by Gina Bisco
Originally published in the ESC newsletter in 2004
In 2004 the Health and Genetics Committee formulated a set of questions on several basic aspects of PennHIP, derived from breeders’ questions to the committee as well as from the information on the PennHIP website and original published research papers. As the PennHIP representative for the ES Club, I made inquiries at University of Pennsylvania to find out if we could get our questions answered directly by PennHIP researchers. Dr. Erin Paster, DVM, kindly agreed to answer our questions in some detail, as well as to allow us to publish his answers in the newsletter. Dr. Paster was, at that time, a PennHIP postdoctoral fellow at the University of Pennsylvania Department of Clinical Sciences. Following is the complete text of Dr. Paster’s response, which I received via email dated June 22, 2004. It has not been edited except to add back in our subject headings for the questions.
To see the full significance of these questions and the answers, you may need a refresher on (or introduction to) the PennHIP method. If you have web access, the best introduction will be had by visiting the PennHIP website.
For those breeders and owners who are familiar with the PennHIP method and have already been struggling with making real life decisions, Dr. Paster’s very direct answers should help clarify what PennHIP officially recommends.
New questions may arise from these answers, but that is not a bad thing: that’s progress!
Answers to ESC Questions on PennHIP
by Dr. Erin Paster, DVM
(1) Regarding follow-up evaluations for dogs PennHIP evaluated prior to age 13 months.
Question (a) Does PennHIP recommend that potential breeding dogs that have been PennHIP evaluated prior to 12 months of age be followed up with another evaluation at a later age (12 months or later) in order to reduce the underestimation of the DI?
The best answer to this question can be found in a document I recently composed for a code of ethics:
PennHIP protocol in the screening and selection of breeding dogs:
The PennHIP evaluation is a useful clinical screening tool for hip dysplasia in the selection of breeding dogs, and can be performed as early as 16 weeks of age. Dogs with a DI of < 0.3 are essentially considered unsusceptible to hip dysplasia, whereas dogs having DI measurements of >0.3 are considered hip dysplasia susceptible. The larger the DI (ie. the more the laxity in the hip), the more the risk for the development of osteoarthritis. To make genetic change, it is recommended to breed dogs in the 60-100th percentiles relative to their generation and breed, (i.e., the top 40%). This strategy will systematically select against the laxity genes which result in hip dysplasia development. By following this ‘moderate breeding strategy’ the threshold for breeding in successive generations will get tighter and tighter. Of course if more rapid genetic improvement is desired, breeders may opt to mate only the very tightest members within a generation and breed (say the tightest 10%). That these strategies will actually work in the real world has foundation in the works of Falconer who authored a comprehensive book of quantitative genetics (Falconer, D.S. Introduction to Quantitative Genetics, 3rd ed.).
Hip dysplasia is a developmental disease. Dogs are born with normal hip congruity, but if genetically predisposed, with growth develop varying degrees of hip laxity associated with hip dysplasia. The timing for the development of hip laxity appears to vary by breed and like your serum cholesterol, there is some individual variation over time. Research has shown that the degree of laxity variation diminishes substantially after 1 year of age. In other words, there is greater possible change in measured hip laxity within a given dog in the interval between 16 weeks and 2 years of age than between 1 year and 2 years of age. Nevertheless, DI at 16 weeks is still a good predictor of later joint laxity; just not as good as at 1 year of age. One needs to remember that in comparison to the hip extended method (and the OFA interpretation there from) the PennHIP scores have much less variation over these same time intervals. While not perfect, PennHIP is the most accurate and precise measure of hip integrity.
Because some degree of biological variation in score is inevitable, PennHIP recommends evaluation at 16 weeks for an initial assessment of hip laxity, and that all dogs be evaluated at a later date preferably at 1 year of age or older. Particularly for dogs in the tighter half of the breed there is added emphasis on re-evaluation because such dogs have the potential to be used for breeding and it is important to confirm the previous prediction. If only one PennHIP evaluation can be performed, dogs should be evaluated at 1 year of age or older, although it is recommended to monitor the status of hip dysplasia with PennHIP throughout life. Follow up radiographs are essential to determine the relationship of hip score to the later development of osteoarthritis in each breed (see graph below, from: Evaluation of risk factors for degenerative joint disease associated with hip dysplasia in German Shepherd Dogs, Golden Retrievers, Labrador Retrievers and Rottweilers. JAVMA 219(12): 2001). If you would like to have similar graphic information for your specific breed, we ask that as a breeder you provide PennHIP with the necessary hip information by evaluating dogs well into adulthood. The graphs below represent thousands of dogs from each breed.
QUESTION (b) Does PennHIP recommend that a longitudinal study be done in the English Shepherd breed to establish the age at which the English Shepherd’s DI score will not change?
