Multiple congenital ocular anomalies

The Silver variant can affect both a horse’s coat color and eye development

The Silver variant lies in PMEL, also called PMEL17 in some documents. This gene contributes to cellular structures involved in pigmentation. Research links the Silver variant to a group of developmental eye abnormalities, rather than a coat change alone.

Pleiotropy means that one genetic change has several effects. Silver is an example: the same DNA variant can dilute black pigment in the coat and affect eye development. The way the horse’s coat looks cannot measure the condition of its eyes. A desirable color effect is not evidence that the associated ocular findings are absent.

References [2] · [1] · [5]

Eye abnormalities are usually more extensive with two copies of the Silver variant

With one copy, reported abnormalities are often more limited, particularly cysts in structures such as the iris or ciliary body. These fluid-filled formations can be difficult to detect without a specialist examination. Their presence does not automatically cause a problem obvious to the owner.

With two copies, several abnormalities may occur together: cysts, an enlarged cornea, abnormal iris or retina, or cataracts. Vision can be impaired, sometimes severely. This dose relationship explains incomplete dominance: one copy can have an effect, while two are generally associated with a more extensive form.

References [1] · [2] · [4]

Horses with the same Silver DNA result can have different vision

Horses sharing a genotype do not all have identical signs. Incomplete penetrance means abnormalities are not always demonstrated in every horse carrying the relevant genotype. Variable expression describes differences in the type or severity of findings when they occur.

Studies also emphasize the difficulty of detecting small cysts or examining certain structures. An apparently normal observation therefore cannot guarantee that every subtle abnormality is absent. Conversely, not every eye abnormality is caused by Silver. The DNA result alone cannot establish how much an individual horse can see.

References [3] · [1] · [4]

A chestnut horse can carry the Silver variant without a visible coat change

Silver acts on black pigment. Where that pigment is present in the hair, the variant can lighten the mane and tail and alter black areas. In a chestnut, the visible dilution can be hidden because black pigment is not expressed in the coat hair.

That masking does not remove the variant or its association with eye abnormalities. A chestnut can therefore carry one or two copies and transmit them without looking Silver. Combined coat effects can further complicate appearance. Coat observation and an eye examination answer different questions and neither should be treated as a substitute for the other.

References [1] · [4]

Silver-associated eye abnormalities occur in several horse breeds

MCOA was studied extensively in American mountain-horse populations carrying Silver, including Rocky Mountain Horses. Published work also describes cases in ponies and Comtois horses. The laboratory lists many populations in which testing can be relevant.

Silver in a breed does not mean every member carries the variant or every carrier has identical abnormalities. A small case study can explain clinical findings without establishing a frequency for an entire breed. Population information helps identify questions to investigate, while the individual horse’s result and examination remain central.

References [2] · [4] · [6] · [1]

An eye examination assesses sight in a horse carrying the Silver variant

Silver/MCOA testing detects the Silver variant in the PMEL gene. On this test, N denotes a version without the tested variant and Z denotes Silver. N/N means no Silver copies, N/Z means one Silver copy and Z/Z means two. A coat-color test for Silver and a test offered for MCOA can therefore analyze the same variant: their results are not additional copies to add together.

A veterinary ophthalmologist examines eye structures to establish which abnormalities are present and what they mean for the horse. A front-facing photograph cannot reveal every cyst. A result without the tested variant does not exclude other causes of cataracts, inflammation or impaired vision.

References [1] · [4] · [3]

What eye care does a horse carrying the Silver variant need?

MCOA eye abnormalities develop before birth. That is what congenital means, although subtle abnormalities may go unnoticed.

Two horses with the same DNA result can have different eye abnormalities. The veterinarian examines their eyes to identify these abnormalities, check whether they affect sight and look for signs of pain. The findings guide follow-up and any treatment each horse may need. The number of Silver copies alone cannot determine which treatment is appropriate.

If a horse seems to see less well, has a painful eye or develops a visible change in an eye, it needs a veterinary examination. Another eye condition could also be responsible for that change.

References [1] · [4]

A foal can have eye abnormalities after inheriting the Silver variant from just one parent

If the mare and stallion each have one copy of the Silver variant, both test N/Z. At each conception, their foal has a 25% chance of inheriting Silver from both parents and being Z/Z, a 50% chance of inheriting it from one parent and being N/Z, and a 25% chance of inheriting it from neither parent and being N/N. Z/Z foals generally have more extensive eye abnormalities, but N/Z foals can also be affected. These chances apply to each conception; four foals are not guaranteed to follow that distribution.

If one parent has one Silver copy (N/Z) and the other has no Silver copies (N/N), their foal cannot inherit two copies of this variant. It still has a 50% chance of inheriting one Silver copy, being N/Z and potentially having eye abnormalities. If one parent has two Silver copies (Z/Z) and the other has none (N/N), every foal inherits one Silver copy. All their foals are therefore N/Z and can have eye abnormalities. Understanding what a mating can pass on requires both parents’ DNA results, including chestnuts, and information from their eye examinations.

References [1] · [2]

Questions about the condition: Multiple congenital ocular anomalies

Scientific sources about the condition: Multiple congenital ocular anomalies

This article brings together the research and the university and laboratory resources listed below. Numbers after each section link to the references used. Sources consulted in November 2026.

1. 1.
   Multiple Congenital Ocular Anomalies (MCOA): Silver testing and interpretation

   UC Davis — Veterinary Genetics Laboratory · Test documentation

2. 2.
   Andersson et al. (2013) — Ocular anomalies and Silver coat color result from the pleiotropic effects of mutant PMEL

   PLOS ONE — Swedish University of Agricultural Sciences and collaborators · Research

3. 3.
   Andersson et al. (2011) — Targeted analysis of four breeds narrows the MCOA locus to 208 kilobases

   BMC Veterinary Research · Research

4. 4.
   Equine MCOA syndrome in PMEL17 (Silver) mutant ponies: five cases

   Veterinary Ophthalmology (2011) · Research

5. 5.
   OMIA 000733-9796 — Multiple ocular defects in the horse

   The University of Sydney — Online Mendelian Inheritance in Animals · University resource

6. 6.
   Depecker et al. (2013) — Phenotypic description of multiple congenital ocular anomalies in Comtois horses

   Equine Veterinary Education · Research