Two recent articles by Wood and Lakhani (Animal Health Trust) have been published in successive issues of the Veterinary Record (January 11th & 18th, 2003) that address this particular question. These papers report their analyses of a large data set of hip scores from Labradors and Gordon setters screened over the years using the BVA/KC Hip Scoring Scheme.
Hip dysplasia is a malformation of the hip joint, first recognised and described in the scientific literature in 1937. The BVA/KC Hip Scoring Scheme was established in the late 60s, modified in 1978 and is still operating successfully today with around 9,000 dogs being screened annually. It is a voluntary scheme that relies on the submission of X-rays from dogs that are more than one year of age. These X-rays are submitted to the BVA where they are assessed by a panel of veterinarians who assign numerical values to certain anatomical features from the X-ray. The accumulated score from each hip can range from 0 to 53 and the total score for both hips (the hip score) can therefore range from 0 to 106.
The results of dogs hip scores are recorded on the dogs data on the KC Registration Database, printed on subsequent Registration Certificates of the dog and any of its progeny and published in the Breed Record Supplement. Wood and Lakhani analysed the hips scores of around 29,000 Labrador retrievers born between 1965 and 1999 and around 1200 Gordon setters born before 1998.
These two recent papers are part of an on-going research programme by the epidemiology group at the Animal Health Trust to assess the relationship between a dogs hip score and the hip score of its dam, sire and other relatives, to estimate the heritability of the hip score, work that is supported by the Kennel Club Charitable Trust. A knowledge of such quantitative relationships is important to establish selective breeding programmes to reduce hip dysplasia in dogs. Malcolm Willis in the UK and Swenson in Sweden have also undertaken similar studies. However, none of these earlier studies took account of the age of the dog at the time the X-ray was taken.
These two papers correct this omission and show that there is a significant relationship between a dogs hip score and the age at which its hip X-ray was taken. Interestingly, their analyses also show that the hip score depends upon the month in which a dog is born. For the Labrador retriever data, the breed mean hip score increases from around 15 in dogs screened at two years of age to nearly 18 when 6 year-old dogs are screened. The results from studying the effect of the month in which a sdog was born show that for both the Labrador retriever and the Gordon setter, dogs born between the months of June and October (period 1) have a significantly lower mean hip score compared to dogs born between November and May (period 2). The Labrador data show that the hip score of dogs born in period 2 would be expected to be 1.5 units higher than that of dogs born in period 1; in the Gordon setter dogs born in period 2 would be expected to be between 4 and 5 units higher than dogs born in period 1. The difference in these two figures probably reflects the fact that the Gordon setter breed mean hip score is higher than that in the Labrador.
are the explanations for these observations? We have known
for some time that hip dysplasia may be affected by a combination
of genetic and environmental factors. Some of these age-related
effects could well be reflecting the complex environmental
effects on hip dysplasia. The fact that there is a significant
relationship between the breed mean hip score and the age
of screening is perhaps not too surprising because much of
the evaluation of X-rays undertaken by the BVA is derived
from scoring secondary arthritic changes which might be expected
to worsen with age. The dependence on the month of birth is
Other workers, for example Hanssen in Norway, have reported similar effects of the month of birth on hip scores in several breeds. The difference may possibly reflect the fact that puppies born during the winter months do not have the same opportunity to get the regular daily exercise needed to develop muscular strength and elasticity and may have a greater chance of damaging their hip joints when running on hard or frozen ground.
The results in these two papers are clearly interesting and academics interested in investigating canine hip dysplasia will now be able to build in these new age variables into their models when attempting to estimate values of heritability and the dependence of progeny hip scores on those of the sire and dam. However, will they be amazingly significant to the dog breeder who is attempting to responsibly use the hip scoring scheme to reduce the problem of hip dysplasia within a breed. Well these new observations do not really alter the bottom line for the breeder. The advice remains the same. To improve selective breeding the most important factor remains the genetic factor. The results of studies over the years have demonstrated the benefits of breeding from dogs with low hip scores for the reduction of hip scores and hip dysplasia. Breeders are strongly encouraged to have all potential breeding stock hip scored in those breeds where hip dysplasia is known to be a problem, and, wherever possible, breed low scoring dogs to low scoring bitches. That way the breed mean average hip score will fall over a period of time. As important as hip scoring is in these breeds, breeders should never forget that they are breeding dogs of a particular type and temperament and their breeding choices should always reflect this.