12 Oct Multiple Congenital Ocular Anomalies (MCOA)

Genetic Survey of the PMEL17/ Silver Mutation Which Causes Multiple Congenital Ocular Anomalies (MCOA)

Prepared by Noah Anderson
Chair, Genetics Committee RMHA
Assistant Professor of Biology
Winona State University

**This is an excerpt from the report; read the entire report here**

What do we know about MCOA? 

1. It is 100% caused by a gene called PMEL17, which also causes the silver (chocolate) coat coloration (Andersson et al., 2013). 
2. The eye abnormalities are more common and severe in homozygous silver (Z/Z) horses.  These include but are not limited to: retinal detachment, retinal dysplasia, large, protrusive corneas (pop eyes), dysfunctional pupils that lack typical pupillary response, cataracts, and myopia (near-sightedness: Ramsey et al., 1999).  
3. Horses that are heterozygous (Z/N) for silver typically have no major eye abnormalities, instead, they present with fluid filled cysts within the eye.  Studies have not been able to determine what effect, if any, cysts have on the eyesight of the horse.  Currently, they are thought to be benign.  
4. To date, only myopia (near-sightedness) has been shown to be progressive component of MCOA; Evidence of which has been found in silver Icelandic horses older than 16 years (Johansson et al. 2017).  Other defects have been found to be stable over the horse’s lifetime (Bellone, 2017).  
5. Lastly, it is important to remember that not all eye problems are caused by MCOA.  Horses, just like humans, can acquire or develop eye problems.