Thus far, we only have a longitudinal study addressing correlations of DI over the 16 week to 2 year time period in German Shepherd dogs and Borzois. Ideally, it would be nice to have similar studies in each breed, but time and resources make this extremely difficult, if not impossible. More recent research from PennHIP and other laboratories have confirmed the data and conclusions from these early studies, making extrapolation from the German Shepherd breed to other breeds appear reasonable. Additionally, we know that hip dysplasia is a developmental disease, and thus it is expected that the best correlation of degree of hip laxity (DI) with later scores will occur after skeletal maturity (1 year of age in large breeds). As mentioned above, longitudinal studies are recommended, to establish the relationship of DI with the long-term outcome and development of DJD in each breed. This information is most important and useful in determination of true disease susceptibility rates and will allow us to evaluate improvement in hip status with the implementation of selective breeding.
(2) Regarding follow-up evaluations for dogs with DI at or above 0.3.
QUESTION: For breeding prospects with DI at 0.3 or above but in the tighter half of the breed (done at, say, 12 months of age), does PennHIP recommend follow-up evaluations to check for development of DJD at some later age (if so what age) prior to breeding?
Again, the above answer is useful in addressing this question. We recommend regular follow-up evaluations (for example, every 2 years) to provide the data necessary to construct probability curves for DJD in relation to DI for each breed (see illustration above). The information gained would allow breeders to predict with greater certainty, the probability of radiographic DJD in their breed given a specific DI measurement, and thus give additional knowledge to those when making breeding decisions.
(3) Regarding PennHIP DJD diagnosis, and the CI.
QUESTION (a) Is the PennHIP diagnosis of DJD on the standard hip extended radiographic view based on the same criteria used by board certified vet radiologists?
This is a difficult question to answer, as not all board certified radiologists read for DJD in the same manner. For example, some radiologists read the caudolateral, curvilinear osteopyte (CCO) or Morgan’s line, as an early sign of DJD in the hip, while others choose to ignore it. Radiologists trained at the University of Pennsylvania follow the former criteria, whereas those reading for the OFA follow the latter. The CCO has been statistically associated with contemporaneous and longitudinal development of DJD. Specifically, a dog showing a CCO is 7.9 times more likely to have hip DJD on the same radiograph, and 3.7 times more likely to have obvious radiographic signs of DJD later in life, compared to dogs without a CCO (Evaluation of radiographic caudolateral curvilinear osteophyte on the femoral neck and its relationship to degenerative joint disease and distraction index in dogs. JAVMA 2002; 220: 472-476 and Caudolateral curvilinear osteophyte (CCO) as a risk factor for degenerative joint disease: A life-long study in Labrador retrievers. Vet Ortho Soc, 29th Annual Conference 2002 (abstract)). At the PennHIP analysis center, we attempt to interpret the hip extended radiograph for DJD similar to OFA radiologists. However, clients receiving PennHIP evaluations at the University of Pennsylvania are educated about the CCO. Presently, we feel that it may be best to exclude these dogs from the breeding program, given the information learned from the above studies. The ultimate decision, however, is left to the breeder. A paper from our laboratory was recently accepted for publication by JAVMA, and addresses the CCO and its implication on hip dysplasia prevalence. This paper should be published some time this summer.
QUESTION (b) How does PennHIP use the CI?
The compression view and the CI measurement there from, are only used for comparison with the measurements obtained from the distraction view in determination of hip laxity.
QUESTION (c) Is the CI used in determining presence of DJD?
QUESTION (d) Why is the CI not included on the PennHIP evaluation report?
The real value to breeders and pet owners in the PennHIP method comes from the DI and its comparison to the rest of the breed (generation of percentiles). Therefore we exclude the CI measurement from all PennHIP reports, as it would not provide additional or useful information.
(4) Regarding PennHIP CHD diagnosis.
QUESTION (a) Is there a DI score above which PennHIP considers a dog (of any breed) to have excess passive laxity and therefore be genotypically positive (carriers) for CHD, and if so, what is that DI score?
Dogs with a DI less than 0.3 are considered hip dysplasia unsusceptible (ie. essentially have no risk of DJD development), whereas dogs with a DI > 0.3 are at risk for hip dysplasia/ development of DJD. The risk increases as DI increases.
QUESTION (b) Is a dog classified as having confirmed CHD only if DI is above 0.30 and DJD is present?
The designation ‘confirmed hip dysplasia’ is given to dogs with radiographic evidence of DJD on the hip extended view, irrespective of the degree of joint laxity. It is the hallmark sign of CHD.
QUESTION (c) Can a dog be classified as having CHD based on DI alone, without showing signs of DJD at the time of evaluation? If so, what DI score is sufficient to classify the dog as having confirmed CHD in the absence of DJD?
No. We only determine a dog to have confirmed hip dysplasia if DJD is present on the hip-extended view.
QUESTION (d) Can a dog be classified as having CHD based on evidence of DJD even if the DI is less than 0.3?
Yes, however, this is extremely unusual (<2.5% depending on breed) . From the above graph, it is shown that the probability of the German Shepherd breed to develop DJD with a DI of 0.3 is 0.1, or 10%, and that the German Shepherd breed is more susceptible to developing DJD than other breeds with the same given DI So for other breeds the number of dogs with tight hips and DJD would be expected to be even lower.