Canine Multifocal Retinopathy or CMR in dogs

Canine multifocal retinopathy or CMR is a hereditary health condition that affects the eyes of dogs, and leads to raised lesions developing on the retina of the eye, which can result in folding of the retina. The lesions tend to develop for the first time in puppies that are a few months old, which means that they may have already been sold or passed on to their new homes by breeders before the condition becomes apparent.

Breeding from parent dogs who are themselves either carriers of or affected by CMR means that their offspring may in turn become carriers of or affected by the condition, as well as spreading the gene mutation behind the condition on to subsequent generations.

However, A DNA testing scheme is in place to let potential breeders find out the status of their own dogs prior to making a decision on any mating match, to enable breeders to selectively mate only healthy stock.

CMR has been identified as a risk to certain breeds of dog, for which DNA testing to determine their status is strongly advisable prior to breeding, and prospective buyers of puppies from these breeds are advised to choose only pups from tested stock.

In this article, we will look at canine multifocal retinopathy in more detail, examining how the condition affects dogs, how it is passed from dog to dog, and which dog breeds are at particular risk of the condition. Read on to learn more.

More about canine multifocal retinopathy

Canine multifocal retinopathy or CMR usually begins to present in affected dogs by the time they are around 4 months old, and leads to a range of symptoms that can range from lesions of the eye to mild folding of the retina to the development of retinal detachment, depending on how it affects the dog in question.

CMR is caused by a mutation in one of the dog’s genes, and usually begins with the development of lesions of irregular shapes and sizes in both eyes at once, which usually have a pinkish-grey colouration. As the affected dog gets older, their vision may deteriorate and ultimately lead to total blindness, although for some dogs, the symptoms are self-limiting, and will reach an early plateau and not worsen further to affect the dog’s vision.

How is CMR passed from dog to dog?

CMR in dogs is an autosomal recessive hereditary condition, which occurs when a mutation is present within the VMD2 gene. When parent dogs that possess this mutation are bred from, they can pass on either the affected or carrier form of the condition to their own offspring. However, because CMR is an autosomal recessive trait, just one parent being a carrier of or affected by the condition will not lead to their offspring being affected too; it is the combined status of both parent dogs that determines the status of their subsequent litter.

A parent dog may be either clear (no mutation in place) a carrier (they possess the mutation but are not affected by the condition themselves) or affected, having the gene mutation and active form of the condition.

What the status of any given litter will be can be predicted by knowing the status of their two parent dogs, using the following model:

• Two clear parent dogs will produce a clear litter.
• Two affected dogs will have an affected litter.
• Two carrier dogs will have a litter of 25% clear, 25% affected and 50% carriers.
• A clear dog and a carrier will produce a litter of 50% carriers and 50% clear.
• A clear dog and an affected dog will produce a litter of carriers.
• A carrier and an affected dog will produce a litter of 50% carriers and 50% affected.

What sort of dogs are at risk of the condition?

CMR has been identified as present within the gene pool of a number of different dog breeds, to the point that the condition is considered to pose a risk to these breed’s wider health and wellness. For dogs of these breeds, pre-breeding health screening is advisable, in order to ensure that only healthy dogs who will not pass the condition on are bred from.

The main breeds identified as at risk of CMR are:

• Bullmastiff
• English mastiff
• Australian shepherd dog
• American bulldog
• French bulldog
• English bulldog
• Cane Corso
• Dogue de Bordeaux
• Pyrenean mountain dog

Health testing for canine multifocal retinopathy

In order to find out the status of any given dog – whether they are clear, carrier or affected – a DNA test can be performed using a blood sample or cheek swab from the dog in question. This is analysed by a laboratory to provide information can enable dog breeders to make an informed decision about whether any given pair should be bred from, and what the status of their resultant litter will be.

Breeding carriers with clear dogs is a viable choice in some cases, as their resultant pups will not be affected with the condition themselves – however, some of them will inherit the carrier form of the condition themselves, which means that they should not be bred from, or must be tested and mated with a tested partner in order to inhibit the spread of the condition.

All breeders of at-risk breeds are urged to determine their dog’s status prior to breeding, and potential puppy buyers should ask to see the parent dogs’ test results prior to committing to a purchase.