Will My Molosser Go Blind from Canine Multifocal Retinopathy?

Canine multifocal retinopathy, also known as cmr, is a genetic condition that affects the eyes of various dog breeds.

It was first described in Great Pyrenees dogs about 30 years ago by a group of veterinary ophthalmologists in Canada. It has since been studied extensively due to similarities with a condition in people known as Best Macular Dystrophy (or Best disease).

Three forms of the disease are described in the dogs. It is due to a mutation within the same gene that is specific for each breed. All three forms will result in the same clinical appearance.

The lesions are detectable by eye examination by a veterinary ophthalmologist in dogs older than three months, but the onset can be variable. It is a very slowly progressive disease, with different stages. The clinical appearance will depend on the stage at which the dog is examined.

When the dog is younger, the lesions can come and go, and may not be detectable consistently at each examination until the dog is approximately one year old. The lesions appear as focal round retinal elevation, filled or not with a darkly pigmented fluid. (See Figure B below.) They can develop anywhere along the dog’s retina, but they have a tendency to accumulate in areas where the retina is rich in cones (i.e. day-vision cells). Some dogs may develop more lesions than others; the reason behind this remains not yet well understood. The size of each lesion is also variable, but some can be quite extensive.

Fundus photography of a normal dog (A) and one affected by cmr (B).

As the dog becomes older, the previous area of elevation will focally die, and it may be harder to detect by eye examination, as it then appears shiny and dry (no longer filled with fluid). Dogs affected by a mutation in the cmr gene are unlikely to go blind, since enough retinal cells will remain viable outside of the lesions for the dog to remain functionally visual. Blurry vision as described in people affected by this disease remains unknown in dogs due to lack of proper assessment. This condition is not painful, and no treatment for dogs is currently available to date.

Beside by examination by an ophthalmologist, genetic testing is now widely available online to determine whether or not your dog is affected by the disease. The mode of inheritance is recessive, meaning that only dogs that carry two copies of the mutation will develop the disease, and will be reported as affected. Genetic testing prior to breeding is crucial for good breeding practice, and is highly recommended in breeds known to be affected by this disease (and others).

While eradication of the mutation within the affected breeds is unlikely, frequency of the mutation can be minimized by careful breeding practice. If no other choice, the recommendation is to breed an affected dog with a dog that tested negative (WT/WT or normal) for the mutation, and to avoid breeding of an affected dog with a dog carrier for the mutation.

Eliminating all dogs affected with the mutation from breeding may lead to emergence of other genetic mutations, which may have a bigger impact on the breed’s health (i.e., increase in hip dysplasia or cardiac diseases).

Valérie L. Dufour, DVM, MSc is an Assistant Professor of Translational Ophthalmology at the University of Pennsylvania School of Veterinary Medicine.