My Thoughts on Developing a Wider View of Hip Dysplasia in the Border Collie

C. Denise Wall, PhD

The predominant known genetic disease in Border Collies is hip dysplasia (HD). The two standard procedures used at this time for identification and control of HD are the front extended hip x-ray view as used by the Orthopedic Foundation for Animals (OFA), and passive laxity as measured by PennHIP.

The “gold standard” for determining HD is still the typical, hip-extended (OFA) view on x-ray. This single position is the most efficient to yield information for several determinations. The most important of these is identifying the presence of joint damage called osteoarthritis, also know as degenerative joint disease (DJD). This joint damage is manifest by cartilage and bone breakdown and irregular bony remodeling in response to stresses and inflammatory processes in the joint. DJD is, in effect, the identifiable result of factors that cause HD.

So what are these factors?

It’s widely accepted that more than one gene is responsible for the development of HD. Important inherited risk factors thought to contribute to the development of HD include: 1) looseness or "laxity" of the joint, which allows too much movement and mechanical trauma (banging around) to the joint during use, eventually causing breakdown of the cartilage and bone. 2) poor formation of the joint or "fit", which is how efficiently the ball part of the joint fits into the cup part of the joint. 3) poor pelvic muscle mass, which is the amount of muscle development and strength in the upper back leg/hip area that helps the joint to remain stable during exercise and stress. Of these factors thought to be important, the standard OFA positioning is fairly effective in identifying the fit, inconsistent in identifying laxity (which can be seen in some cases but is missed in many others due to unfavorable positioning to detect it), and the effects of pelvic muscle mass are not really evaluated as a direct measurement. Once again, though, this position is very efficient for identifying the overt physical manifestations of these risk factors, DJD. However, the occurrence of DJD may be late in the process of the disease. For example, a dog known to be at high risk for the development of hip dysplasia may, at one year of age, demonstrate a poor fit of the joint but no DJD at that point in time. The poor fit indicates the dog is predisposed to hip dysplasia and will probably develop DJD at some time in the future. As well, a dog with very lax hips may be predisposed to hip dysplasia but may not show DJD until much later in life. One major problem with the OFA hip view as a screening technique is that it doesn't always do a very good job of detecting laxity, should that be the only indicating factor for the development of DJD. Enter PennHIP.

PennHIP was developed under the premise that passive laxity was the most important factor involved in the development of HD. It was specifically designed by a biomechanical engineer as a precise measurement of the degree of laxity in the hip when a measured force is applied to the hip joint in a fully anesthetized dog. The laxity measurement is called the distraction index or DI. This type of laxity is termed "passive laxity", in that the joint is tested in a way that muscle strength is excluded as a factor affecting laxity within the hip joint. PennHIP's studies have shown that dogs with a DI of 0.30 and lower are at reduced risk of HD and dogs with a DI of 0.70 or higher are at an increased risk for developing HD. Initial studies using breeds such as German Shepherds and Labrador Retrievers showed the laxity measured by the PennHIP DI score was highly heritable in these breeds and that the hips of progeny could be improved over generations by selecting only the tightest hipped (lowest DI scores) dogs to breed from.

Sounds good, shouldn't this be the test to use?

First let's separate the difference between using a procedure such as OFA or PennHIP as a screening tool and using it as a breeding selection tool. A screening tool is one that yields information about an individual. Does this dog have HD -- 'yes' or 'no', for example. A breeding selection tool is one that aids in decisions for selecting breeding stock by having predictive value on what that individual is likely to pass on, for example, good hips. Some procedures are better at screening individuals for a disease but not as good as a selection tool, that is, predicting how that individual will pass on certain traits. OFA ratings are usually very reliable as a screening tool. A passing OFA rating on a dog at least two years old will determine within a 95% confidence limit the dog will not develop HD. Therefore, for a yes or no answer on that individual, OFA is statistically sound as a predictor for HD. As a breeding selection tool, OFA varies in its effectiveness according to the breed. OFA evaluated figures to determine trends in the percentages of dogs rated as dysplastic and OFA excellent in or before 1980 as compared with same breed dogs in 1994-1995. Using both decreased percentages of HD and increased percentages of OFA excellent ratings as an indicator, this study showed some breeds have improved the overall hip status in the study period using this method as much as 260% for increase in OFA excellents and as much as an 82% decrease in HD. Conversely, this same study showed some breeds making little to no progress in improving hip status using OFA as a selection tool. Border Collies are ranked in with the breeds considered to have shown moderate success. Given this information, its effectiveness as a breeding selection tool in our breed could be better.

