEL. PLoS ONE, 8(9). doi: 10.1371/journal.pone.0075639

Johansson, M. K., Fegraeus, K. J., Lindgren, G., & Ekesten, B. (2017). The refractive state of the eye in Icelandic horses with the Silver mutation. BMC Veterinary Research, 13(1), 153. doi: 10.1186/s12917-017-1111-7