Equine Multiple Congenital Ocular Anomalies and Silver Coat Colour

New study on vision in silver horses – participate with your horse!

We now want to examine silver-colored horses to determine how their vision is affected. The goal is to map whether horses with MCOA predisposition, both homozygotes and heterozygotes, develop refractive errors that can affect their vision. Previous studies have shown that some homozygotes have severe visual impairment, mainly due to abnormal pupil function and cataracts, but it is currently unknown whether the vision of heterozygotes is affected.

We are looking for silver-colored horses for a clinical study. Chestnuts with at least one silver-colored parent are also of interest for the study.

We will want to examine the horses' eyes and take a hair sample from them. Therefore, we would prefer the horses to be located somewhere near the Uppsala area. However, other areas may also be of interest if there are more horses in the same area. The study will begin in January 2016. If you are interested in participating with your horse, please contact Kim Jäderkvist Fegraeus, or call 018-671917.

More background information about MCOA and the silver color

A study conducted in 2013 at SLU showed that the gene PMEL, which causes MCOA, is also responsible for the silver color in horses.

Examples of symptoms of MCOA are cysts (blisters) in the cornea and retina, underdeveloped iris, changes in the area where fluid inside the eye drains (chamber angle), abnormal shape of the cornea, and cataracts. There is a milder form that is clinically characterized by the aforementioned cyst formations and a more severe form of MCOA where several malformations are seen simultaneously. A study conducted in 2013 at the Swedish University of Agricultural Sciences showed that the gene, PMEL, which causes MCOA is also responsible for the silver color in horses. MCOA was first discovered in Rocky Mountain horses, a breed with a large proportion of silver-colored horses. The disease has also been detected in a number of other breeds where the silver color occurs, such as Icelandic horses and Morgan horses.

To identify the mutation causing MCOA, the disease-associated DNA region was sequenced. We were able to systematically exclude all genetic variants except two located in the PMEL gene, which perfectly matched the inheritance pattern of the disease. One genetic variant changes the amino acid sequence and the other is located in a non-protein-coding part of the gene. Together with results from other research groups on the function of the PMEL protein, our study shows that the coding change leads to both the eye defect MCOA and the silver coat color.

It has previously been observed that horses that are heterozygous for the mutation, i.e., carry one copy of the disease gene, almost always exhibit the milder form of the disease, while those that are double carriers of the silver gene (homozygous) are more severely affected. Our recommendation is not to cross two horses that both carry the silver trait, as there is a risk of producing offspring with double predisposition. Certain colors (e.g., chestnut) do not reveal whether a horse is a carrier of the silver trait, as the silver color is only expressed on black pigment. However, in Shetland ponies, there is a color similar to silver that is expressed in chestnuts. This color is called mushroom, and of the horses that have been tested for the silver mutation, none have been carriers of the silver mutation.

The genetic background to the mushroom color is still unknown, but it is not linked to the silver mutation. To determine whether a horse is a carrier of silver, a hair sample can be sent to the Animal Genetics Laboratory in Uppsala, which performs genetic tests for the silver color.