PERSPECTIVES ON HIP DISPLASIA

by Charlie MacInnes, Finnshavn, Canada

The conventional wisdom says that hip dysplasia is not a problem in the Cardigan Welsh corgi as a breed.  That does not mean that the breed is completely free of dysplasia.  Crippling dysplasia is, however, rare enough that some breeders do not habitually X-ray their breeding stock.  It is uncommon to see a Cardigan with a crippled or tender rear, or suffering from the early onset of arthritis of the hips.

I believe there are at least 5 groups of factors contributing to hip dysplasia, and these are not mutually exclusive - as will become evident.  That generates the tough analysis problem that the different factors may contribute different amounts, and may interact, so that it is very hard, if not impossible, to sort out the effects of each factor separately.  Relative contributions may vary from one breed to another, and among individuals within a breed.  The result is that it is difficult to see how best to reduce the probability that future puppies will grow up dysplastic.

Hip dysplasia means that the hip joint is unstable or less than optimally functional.   One may argue that the hip joints are the two most important joints of the dog's skeleton.  Virtually all the power which drives the dog forward is generated by the muscles of the hind limbs, and that energy is transmitted to the rest of the dog through the hip joints - hence their extreme importance.  There is more to the joint than just the shape of the bone.  The joint is also held together by ligaments, and to some extent musculature can also contribute to further soundness of the joint.  One vet I know who also breeds and shows (Gordon Setters) told me that she believes that loose ligaments contribute as much to dysplasia as the morphology of the hip sockets.  Since OFA looks mostly at bone morphology, but PennHip also measures the tightness  or laxity of the ligaments, score one plus for PennHip.  PennHip also gets another plus in my book from a policy.  When you sign the form to have a dog subject to the PennHip procedure, you agree that, no matter what the X-rays look like, they will be submitted to PennHip, and will get into the database.  I have several times stood with my partner and her vet and looked at a fresh X-ray, and we decided - no, there is no point submitting this to OFA, it won't pass.   While the PennHip system is better in that regard, it is not perfect.  Only when breeders and owners submit results from every puppy in every litter will we get true statistics.  The definitive studies on dysplasia, mostly in German shepherds or Labradors resulted from X-raying of all members of a pedigree group.  A third plus for PennHip is that it can be done at much younger age - and still produce valid results.  With both OFA and PennHip, it is a plus if you can find a vet who has experience with dwarf-legged breeds, as the heavy muscling combined with short legs create unusual problems.

So what are the 5 factors?
(1) Nutrition
(2) Exercise
(3) Environment
(4) Genetics
(5) Basic design of dogs

Let's examine these one at a time.

(1) Nutrition of the puppy can cause hip problems.  The diet fed to wild wolf pups (the ancestors of all modern dogs) is of very good composition, but it is probably limited in quantity.  When dry dog foods first came on the market, they were manufactured by companies such as Purina and Gaines, which had extensive experience in feeds for commercial livestock.  When you grow a chick or a piglet or a beef calf, the priority is on getting them to grow as fast as possible.  Since they will be marketed quite young, the consequences of this high nutrition are not potentially as serious as in puppies, where we want the dog to function well for a long life.  (Our dogs live much longer these days than modern wolves and coyotes last in the wild!).  Twenty years ago breeders were told to keep the protein content as high as possible, and make sure the diet contained lots of calcium and phosphorus.  This is now thought to have resulted in puppies growing too big too fast.  That might put extra strain on the hips.   Remember that growth is a system, and many different parts grow at different rates.   Thus, a puppy growing too fast might be too heavy for the ligaments, resulting in the latter being stretched or otherwise damaged.  I have never seen a scientific study of that in dogs, but if you raise arctic-nesting geese or swans on too rich a diet, the wing bones grow faster than the muscles to support them, the wing tips droop, and twist outwards, resulting in a condition called "paddle wing" - and a bird that will never be able to fly.

I was heartened to learn - at the seminar arranged by the committee who staged the CWCCA supported entry in Franklin, Tenn (Nashvillle) in March 2004 - that the major dog food manufacturers have responded to this nutrition problem, and that today's large breed puppy foods do not have the very high protein etc. levels.  They are in fact very similar in major nutrients to adult foods, but differ a bit in calcium and vitamins.  It is also worthy of note that companies produce at least two puppy rations, recognizing that large breeds have different requirements than small breeds.  Question - should Cardigan puppies be considered large breed or small breed?  I vote for large breed.