When PennHIP is used as a screening tool, it can predict with very good accuracy that dogs with values of 0.30 or lower will be unlikely to develop HD. And that many dogs at the other end of the range (DI of 0.70 or higher) with very loose hips will develop HD. However, most dogs of all breeds will fall somewhere in the middle or the "gray zone" for these predictions. So one cannot accurately predict whether one’s dog will develop HD or not in most instances using this technique. PennHIP’s recommendations are to determine a breed average DI and select dogs below the 50th percentile for breeding. Over time, they claim, the hip status of the progeny will shift towards tighter and better hips. If this premise holds true for our breed, PennHIP is probably best used as a selection tool for choosing dogs to breed tighter hips in future generations than a yes or no tool for determining HD in an individual

Then what about Border Collies and PennHIP?

Interestingly, when using strong selection pressure only for their specialized performance, Greyhounds, Salukis and other sight hounds coincidentally develop nearly perfect, very tight hips over generations. Breeding selection based on efficient sled-dog performance is another type of work-only selection that results in good hips. Even when Labrador Retrievers (a breed with a fairly high predisposition for HD), are used as sled dogs, and selected and bred based entirely on their efficiency as workers, they will coincidently breed good, tight hips over time. Therefore, in these types of performance areas, selection for efficient function alone is strongly associated with incidental selection for good, tight hips.

So why, after all the years of strict selection for herding work, do Border Collies still have hip dysplasia? Why have they not naturally gravitated toward the perfect hip status over all these generations like the racing Greyhound? Perhaps, in our breed, one of the risk factors for HD, laxity, is something that has been inadvertently selected for in their performance. On the list of breeds that maintain or convert to good hip status when breeding selection is based solely on performance, are breeds that run or pull straight ahead, or mostly so. Border Collies, on the other hand, do a great deal of quick flanking and lateral movement, quick starts and stops, pivots on the hind legs, and traveling in a crouched position in their work. It seems likely that there needs to be some "give" somewhere for efficient performance, and that give has been selected for. In the same way slight cow hocks allow for efficient performance and have been selected for through the many generations of working stock, so perhaps has some laxity in the hips. If we decided to PennHIP every breeding Border Collie, and not accept anything over a 0.30 DI for breeding, aside from the other hazards of such a short sighted plan, we might unwittingly be left with a breed of dogs lacking the flexibility needed for their work, or ones with stifle and hock injuries from stress on those joints, since the now perfectly tight hips are no longer helping absorb the stress.

The problem with using a one factor, "one-size-fits-all" procedure with a disease caused by a combination of factors such as HD, is that all breeds do not have the same percentage contribution of these factors. All breeds of dogs are built somewhat differently, are of different sizes, activity levels, and selected using different standards as the primary criteria. Some of these standards involve performance criteria as a major selection criteria and some don't. In dogs bred to look a certain way, the selection against such factors as favorable pelvic muscle mass may change the percentage contribution of that particular HD factor in that breed. It's not surprising then, that adherence to a certain screening and selection technique would be more or less effective in decreasing the incidence of the disease in one breed versus another.

In reality, for our breed, the degree of tightness the researchers at PennHIP have determined in their test breeds will almost certainly ensure no HD, is a value very few Border Collies have. Many, if not most, researchers in HD admit that some breeds are more tolerant of passive laxity than others. In other words, these tolerant breeds tend to have a lower incidence of HD than might be expected from their passive laxity scores. Border Collies appear to be a fairly laxity-tolerant breed. Table 1 compares the OFA figures for HD and PennHIP data in four herding breeds. Although OFA HD statistics are undoubtedly skewed low by pre-submission screening, it's likely these skewed values are constant across the board for these breeds, making the comparisons between them here valid on a relative scale. The figures in the table indicate the Border Collie is more tolerant as a breed to passive laxity than the German Shepherd. Comparing the Australian Shepherd with the Border Collie, the mean PennHIP DI is very similar, 0.49 vs. 0.50, respectively, yet the reported incidence of HD in the Australian Shepherd is about half that of the Border Collie. Australian Shepherds, as a breed, appear even more tolerant than Border Collies to similar passive laxity scores. It's also of interest to note the highest mean DI (0.56 of the Shetland Sheepdog) has the lowest incidence of HD, and the lowest mean DI (0.43 of the German shepherd) has the highest incidence of HD of these four breeds. These data suggest that average breed passive laxity scores do not directly correlate with the breed incidence of HD as determined by OFA. It appears the passive laxity score that would almost certainly doom a German Shepherd to HD may not doom a Border Collie with the same DI score, depending on the strength of other factors that may be present in that individual. From these figures, one could also speculate that herding breeds similar to the Border Collie, such as the Shetland Sheepdog and the Australian Shepherd, have further mitigated the effects of lax hips through selection for HD protective factors present in these breeds.

