Merle Eye Anomaly in Dogs

A variety of coat and iris colors in dogs are the result of the Merle gene. A dilution gene, its presence can lighten the color of the dog’s coat, often creating a dappling effect as well as vary the eye’s colored part (iris). When it comes to the eye, a variety of colors can be seen as a result of the Merle gene, ranging from pale, light blue to green to amber. This same gene can also be responsible for developmental defects in the eye or eyes. The following breeds, among others, are known to have the Merle gene:

• Australian Shepherd
• Rough and Smooth Collies
• Shetland Sheepdog
• American Foxhound 
• Catahoula Leopard Dog

The ocular effects of the gene can be minor, as in a blue iris, through severe as in complete blindness. The front or back part of the eye, or a combination of both can be impacted.

The Merle gene can lighten the color of a dog’s coat as well as lead to developmental defects in one or both of his eyes, ranging from minor to significant.

Dogs with the Merle gene (heterozygous or homozygous) can experience various symptoms with those considered double merles having the most severe eye abnormalities. These include:

• Diluted or dappled coat
• Varied color of the iris
• A small eye (microphthalmia)
• Pupil that is off-center (corectopia)
• Cataracts
• Blindness
• Congenital hearing loss

It is found that double merles that have an excessive white coat particularly in the region of their head will experience the more significant abnormalities.

Types

The Merle gene anomaly can be minor, as in the blue iris. The blue appearing in your dog’s eye can be a part of an eye that is otherwise brown and won’t impact his site. On the most severe end, the Merle gene anomaly can cause blindness. 

The front or back of your dog’s eye can be impacted; though the condition can impact both parts. Merle Ocular Dysgenesis is the name used when the entire eye is impacted.

It is known that congenital anomalies in eyes can be inherited. In each dog, there are two copies of a gene; one from each of his parents. We will call the Merle gene “m” and the non-merle gene “M”. If your dog gets two copies of m, the gene for Merle, it is considered homozygous (mm) or double merle. A dog with two merle genes will be mainly white in color. If the dog inherits one Merle gene and one non-merle gene, he is considered heterozygous (Mm). The one Merle gene copy will dominate the non-Merle gene; the one copy will dilute your dog’s coat and lead to the possibility of different colored eyes. If your dog has no Merle genes, (MM) he will be a typical, full colored dog.

In Australian Shepherds, there are various eye abnormalities that result from the Merle gene that happen from an autosomal recessive trait, indicating that the condition is not sex-linked. A recessive trait is only expressed when homozygous, meaning that dogs with these abnormalities must be double Merle (mm). Double merle dogs may also experience different degrees of congenital hearing loss.

A diluted coat will tip off your veterinarian that your dog may have at least one Merle gene. When it comes to the Merle gene and ocular dysgenesis, any abnormalities that result will be present at birth, allowing for diagnosis at as young as six weeks of age. Your veterinarian will conduct a physical examination of your dog and may recommend that he be seen by a veterinary ophthalmologist, who can look for structural abnormalities in his eyes. 

Dog’s with one more two copies of the Merle gene may show changes in their iris (including thinning), a pupil that is off-center, iris coloboma (abnormal development of the iris that looks like a notch or cleft at the pupil’s edge) and the lens of his eye may be out of place. Persistent pupillary membranes or PPMs may be seen; these may be viewed as strands or sheets of tissue that start from your dog’s iris and connect to another part of the iris, lens or cornea. These can be minor or cause significant vision problems. Cataracts (focal or complete) can also occur as a result of the Merle gene. 

The back portion of your dog’s eye can also be impacted by the Merle gene. The sclera, which is the white of the eye, can be unusually thin (called scleral ectasia), leading to a staphyloma, which is a bulge that can be present in the front or back of your dog’s eye. The vascular layer at the back of your dog’s eye may not fully develop, call choroidal hypoplasia. Anomalies at the back of your dog’s eye can impact the optic nerve, in severe cases causing blindness. The development of the retina may also be impacted and the reflective layer at the back of your dog’s eye can be missing. 

The examination will determine whether there are eye abnormalities present in your dog and how severe they are. Your veterinarian will take those into account when considering whether your dog has at least one copy of the Merle gene.

Whether treatment is possible, and the type of treatment that is recommended, will be dependent upon the conditions that have resulted from the inheritance of the Merle gene. For some of the abnormalities that occur, there are no treatment options, while for others, treatment may be possible. 

Dogs that have the Merle gene should not be bred, even if they are only displaying a mild form of the condition as they can still pass the gene on and their offspring can experience it in a more severe form. Merles should never be bred with one another as this would lead to an increased chance of double merles.

Recovering from treatment and managing your dog’s condition will depend on the extent of the problems he is experiencing as a result of the Merle gene. It is important that you attend follow up appointments as recommended by your veterinarian so that changes in your dog’s condition can be monitored and treatment provided when possible.