Health corner: degenerative myelopathy

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Source: this article was originally published in La lettre électronique du CBF (the CBF's electronic newsletter), n°50, 10 August 2017, p. 30-33. © E. W.

What is it?

It is a serious neurodegenerative disease, both in its extent and in its consequences for the animals affected.

It is characterised by a progressive degeneration of the spinal cord in the thoracolumbar region, which affects dogs from 8 or 10 years of age onward. More rarely, it can affect younger dogs.

It results in progressive paralysis of the locomotor system. Life expectancy is then 18 to 24 months. The outcome is fatal. There is no known treatment.

Which breeds are affected?

A study of 33,747 dogs from 222 breeds showed that the mutation is very widespread and common across all breeds.

We have no track record regarding its penetrance in the French Bulldog breed.

The only figures we have are the results of tests recently recorded by the SCC, showing that out of 29 dogs tested, 17 are homozygous normal, i.e. « CLEAR », while 12 are heterozygous for the mutation (5 of them from the same kennel affix), i.e. « CARRIER » and therefore able to pass the mutation on to their offspring.

How is it transmitted?

The mutation is transmitted through the use of a dog that is a carrier of the mutation or affected by it.

It has been able to spread widely because the signs of the disease appear after the dogs' active breeding period, and, as far as our breed is concerned, until very recently screening was only included in the SCC's "comfort" tests and not yet in the predictive tests whose results are recorded on the pedigree.

As a result, a well-regarded stud dog, or even more so a champion, that is AFFECTED, may have a very large number of offspring — potentially a hundred or more — to whom he will unfortunately have passed on the mutation. They will in turn pass it on, and a certain proportion will develop the disease around ten years later.

Aggravating factor: inbreeding increases the risk. The risk of DM is 7 times higher in the case of a father × daughter or mother × son mating.

The recent ban issued by the SCC on this type of mating should minimise this risk, at least for dogs registered with the LOF.

How can a breeding dog's status be known?

A genetic basis has been identified on the SOD1 gene.

A dog will necessarily correspond to one of the 3 genetic profiles described below: homozygous normal, heterozygous for the mutation, homozygous mutated.

For ease of reading, we have adopted the following simplified terminology:

• Homozygous normal = CLEAR (SCC result: 1.1). A "homozygous normal clear" dog does not carry the mutation; it will not pass it on and will not develop the disease.
• Heterozygous for the mutation = CARRIER (SCC result: 1.2). A "heterozygous for the mutation" dog carries the mutation and is therefore able to pass it on to all or part of its offspring depending on the status of the mate (clear, carrier, or affected), but will not itself develop the disease.
• Homozygous mutated = AFFECTED (SCC result: 2.2). A "homozygous mutated" dog will pass the mutation on to all of its offspring, regardless of the mate's status, and depending on that status, some puppies will be affected and will develop the disease.

A few clarifications

DM is an autosomal recessive disease. The characteristics of a genetic disease with autosomal recessive inheritance are as follows:

• an individual (animal or human) is said to be homozygous when it carries two identical alleles at a given gene, and heterozygous when the alleles differ;
• a disease is said to be recessive when the defective gene is only expressed in the homozygous state;
• it affects any individual (animal or human) regardless of sex;
• inbreeding increases the risk;
• its penetrance is incomplete, which means that not all homozygous mutated dogs (AFFECTED) will necessarily develop the disease, but it is currently impossible to determine which of the homozygous mutated dogs will eventually develop the disease.

The mutation will not affect a heterozygous dog (CARRIER), but it will pass the mutation on to its offspring.

How can it be screened for?

The genetic screening test for DM is available from the SCC, and from various laboratories such as ANTAGENE and GENINDEXE, to name only the best known among breeders.

Why was this test made compulsory?

It was the request to register on the CBF's list of recommended stud dogs at rating 3 or above of a « CARRIER » stud dog that alerted us, at the same time as the SCC was asking us, on the one hand, whether the CBF accepted the registration by the health department of test results it had already received, and on the other, on the eve of the Nantes Championship, to specify the tests required for the award of "champion" titles for the year 2017.

As the DM test had, at the SCC, moved from a "comfort" test to a "predictive" test, we felt it was consistent to give the breed association's agreement for the SCC to register the results of tests already carried out spontaneously by a number of breeders and, in the same vein, to require this test for the validation of the champion title.

Some breeders posted rather unflattering comments on social media about the CBF and its board, considering this test ill-advised.

We have therefore selected some essential information from well-informed sources, and taken note of the veterinary and cynological position, in order to share it with our members and explain our position regarding DM test results.

The CBF, as the breed association, has therefore notified the SCC that, for the year 2017, it will give its approval to the validation of champion titles for French Bulldogs that are CLEAR for DM, and we would like to clarify that this should be understood as applying both to dogs that are « homozygous normal clear » and to dogs that are « heterozygous for the mutation », in other words clear carriers.

This decision applies solely to the validation of champion titles for the year 2017. For 2018 and subsequent years, the decision will be made each year by the board, based on the evolution of the statistics provided by the SCC, and will be published on the official website.

All other breeding dogs are nonetheless affected by the efforts that must be made to reduce the penetrance of this mutation in our breed, especially as its state of health is already the subject of intense criticism, and we have committed to addressing this before the cynological and scientific authorities. This is therefore not the time to look away.

What are the CBF's recommendations?

Do not give in to panic, nor take literally the advice for drastic eradication that can be read on social media, as we have a large population of breeding dogs, including titled, rated, and tested stud dogs, which allows for serious work to be carried out without losing entire bloodlines.

As the mutation is now identifiable through DNA analysis, any litter project can be planned with knowledge of the breeding dogs' genetic profile, the aim being not to produce homozygous mutated puppies, carrying 2 copies of the mutated allele, and therefore AFFECTED.

Every responsible breeder, concerned about the future of the breed as much as about the reputation of their own kennel, should now make it a priority to factor DM into their breeding choices and take into account the breeding dogs' genetic profile for the SOD1 gene before deciding on a mating, with the goal of eliminating the mutation.

N.B.: puppies from the mating of two "homozygous normal clear" breeding dogs will automatically be "homozygous normal clear".

After reviewing the work of Professor Grandjean (Unité de Médecine de l'Elevage et du Sport at the ENV d'Alfort veterinary school): « An excessive reduction in the number of potential breeding dogs leads to a genetic impoverishment that is highly detrimental to the breed », « implementing an eradication policy within a kennel or a breed club is difficult: one must avoid falling into extremes (ignoring the defect, or excluding too many breeding dogs) ».

Furthermore, according to information obtained from Antagene, for recessive diseases with a prevalence above 10 % (the majority of breeds), in order to preserve the breed's genetic diversity, heterozygous clear-carrier dogs can be used for breeding provided matings are adapted accordingly.

For the past 5 years, with no notable decline, 6,500 puppies have been born each year. This should give us the means for intelligent selection.

In summary

• Have breeding dogs tested.
• Find out the genetic profile of the stud dog and the dam before any mating.
• For a clear female (DM 1.1), preferably choose a stud dog with the same status, so as to produce only homozygous clear puppies.
• For a female heterozygous for the mutation, i.e. a clear carrier (DM 1.2), preferably choose a homozygous clear stud dog, and have the whole litter tested, so that only homozygous clear puppies are intended for breeding.