Herman A.W. Hazelwinkel, DVM, PhD, DECVS, DECVCN
The International Elbow Working Group has learned from the world wide experiences in screening for hip dysplasia (HD). Apart of the lack of agreement on radiographic protocols there is also a lack of agreement on interpretation and grading for HD, despite the effort of the Federation Cynologique International (FCI) to make the final scores comparable all over Europe. This is due to the fact that the acceptance of the quality (positioning and exposure) and amount (position with extended femores, sometimes in addition frog leg position) of the films, but also the grading of the amount of laxity (either natural or forced outside the acetabuli), acetabular depth and osteophytes are less well harmonized. In some countries one person judges (per breed or per kennel club), whereas in other countries three screeners grade the radiograph--at the same time or in sequence to come to a final scoring. The different national breeders clubs are organising the HD-screening, influenced by the large breeders of whom sometimes the interests in screening results for HD are not parallel to the interest of the breed on the long run; as a consequence more often it seems that those breeder clubs chose for simple, cheep and quickly (one radiograph of any quality screened by one), with all consequences connected to that. Due to the lack of harmonisation of the method and the historical deviation of national grading schemes, a certificate for HD is for breeders and veterinarians very unclear and sometimes misleading.
SCREENING FOR ED
The screening for ED differs also in different European countries as far as accepting quality and quantity of radiographs, and the judging (individual or panel) is concerned, whereas the method of screening is harmonized by the International Elbow Working Group (IEWG, see http://www.iewg-vet.org/). The early recognition by the founders of the IEWG to harmonize the grading system gives a much better start than the individual or national HD-scoring systems. The IEWG is very grateful to the WSAVA to be a affiliate association of this world wide veterinarian organisation. The IEWG is now in a process to get a certificate functioning in the veterinary world where the grading for OA is given per elbow joint as well as the possible different forms of ED as visible on the radiograph(s) (i.e., fragmented coronoid process [FCP], osteochondrosis/itis dissecans [OCD], joint incongruity [INC], and ununited anconeal process [UAP]). Since in some countries ED screening is performed on one view (ML-flexed), whereas in other countries more views are obliged in order to have a lower false negative scoring rate and to visualize entities which are not visible on the ML view alone (e.g., OCD of the medial humeral condyle), the certificate gives insight to the buyer or kennel club which views were used for screening, in order to take this into account when comparing different dogs screened by different systems.
Although breed clubs and national kennel clubs take the initiative to screen for ED and HD as well as for other skeletal and nonskeletal hereditary diseases, this does not imply that all dog owners cooperate without hesitation. They criticize the limited success rate of screening for decades, without realizing that it is not the screening but rather the consequences drawn from it, which can make the difference. In addition, whole series of unacceptable tricks are in use to earn a better score for the dog, without taking the influence on the breed into account. Due to the old fashioned (i.e., with plane radiographs) and limited (only one view) techniques many positives will be missed, in addition to the dogs with the negative phenotype but positive genotype raised under optimal environmental circumstances. Only for some breeds there is insight in the h2 for elbow dysplasia, and even less for the different entities. Guthrie and Pidduck (1990) published a h2 of 0.77 and 0.45, respectively, although Studdert et al (1991) published a lower h2 (0.27) for the latter breed. A h2 above 0.2 for ED suggests that genetic selection should be effective to improve the medium value within the breed. Long term follow-up studies, both in dogs with different grades of HD as in dogs with or without ED have shown the benefit of breeding programs and the detrimental effects when dogs with ED or HD were used.
The only way to overcome the disadvantage of screening the phenotype with a technique of low precision to rule out the genotype of skeletal diseases like ED is to develop DNA-screening tests. These tests will allow for excluding homozygote positives from breeding, followed by excluding the heterozygotes when the breeding stock allows for.
To investigate the involved gene(s) coding for FCP, DNA samples of Labradors have been collected from complete litters which were also radiographed for ED. The diagnosis FCP has been confirmed in these dogs by surgery. From these dogs we selected 13 families with at least two affected siblings. The new method of sib-pair analysis was used to investigate the linkage of candidate genes to the phenotype of FCP, making use of the Mendelian rule stating that two siblings share on average 50% of their alleles, however in a gene locus responsible for a disease, affected siblings will share more than 50%. This principle was used to test the candidate genes for FCP. Since histological scans of FCP show defects in the collagen development of the joint, genes coding for collagen constituents were selected as appropriate candidate genes. Affected littermates shared approximately 50% of alleles of markers close to the collagen genes COL1A1, COL1A2, COL2A1 as well as VDR, ruling out a role for these genes in FCP (Salg et al, 2004). Additional candidate genes are investigated, making use of the knowledge about the dog genome, which became available recently. The DNA-screening method needs a lot of investment in time and money (we acknowledge the cooperation of the Royal Netherlands Blind-guide dog Association and Hill's Pet Food for their financial support in this study), but may finally lead to a precise testing, irrespective of the age of the animal, the quality of the radiographs or the tricks to cover ED for screening.
Till this and other tests are developed, the veterinary profession has to uniform the screening methods to solve the paradox that the better the screening, the more likely to be positive, and thus the greater the chance to be excluded from breeding and the harder to sell the dog or its offspring. Certification should at least make it transparent how the animal has (not) been screened. Veterinarians associated with the WSAVA are in the strong position to implement the certificate as has been designed for dog owners when dogs are sold within or outside the country, to offer insight to the potential buyer if the animal has been tested and if so, according which protocol.
1. Guthrie S, Pidduck HG. Hereditability of elbow osteochondrosis with a closed population of dogs. JSAP 32, 460-464, 1991
2. Studdert VP, Lavelle RB, Beilhartz RG, Mason TA. Clinical features and heritability of osteochondrosis of the elbow in Labrador retrievers JSAP 32, 557-563, 1991
3. Salg KG, Imholz S, Everts RE, Leegwater PAJ, Hazewinkel HAW. Genetic research in Fragmented Coronoid Process in Labrador retrievers, Proceeding Voorjaarsdagen 2004, Amsterdam, The Netherlands
4. Elbow screening protocol: http://www.iewg-vet.org/
5. The members of the extended board of the IEWG are acknowledged for their effort over the last 5 years to develop the certificate as presented here to the WSAVA-members.