Handling an Increased Risk of Inherited Disorders in Pedigree Dogs: Past, Present and Future
Department of Clinical Sciences, Swedish University of Agricultural Sciences, Uppsala, Sweden
Although various types of dogs defined as breeds have been recognized for much longer, it is less than 150 years since national kennel clubs were established and stud books instituted to regulate breeding of pedigree dogs (PGD). An increased international trade in dogs of various breeds started and geographical barriers were replaced by regulatory barriers between different populations of dogs. Selection for size, shape and variations in temperament has created breeds of dogs for an overwhelming wide range of functions.1
During the past century the burden of "unhealth" in humans as well as companion animals in western societies has changed from infections and malnutrition to congenital, developmental and degenerative disorders. These entities are to great but variable extent influenced by inheritance.
Inherited disorders do occur in all populations of mammals including man as well as dogs, to greater extent in isolated and inbred populations, but are documented in mixed breed dogs as well as in so named pure bred dogs. Some of them accumulated to much greater extent in certain breeds but others of about the same prevalence.2
Even if some inherited disorders are much more common in pedigree dogs, the advantage is that by known pedigree we can follow the segregation and have possibilities for more sophisticated measures to select against them.
There are mainly two problems that are overrepresented in PGD: related to conformation3 and related to small closed populations4. To counteract these problems screening potential breeding stock for desired as well as undesired features has been practiced since long time.
One method that has been practiced since long in animal breeding is to screen for advantageous phenotypes at various gatherings as shows and trials. In the past also screening for disadvantageous phenotypes as inherited disorders have been instituted.
Ideally, such a screening procedure should be easy and inexpensive to perform and thereby widely used in the selection of breeding stock.
Phenotypic screenings for diseases are based on the possibility to evaluate and indicate "affected as well as non-affected" individuals and thereby depicture the population.
More structured screening programs for specific entities as hip dysplasia and inherited ocular disorders can be dated back to the mid-19th.
Most phenotypic screening programs are based on an ability to predict at an early age if a dog is going to develop an inherited disorder. Hip dysplasia is a typical example, where a standardized radiological procedure was proven to indicate if a dog would develop clinical signs by a deviation from normality regarding laxity and form of the hip. The origin of hip screening is a study from the Kennel of Swedish Armed Forces in which early screening for hip status predicted clinical signs of hip dysplasia.5
In the late 1960s, the British Veterinary Association (BVA), in conjunction with the Kennel Club (KC), started a scheme to assist dog breeders in the eradication of the inherited eye disease, progressive retinal atrophy (PRA). Later, the International Sheepdog Society also became involved because of their concerns about inherited eye diseases in border collies.
Unfortunately, screening programs for hip dysplasia as well as hereditary eye diseases despite similar function have evolved somewhat differently in UK, the rest of Europe and in US.
Both WSAVA and FCI have actively sought to harmonize national certificates and to implement international certificates for exchange of results from screening of hip, elbow and eye examinations.6
After screening programs for hips and hereditary eye defects instituted in the mid 19th century, a number of phenotypic screening programs have evolved on i.e., heart disease and patellar luxation.
Breed Disposition to Complex Diseases
By access to epidemiological data from kennel clubs and pet insurance companies, it is becoming quite evident that breed-specific accumulation of detrimental genes goes not only for single gene diseases. Likewise, and probably to an even greater extent, also multifactorial ones, previously unknown to exhibit significant genetic components, do segregate within breeds.
Molecular Genetic Testing
Since the revealing of the canine genome, molecular genetic testing for numerous diseases has been developed.7 Well-organized registries on results from gene tests as well as screening programmes, based on phenotype and openly accessible to the public, are important tools, which cynological organisations and the veterinary profession should share the responsibility for.
Exaggerated Anatomical Features
Selection for size, color and shape has unfortunately also encouraged recognition of exaggerated features, fixed as breed characteristics. Breed standards formulated in the past and judges at dog-shows did support this fashion, unaware of negative consequences for health and wellbeing. Creation of new breeds and reestablishment of old, almost extinct breeds, from just a few founders has happened repeatedly over the last century. That has resulted in too small and "regulatory isolated populations" in which detrimental genes may be accumulated and then propagated by inbreeding and too widely used sires. Exaggerated features and the accumulation and propagation of detrimental genes - have been brought to attention by vets and geneticists quite recently from an evolutionary perspective. Unfortunately, it takes a much longer time and greater efforts to repair it.