(2)  Exercise.  Too much exercise can damage the hips joints.  (My ex partner went for seven years without hearing of dysplasia in any of the pups she produced.   Then she had three in one litter.  All were fed high powered puppy food, and two of the three were being trained for field work, so were doing a lot of fast retrieves at 3-8 months old.  One of these was retrieving on an asphalt playground for over an hour a day! Enough said, for the moment.)  It is also true that too little exercise, if it results in poor muscle tone, can result in strain on the ligaments.

(3)  Environment.  Labrador people of 20 years ago believed that raising puppies on a linoleum floor made them more likely to end up dysplastic.  This theory is that wet (pee) linoleum is slippery, and if the pup's legs slip out sideways, that will strain the ligaments.  Linoleum is not the only possible culprit - lots of flooring is slippery when wet, and most of those that aren't get pretty smelly with all that pee.   The wild wolves, of course, are on surfaces that mostly don't get slippery.

One Cardigan I heard of recently (sire OFA excellent, dam OFA good) had to retire from competitive flyball and agility at six when his hips degenerated and became painful.   But then, in one two-day tournament he made 40 runs, 4 jumps per run.  Is that too much?  We really do not understand how much hard exercise Cardigans got when they worked on Welsh hills farms in 1800!!  What was our dog designed to do?

(4)  Genetics.  There are three major reasons that it has been hard to improve the genetics of hips.  The first is the three factors listed above.  The second is that dysplasia is affected by many genes - it is multifactorial.  Someone said that this is a cop-out for those who wish to ignore dysplasia.  I think that overstates the case, but it certainly does make progress frustratingly slow.
What does multifactorial mean to a non-geneticist?  Let me use a simple example, or at least one that is probably simpler than the true situation.  Imagine there are 6 different loci which affect hips.  At each locus one allele codes for a contribution to good hips, the other a contribution for poor hips poor hips, So A gives a unit of good, a a unit of poor.  However, each locus makes only a partial contribution, so it is actually impossible to identify the individual loci, because their effects are very similar.  The effects are also additive.   So, a dog which had the genotype AABBCCDDEEFF would have the best possible hips, whereas one that was aabbccddeeff would have dreadful hips.   (My ex-partner bought a lab puppy - both parents were HD clear, and this was from a repeat breeding - 8 of 10 puppies in the first litter had been subject to preliminary X-rays.  This pup was missing one hip socket - there was just a clean sweep of bone without even a ledge for a joint.  She sent the X-ray to the breeder who phoned to say thank you - if she had not seen the plate herself she would not have believed the description!).  Well, as you can see, an overwhelming majority of dogs will not be perfect, but for this example, as long as six or more of the genes are "good", then the dog will have workable hips.  So a dog which is AaBbCcDdEeFf, or one which is AAbbCCddEEff, or one which is AaBBCcddEeFf will have hips that test out the same.  When you start making crosses, you will see that the results are very variable too.  But, if only those which have fewer than six “good” (capital letter) genes can be identified as affected and removed from breeding programs, theorists will tell you that the “bad” (lower case) genes will never be eliminated.

There have been a lot of experiments in population genetics to see how much progress you can make.  The classic result is that you make rapid progress at first, then improvement flattens off for many reasons.  In Drosophila (fruit fly), the early experiments worked on the number of bristles on the thorax - since no one knew the function of those bristles, they were thought to be neutral in terms of natural selection.  But, as the bristle number became higher, other problems arose, the most serious of which was disastrously low fecundity.  Really hairy fruit flies just didn't produce young.  A member of my PhD committee suggested that one reason for this was that, when you select severely, you probably throw away whole chromosomes just because they did not have the best bristle genes.  In throwing away whole chromosomes, you lose a lot of "good" genes, having nothing to do with bristle number, just because they happened to be on the same chromosome as the poorer bristle number genes.  So, for those who do not want to see us lose genes for Cardigan type just to help get rid of dysplasia, they are backed up, in the broad sense, by a huge amount of research results on a variety of species.

There is a theoretical possibility which I shall mention, but not discuss.  We could have two kennels which put a high priority on selecting for good hips, while line breeding within their own programs.  Both might achieve better hips, yet crosses between the two lines could be disastrous.  Using the example above, kennel A might, by pure chance, have worked on increasing the frequencies of A, B, and C, while kennel B might have concentrated on D, E, and F.  I actually heard of a similar case involving temperament in Golden retrievers.

