+++ Your dog suffers from tricuspid valve dysplasia? Please participate in our research project! +++

One can say that the frequency of heart diseases in a purebred dog is increased by 3.4 times that of a half-breed. With purebred breeders looking to select breed-specific features, faults in their ancestry can manifest themselves in the subsequent offspring. This is why one must be careful when looking at the pedigree of a dog and to be careful when noticing early deaths of ancestors or relatives.

Until today there are no sires or specific lines as source of the tricuspid valve dysplasia known. However tricuspid valve dysplasia is noted in the CIDD (CIDD: TVD) as “...the most common birth defect of heart in Labrador Retrievers...”. It is a fact, that tricuspid valve dysplasia is a well-known heart disease among animal cardiologists (especially with Labrador Retriever) and that a genetic cause is assumed – although with a very complex inheritance:

For the tricuspid valve dysplasia „in Labrador Retriever [...] an explicit breed disposition with familial accumulation“ exists and „a genetic cause of the tvd on chromosome 9 is assumed“.

That is described in various scientific sources (here) and most recently in the reference book „Praxis der Kardiologie Hund und Katze“, edition 2017, by Dr. Jan-Gerd Kresken (source of the upper cite).

To judge whether a certain problem actually represents a significant problem in regard to the overall population concerned or not, it not only requires some incidental findings but focused mass screening with an average sample. A basis for this could be provided by the Three Phase Model, developed by the VDH e.V. in cooperation with the Collegium Cardiologicum e.V.. However this needs a cooperation by the dog owners, breeders and primarily the associations.

The fact, that there is a problem (or simply not), cannot be changed by repression. Conversely: If there is a problem, continuous repression only contributes to deeper and deeper manifestation over time and maybe it developes to a – possibly not at all containable –  problem all the more. Additionally, repressing a potential problem becomes all the more dangerous, if (as for the tricuspid valve dyspasia) an autosomal dominant heredity disease with reduced penetrance is described in regard to the underlying genetics [Andelfinger et al. (2003)].  

Because it really is important to understand, what this kind of inheritance means, I would like to explain it once again, as precisely as possible:

When mating two dogs, every puppy receives one chromosome of each of its pairs of chromosomes from the mother and the others from the father. The chromosomes are the carriers of the genes, that’s why every puppy accordingly receives 50% of its genes by its father’s chromosomes and 50% of its genes by its mother’s chromosomes.

An autosomal dominant inheritance is characterised by already developing a particular attribute in the offspring, when the responsible gene occurs on at least one of the two chromosomes of the relevant pair of chromosomes. That means the attribute occurs, when the offspring received a gene with this attribute from at least one of its parents (or even both).

The same applies for defective genes: an autosomal dominant heredity disease already occurs in the offspring, when the pair of chromosomes relevant to the disease contains at least one chromosome with the defective gene.

For a better understanding I want to illustrate the possible heredity patterns visually.

For convenience only I label the genes as follows:

h – healthy gene

D – diseased gene (written in capitals because it’s the dominant gene)

One pair of chromosomes therefore can consist of the following gene combinations:

hh  or  hD  or  DD

Consequentely there are the following 6 potential combination possibilities when mating two dogs.

If both parents have two healthy chromosomes, the following distribution can be found for the offspring, statistically speaking:

• 100% of the offspring is healthy (hh)

If one of the parents has one chromosome with the defective gene and the other parent has two healthy chromosomes, the following distribution can be found for the offspring, statistically speaking:

• 50% of the offspring is affected with one defective (hD)
• 50% of the offspring is healthy (hh)

If one of the parents has two chromosomes with the defective gene and the other parent has two healthy chromosomes, the following distribution can be found for the offspring, statistically speaking:

• 100% of the offspring is affected with one defective gene (hD)

If both parents have one chromosomes with the defective gene, the following distribution can be found for the offspring, statistically speaking:

• 25% of the offspring is affected with two defective genes (DD)
• 50% of the offspring is affected with one defective gene (hD)
• 25% of the offspring is healthy (hh)

If one of the parents has two chromosomes with the defective gene and the other parent has one chromosome with the defective gene, the following distribution can be found for the offspring, statistically speaking:

• 50% of the offspring is affected with two defective genes (DD)
• 50% of the offspring is affected with one defective gene (hD)

If both parents have two chromosomes with the defective gene, the following distribution can be found for the offspring, statistically speaking:

• 100% of the offspring is affected with two defective genes (DD)

You are likely to consider, that the combination possibilities 2 to 6 won’t occur, because affected dogs won’t be used for breeding.

In principle this appears reasonable to consider. But admittedly in regard to the tricuspid valve dysplasia  a reduced penetrance additionally is described besides the autosomal dominant inheritance. Unfortunately this makes the whole case even more unpredictable.

This reduced penetrance namely means, that even in a dog with one or two defective gene/s at the relevant pair of chromosomes (the dog being affected in regard to the genetic code) the disease does not unavoidably also has to develop in the physical appearance – the dog nevertheless could occur healthy in regard to its body appearance and in the ultrasound examination. That’s why the combination possibilities 2 to 6 unfortunately can occur in breeding, unintentionally and without our knowledge.

What does that mean precisely?

• A dog, that is affected by tvd (verifiable via heart ultrasound), definitely carries at least one chromosome with a defective gene at the pair of chromosomes relevant for tvd – he is affected in regard to the genetic code as well as in regard to its body appearance.
• A dog, that occurs healthy in the heart ultrasound examination – therefore shows no tricuspid valve dysplasia in its body appearance – nevertheless  could  be affected in regard to the genetic code (with one or two chromosome/s with defected genes) and could unknowingly pass these disease on to its offspring because of the reduced penetrance of the inheritance.

Under the condition of the assumption, that the tricuspid valve dysplasia is a disease with autosomal dominant inheritance with reduced penetrance, consequentely there are in sum the following potential hazards concerning Labrador breeding – as long as no genetic test exists:

• at least(!) one of the parents of a tvd affected dog (verifiable via heart ultrasound) itself is affected in regard to the genetic code at least, without this unavoidably being shown in a cardiac examination
• statistically speaking it most likely can be expected, that there are further affected siblings (in regard to their body appearance or at least in regard to their genetic code) among the siblings of a tvd affected dog (verifiable via heart ultrasound)
• even a dog with a clear heart ultrasound examination nevertheless can be affected in regard to its genetic code (with one or two chromosome/s with a defective gene) and can pass the disease on to its offspring

According to these informations – and especially if one considers, that the tricuspid valve dysplasia isn’t the only heart disease relevant for Labrador Retriever – it actually should be clear,  why we feel it is strictly necessary

• to establish the heart ultrasound as an obligation in breeding,

as well as

• to send in findings and blood samples of as many breeding dogs as possible (both, clear and not clear findings) for the research project

and 

• to engage in education about the tricuspid valve dysplasia (and further heart diseases) similar to the education about hip joint dysplasia and elbow joint dysplasia, to cause the highest possible rate of ultrasound examinations in the offspring including participation in the research project.

Because that’s the only way we sooner or later will be able to describe the actual extent of the disease and – if the inheritance is to be confirmed – to interfere in breeding before it is too late, at best.

Even though we had our differences in recent years, we are continuously always willing to cooperate with the responsible associations regarding the heart health in Labrador Retriever.

+++ Your dog suffers from tricuspid valve dysplasia? Please participate in our research project! +++