In 1967 at the world congress organized by WSAVA in Paris, a detailed presentation was given on breed standards that encouraged exaggerated anatomical features.8 Since then, work within FCI as well as the US and UK kennel club have resulted in changes in breed standards for several breeds.
Cooperation to Enhance Canine Genetic Health
It is quite evident that breeders and vets as well as kennel clubs and the veterinary profession do share a responsibility for canine genetic health. Further formal cooperation between the veterinary profession and cynological organizations on national and international basis have great advantages and organizations as WSAVA, FECAVA and FCI are suitable as international leader in that process.9
Besides efforts to control spread of detrimental genes currently known of, there is an urgent need for measures to prevent the history from being repeated. As long as dogs are bred in too small and genetically isolated populations, new problems will occur and easily propagate. Breeding "purebred" dogs in the future relies on measures to counteract wisely the isolation in too small populations of breeds and their varieties. Rare breeds that are, from an evolutionary perspective, varieties, would probably benefit from less rigorous regulatory barriers.
Proposed Further Activities
To prevent from their further propagation and new ones to be emerging there are a number of changes in breeding of PGD - summarized in the following - that could and should be instituted.
Do Not Breed PGD in Too Small Populations
That is achieved by
1. Counteracting division of existing breeds in varieties
2. Counteracting the establishment of new breeds with insufficient numbers and unrelatedness
Do Not Breed PGD with Exaggerated Anatomical Features
That is achieved by
1. Making sure that existing and new breed standards do not contain wording that can be interpreted in a way that it promotes exaggerated anatomical features.
2. Information and education of judges and breeders about the increased risk by exaggerations and how to reward and select breeding stock with less exaggerated features.
1. Stockard CR, Andersson OD, James WT. The genetic and endocrine basis for differences in form and behavior, as elucidated by studies of contrasted pureline dog breeds and their hybrids. The Wistar Institute of Anatomy and Biology. 1941, PA, Pennsylvania.
2. Bellumori TP, Famula TR, Bannasch DL, Belanger JM, Oberbauer AM. Prevalence of inherited disorders among mixed-breed and purebred dogs: 27,254 cases (1995–2010). J Am Vet Med Assoc. 2013;242(11):1549–55.
3. Asher L, Diesel G, Summers JF, McGreevy PD, Collins LM. Inherited defects in pedigree dogs. Part 1: disorders related to breed standards. Vet J. 2009;182(3):402–11.
4. Summers JF, Diesel G, Asher L, McGreevy PD, Collins LM. Inherited defects in pedigree dogs. Part 2: disorders that are not related to breed standards. Vet J. 2010;183(1):39–45.
5. Henricson B, Olsson S-E. Hereditary acetabular dysplasia in German shepherd dogs. J Am Vet Med Assoc. 1959;135(4):207–10.
6. Hedhammar Å. Cynology and small animal veterinary medicine: actions by FCI and WSAVA to promote canine genetic health. The FECAVA Symposium 2004. Eur J Companion Anim Pract. 2005;15:22–25.
7. Karlsson EK, Baranowska I, Wade CM, Salmon Hillbertz NH, Zody MC, Anderson N, Biagi TM, Patterson N, Pielberg GR, Kulbokas EJ 3rd, Comstock KE, Keller ET, Mesirov JP, von Euler H, Kämpe O, Hedhammar Å, Lander ES, Andersson G, Andersson L, Lindblad-Toh K. Efficient mapping of Mendelian traits in dogs through genome-wide association. Nat Genet. 2007;39:1321–1328.
8. Anonymous Report of the World Small Animal Veterinary Association Committee appointed to consider breed standards in relation to the health and welfare or dogs. J Small Anim Pract. 1969;10(3):135–41.
9. Hedhammar ÅA, Malm S, Bonnett B. International and collaborative strategies to enhance genetic health in purebred dogs. Vet J. 2011;189(2):189–96.