There is a whole different way to look at the problem of dysplasia being multifactorial.  Genes do not work in isolation.  It may take several genes to make a hip socket, yet it is unlikely that any of them work just on the hip socket.  Some genes regulate the amount of action of other genes.  Thus, growth is turned on or off, and its rate is speeded or slowed, by genes acting on other genes.  One consequence of this knowledge is that there are probably not genes for good or bad hips, in isolation.   So getting growth speeded up may be good in one part of the body, but bad somewhere else, for example, in the hips.  That means that in selecting for some good characteristics, we may inadvertently end up with poorer hips, because we certainly do not know all the linkages between genes.

It has now been 50 years since the first experiments began on selecting breeding stock to have better hips.  The experimenters have had little trouble deriving breeding lines that are seriously dysplastic.  However, the desirable result, a pedigree which never produces dysplastic hips is still a holy grail for breeders.  My own summary is that genetics does contribute to hip dysplasia, but, perhaps by chance, not a great deal among Cardigans of today.  We need to be vigilant.  However, I have been around a lot of Cardigan puppies, and I have never seen one having the sorts of problems that are all too common in labradors and goldens.   If you watch puppies of those in the show ring going away from you, you will see suspicious movement, and that is a pretty good indicator of dysplasia.   Interestingly, Norwegian elkhounds as a breed have mediocre hips, but it appears that no one told the dogs that was supposed to slow them down.  The problem on X-ray is relatively shallow sockets, but most do not develop disabling arthritis.   Maybe they have exceptionally good ligaments?

(5) Basic dog design.  This is a large and complex subject.  I start by referring to the auto manufacturers' definition of a "platform".  A platform in auto parlance is a basic design of frame, body, suspension and running gear.   Chrysler had the K-car in the 1970s and 80s.  They built sedans and station wagons on the K platform, and it also served as the heart of the first minivans.  An engineer will tell you that you can put a variety of bodies and engines onto the same basic platform, but you have to pay close attention to the limits.  If the engine is too powerful, or the body too heavy, etc, your vehicle will become accident prone, quite possibly due to failure of the frame.

The prototypic platform for our modern dogs is a smallish wolf.  Now, both wolves and coyotes are relatively lightly muscled for size of their skeleton.  That is, the bones are robust in comparison to the muscle and other weight-producing parts of the body.   Remember that a wolf sometimes has to body-slam a running deer.  We know well that dogs that deviate in particular directions from the skeleton:weight ratio of wolves are more prone to hip dysplasia.  So all the giant breeds are disasters in terms of hips on most individuals.  In breeds relatively similar in height and length to wolves, the rule of thumb is that the lightly built breeds have better hips than heavier breeds.   Thus, greyhounds seem free of dysplasia, while labs and goldens are cursed.  Siberian huskies mostly have good hips, while Alaskan malamutes are more frequently dysplastic.

Most small dog breeds are thought to be free of hip dysplasia.   So which is the Cardigan, large breed or small breed?  Perhaps it is a sturdy small breed.  If one thinks of what a Cardigan would be without dwarf legs, it would be middle-sized.  The Cardigan has muscles of the hind leg which are unusually wide in relation to their length.  It appears that, when the leg bones shortened, the muscle mass stayed the same, and therefore the only solution was very wide muscling.  Maybe that contributes to the relative stability of Cardigan hips.  It is also possible that the great athleticism of Cardigans helps.  No one told the dogs that having short legs was a disadvantage, but they surely have compensated by working a lot harder – hence the hardness of the muscles of the hind leg of working corgis.

Conclusion: There is a lot more to hip dysplasia than just the morphology of the joint itself.   There is a lot to think about!!  Then, you must put X-Rays as an item in the budget.  Finally, given all the features we try to select for, how important are good parent X-rays?  There are probably over 50 features we select for, some have high priority, some lower.  But, surely PRA clearance, avoiding monorchidism, and other medical problems are just as important as good hip X-rays.  I wish we were all so lucky as to find dogs that passed all of these, but it the real world, no individual dog is perfect.  Then, we want to select for sound structure, all over the dog, not just the hips.  And we want it to look like a Cardigan corgi. 

The choices are never easy.

December 2004

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