Inherited diseases: the adequate breeding strategy (1)
In relation to the concerns regarding pedigree health, as well as to the growing number of genetic tests available, there is an increasing demand from breed clubs about guidelines regarding the management and selection against inherited diseases. Indeed, the commercialization of a given test does not always mean it allows to indentify all carriers of the targeted disease. Also, removing all carriers may create genetic bottlenecks and can, in some cases, be the surest way to favor the emergence of other diseases. It is therefore very important for breed clubs to have a clear view of their situation, before deciding a given strategy. Here are some elements that should be taken into account for clubs confronted to such interrogation.The first step should be to assess the priorities within the breeds, by answering some questions relative to the disease and capacities to handle it.
What is the impact of the disease on dog welfare?
- It is first important to assess the consequence of the disease on the animal'health. For instance icthyosis, which cause some mild skin problem in Golden Retriever, should not be considered with the same importance as cerebellar ataxia in American Staffordshire Terrier, which often leads to premature euthanasia on the affected dogs.
What is the incidence of the disease within the breed at a national or international scale?
- The importance that should be given to a given disorder is fully dependent on its incidence within the breed. It is one of the reasons why regular health surveys have to be implemented within dog breeds. It is also important to take into account the international context, in order to know if imported dogs may bring a risk, or on contrary constitute an opportunity relative to the fight against the disease.
What is the inheritance mode of the disease?
- The inheritance mode of a disease can vary from very simple (one single allele, recessive or dominant, with full penetrance and early expression) to extremely complex (several allele and genes involved, incomplete penetrance, late expression, and variable incidence across subpopulations). The way the breeding strategy should be implemented will of course depend on this inheritance mode.
- The existence of genetic test can greatly simplify the task for breeders and clubs, especially in relation to recessive and/or late expression disorders. It is however important to verify if the efficiency of the test has been assessed in scientific literature for the population of interest. Also for more complex diseases (with incomplete penetrance for instance) a genetic test can only indicate an increasing risk for the dog to develop the disease, which can be interesting anyway, especially if the disease has a heavy effect on health.
What are the demographics of the breed?
- Depending on the population size of the breed, the capacity to handle a given breeding programme relative to its impact on genetic variability is not the same at all. The selection pressure should be much less important if dealing with a small breed than if considering an international breed with thousands of litters produced every year.
Are there other diseases or problems to be considered within the breed?
- Many breeds have to deal with more than one inherited disorder, with consequences that are more or less important for welfare. Of course, breed clubs may want to select also traits that are not related to breed's health, such as behavior, working capacities or morphology. Breeding strategies may represent constraints for breeders; for instance imposing a multiplication of genetic tests for large number of diseases may be counterproductive and push breeders away from the breed. It is of paramount important to assess which level of priority should be given to which selected trait.
What is the timeframe considered for the breeding programme?
- Elimination of the disease is not always reachable, especially for complex diseases. Therefore clubs should decide what is a realistic objective (elimination or a given reduction of incidence) and how long should it take to reach it.
Once the situation and objectives are assessed, it is important to consider the measures that could be used to reach those objectives. Basically, these measures can be classified according to the fact they constitute constraints or incentives for breeders. They are also largely dependent of local Kennel club rules and national legislation; for instance, AKC cannot ban a dog from breeding. These contrasts across countries may appear therefore problematic when considering breeding programmes implemented at an international scale.
Removal of individuals (affected or carriers individuals for instance) from breeding will be by far the most simple (and probably the most efficient) measure to implement, especially if adequate genetic tests are available. The question on if the affected or carriers should be removed and at which timeframe, is dependent of the context (examples will be provided in the next post). In absence of a genetic test, it is possible for instance to remove parents of affected individuals, or eventually to implement genetic indexation, as it was made for hip dysplasia in Sweden or UK for instance, and to base the breeding choice on estimated breeding values. The problems with reproducer removal are related to the fact that, as stated earlier, local rules do not always allow its implementation. Also, as a constraint, it can be unpopular among breeders.
Others measures can be taken to encourage breeders to select against the disease, such as through awards, communication and education. As an example, in France dog are classified in a selection grid specific to each breed, including six levels according to the performance and health status of the dog and its offspring. The efficiency of such measures is in practice quite difficult to assess. It also depends a lot on the participation of breeders.
This post shows the different factors that have to be taken into account for the choice of a given strategy. There is, however, a need for more developed guidelines and case studies. In the next post, we will show some simple illustrations…
Credit picture: I. Horvath
- Is there a reliable genetic test available?
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