Canine hip dysplasia (CHD) is a developmental trait primarily affecting medium and large breed dogs. CHD is characterized by faulty conformation and laxity of the hip joint that usually affects both hips. It can be detected radiographically as subluxation of the affected hip. The eccentric load on the articular surface of the femoral head (ball) and acetabulum (socket or cup) leads to erosion of the cartilage, inflammation in the joint, and debilitating pain. Clinically, the osteoarthritis or degenerative joint disease that results from hip dysplasia is characterized by hind limb lameness, reduced exercise tolerance, reluctance to jump, and poor hind limb muscle mass.
CHD in dogs is an inherited, polygenic trait in which mutations in several genes called quantitative trait loci (QTLs) contribute to its clinical expression. Many dogs with normal hips on radiographs carry at least a modicum of the trait-causing mutations but not all that are necessary to cause physical expression of the trait. CHD is a quantitative or complex trait that is expressed as a continuum from imperceptible to severe forms. This continuum of trait expression is due to environmental influences (such as plane of nutrition and exercise, as well as other unknown factors) which interact with the genetic constitution to affect the degree to which the trait is manifested. CHD has a heritability between 0.20-0.7. This means that between 20 and 70% of the physical appearance of the hips of each dog in a pedigree can be attributed to its genetic relationships within the pedigree.
It will take a concerted effort to rid breeds of the genetic mutations that cause CHD or conversely, to introduce protective alleles at the loci that cause good hips. Selective breeding based on current radiographic methods can reduce the frequency of CHD in a population. Breeding two dysplastic dogs can yield a 75% incidence of hip dysplasia in offspring, while mating two unaffected dogs can yield a 25% incidence of the disease. Selective breeding using normal dogs from normal parents and grandparents, as well as progeny testing, should decrease the incidence of CHD. The message here is that until we have a genetic test for CHD so we can detect genetically susceptible dogs, the best indication of a dog's genetic makeup is where it came from (its' parents and grandparents), what it produces (its' offspring), and the phenotype of its' siblings or half sibs. Dogs with normal hip radiographs that carry some of the mutations that cause CHD but perhaps not the major ones, when bred to a mate that also carries some of the mutations for CHD, may produce affected offspring. To test whether a breeder carries some of the mutations (even if the dog has OFA-good hips), it should be bred to sires or dams with good hips and the proportion of affected offspring recorded (progeny testing). As many as 15-20 offspring should be produced to be reasonably sure that the parents do not carry important mutations. This is an unreasonable burden for dog breeders to bear. Breeders should attempt to breed dogs with the best hips in their colony as well as to dogs with the other optimal breed characteristics and temperament.
We have been searching for the genes that contribute to hip dysplasia. When an owner brings their dog to the Cornell University Hospital for Animals for hip radiographs, they are asked to donate a small sample of blood from their dog for DNA isolation. This DNA and the hip radiograph measurements are then used later to discover and confirm the mutations that contribute to hip dysplasia. Once we know which genes and biochemical pathways lead to good and poor hip conformation, we can develop novel treatments which can be applied at an early age.
If you'ld like to support this research, and help us find new diagnostics and treatments for this disease, please click here and choose CVM DNA Bank in the area for designations.