Figures for OFA from http://www.offa.org/hdstats.html. Figures for PennHIP from PennHIP Quarterly Newlsletter April 2001 (Permission to publish granted).

So what’s going on with our breed?

The good news is that although it appears Border Collies may, as a breed, have somewhat lax hips, predisposing them to HD, many also have the protective factors such as increased pelvic muscle mass and good hip formation and fit. Increased pelvic muscle mass and good hip formation are factors not taken into account during PennHIP laxity measurements and not always measured accurately in the standard OFA view. So perhaps the goal should be to find a breed specific test to determine functional soundness in our dogs rather than to put total stock in an all breed, one-size-fits-all approach or in procedures tested in breeds whose primary functions are unlike ours. The identification and selection for those HD protective factors prevalent in the Border Collie breed may well be the most effective strategy to preserve the working characteristics in our breed while lowering the incidence of HD.

Although there’s currently no effective technique for measuring pelvic muscle mass and strength as a screening procedure, there is a promising new technique examined in several well-thought-out experimental studies that appears to be a good measurement for functional soundness, or that which takes into account pelvic muscle mass and strength to support the joint, fit, and joint laxity. This procedure measures the degree of laxity in a sedated dog by using hip x-rays taken while the dog is in a kneeling position. This angle is more favorable for identifying strengths and weaknesses in the joint in a more natural position. This type of measurement, called dorsolateral subluxation (DLS), identifies a type of laxity that is inherited differently from the passive laxity measured by PennHIP. There's a pilot study being planned to evaluate this technique in Border Collies. The plan is to study dogs initially between 8-12 months of age and follow up at 24 months of age. The OFA view will be incorporated in this study to compare the results of the DLS technique to the known standard.

Perhaps in the near future, this new procedure will be the comprehensive tool we need to both predict whether our dogs as individuals will develop HD and as an effective selection tool for improving hips in future generations while allowing breeders to preserve the working traits so important in our breed.

The intention of this article is not to endorse or reject any particular test or procedure. The purpose is to introduce some new thoughts on HD and suggest differences in effectiveness of the current testing procedures based on the breed specific differences. This information is presented with the hope that we will find the most effective, efficient tools to improve the genetic health of our breed and learn to use these tools in a way that benefits the breed overall and preserves it for what it's meant to be, the working Border Collie.


References:

Corley, E.A. Role of the Orthopedic Foundation for Animals in the control of canine hip dysplasia. Vet Clin North Am Small Anim Pract. 1992 May;22(3):579-93.

Corley, E.A., Hogan P.M. Trends in hip dysplasia control: analysis of radiographs submitted to the Orthopedic Foundation for Animals, 1974 to 1984. J Am Vet Med Assoc. 1985 Oct 15;187(8):805-809.

Farese, J.P., Todhunter, R.J., Lust, G., Williams, A.J., Dykes, N.L. Dorsolateral Subluxation of Hip Joints in Dogs Measured in a Weight-Bearing Position With Radiography and Computed Tomography. Vet Surg. 1998;27:393-405.

Smith G.K., LaFond E., Heyman S.J., Cofone M.A. and Gregor T.P. Biomechanical characterization of passive laxity of the canine coxofemoral joint. Am J Vet Res 1997;58:1078-1082.

Smith G.K. Advances in diagnosing canine hip dysplasia. J Am Vet Med Assoc 1997;211:1451-1457.

Todhunter, R.J., Acland, G.M., Olivier, M., Williams, A.J., Vernier-Singer, M., Burton-Wurster, N., Farese, J.P., Grohn, Y.T., Gilbert, R.O., Dykes, N.L., Lust, G. An Outcrossed Canine Pedigree for Linkage Analysis of Hip Dysplasia. J Heredity 1999;90(l), 83-91.

Copyright 2001, 2002. All rights reserved by C. Denise Wall. No one is permitted to copy the photos, graphics or text on these pages without written permission.