Dealing with Hip Displasia, PRA, et
al.
by Patrick Ormos
Recently I have received several inquiries about hip dysplasia. It seems that we are going through another round of mild hysteria about this condition. Let me try to delineate some ideas about dealing with this problem in our breed.
The predisposition to a susceptibility to this constellation of problems is what we term in shorthand as hip dysplasia. HD is NOT an inherited condition, rather one inherits the predisposition to a susceptibility to its development. HD is NOT a matter of a simple dominant/recessive mode of inheritance. HD is heavily influenced by the manner in which the puppies are raised, as well as by the genetic inheritance which they carry. Nor is HD a simple question of yes or no - rather, HD manifests itself along a continuum from excellent to extremely poor.
Questions of diagnosis are best left to fully trained veterinarians who can diagnose from a quality set of X-rays of the dog. However, questions of whether or not to breed the dog do not belong to the veterinarian's field of expertise. Few veterinarians have the experience with a single breed and all the characteristics which must be balanced in any decision to breed a dog which experienced breeders have. The knowledge to advise a breeder is the veterinarian's field. The responsibility to use that advice and make informed decisions about breedings is the breeder's, not the veterinarian's.
A caveat here, as far as I know, OFA has not done adequate research on the effects of achondroplasia on hip socket formation and function. Anyone who has access to such research would do us a favor by informing us. Thus it would seem to me that it would be illogical to expect exactly the same kind of hip formation in an achondroplastic breed as in a Greyhound. OFA has said that they read all X-rays the same, regardless of the breed in question.
So, as a responsible breeder, what do I do when faced with this question? First, may I suggest that you read Malcolm B. Willis' book: Practical Genetics for Dog Breeders. I have found this to be an excellent introduction to the issues facing dog breeders, and to the informed use of scientific data and methods.
Willis makes a cogent argument
about the different methods of grading hips, and points out that the British method (a
numerical grade/score) actually gives us more information than the OFA Pass/Fail method
(even if recently modified to Excellent, Good, Fair and Fail.) His argument is based in
the mathematics of population genetics (statistics.) Now, understand that I am not a
mathematician (ask my banker.) But even I can understand his graphs and his point. PennHIP
provides a different way to score hips, as do various other programs in other countries. I
see no purpose in arguing for one method as being superior to others. The issue is that
there should be a consistent use of one method which will allow breeders to judge progress
or regress.
A polygenic trait like HD occurs with normal statistical distribution in a population. The median line is at the midway point, where half the curve falls below and half above. Polygenic traits are very slow to respond to selection criteria. That is, while we can choose for good hips, and against bad hips, we will only slowly change the distribution. Further, it is the median which will change. We will still have a small percentage with excellent hips, and a small percentage with dreadful hips - but the median, the average state of hips in the breed will have improved slightly ... and will continue to improve as we continue to select for good hips and against bad hips.
So, how does that translate into practical breeding advice? First, you have to have your goal clearly in mind. What do I want to produce? Dogs which have clean hips as a major priority? Then what happens to breed type? Where does temperament come in? What about color? Etc.
Breeding good dogs is an art, rather than a pure science. Like all the other arts, science is of great value in helping us to understand what we are doing in breeding - but in the end the actual choices become a question of art. Breeding good dogs is a balancing act in which one selects for dozens of characteristics, rather than just one. Breed type, structure, movement, temperament, genetic questions (HD, Hypothyroidism, PRA, etc.) all figure in my ultimate decisions, as I try to balance the demands of one off against another. Single issue breeding, whether it be about hips, color, coat length, or anything else will ultimately lead to failure in terms of overall good breeding. Yes, please X-ray your dogs, and please discard dogs with dreadful hips from your breeding program - but think carefully about those dogs with mild to fair hips - are they of such quality as to make their exclusion a detriment to the breed, or are they just fairly good dogs with fairly nice hips. A dog who is outstanding in several areas should be carefully considered, even if they fail in another area.
It is clear from the literature and from shared experience that breeding only from OFA excellent/good parents will NOT result in only OFA excellent/good progeny. That is the nature of polygenic inheritance. However, it will result in a higher incidence of better-than-average hips for that breed, thus exerting a positive influence on the breed for that trait.
PRA
A series of articles about one person's experience in breeding her first litter has set off a wave of PRA-panic throughout this country, and abroad. While the author of these articles shows a careful analysis of the situation, others seem to have gone overboard!
Certainly it is clear that the author had the extraordinary bad luck to have three of her four puppies grow up to have PRA, with a very strong likelihood that the fourth is a carrier... nonetheless, the incidence of this disease in Cardigans is very, very low.
PRA seems to be carried on a single gene, and to be subject to the simple dominant/recessive mode of inheritance documented by G. Mendel. It would seem to be a recessive gene. Thus, while we can reduce the incidence of this disease, we can not (at the current level of technology) eradicate it! Irish Setters have been able to develop a blood test which will determine if any IS carrying the gene. We do not have such a test available to us.? Nor do we have the money currently available to fund such research. Should that happen in the future, I will be among the first to see that all my dogs are tested.
Our only hope as breeders is to carefully evaluate our stock's pedigrees to see if we might have a carrier or an affected in the background, and to carefully breed to avoid (hopefully) such a problem. Testing of our dogs would seem to be the only appropriate means of increasing our knowledge and avoiding the problem. Late onset PRA will usually develop before the dog reaches five years old, thus a thorough eye exam every year until about age six would seem to be indicated. Obviously, we will want to avoid breeding to known carriers or affecteds.
I must stress that EVERY Cardigan has the possibility of being a carrier. There are NO PRA-free bloodlines in this breed. Until a test is developed which will indicate the presence/absence of the recessive gene, no one can claim that none of their dogs carries the gene. Certainly we can express the statistical probability of the presence/absence of the gene; Ch. XX has a 75% chance of being free of the gene, etc., but no one can say at this point in time that they are 100% sure that Ch. XX does not carry the gene.
On the other hand, the presence of only one puppy who turns out to have PRA, even if it is only one out of a thousand, guarantees the presence of the recessive in BOTH parents. It would be unethical for the owner of any dog who is known to be a carrier to not inform others of this fact.
Dogs carry lots of genetic "junk." As we continue our programs of line-breeding and in-breeding, more and more of this "junk" will come to the surface. We can calmly take stock of what appears and try to deal with it, or we can panic and run around having "witch hunts." The latter is of no help to the breed. At least, that is this person's opinion.
Published in the CWCCA 1994 Handbook and reproduced with the kind permission of the author.
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As I review this article in
2004, it is important to note that a genetic test has been developed. There is now no
excuse for a breeder to not test their stock and know for certain whether or not their
bloodline carries PRA. One of the issues which has developed as a result of this test is
what to do when you discover that your cherished and very high quality dog is a carrier.
A well-known dog in the USA proved to be a carrier. Her breeder discovered that all of her stock were carriers and had to face the question of whether to abandon the bloodline, or find a way of breeding around the problem. She chose to breed around the problem. With the help of other well-known breeders, she bred her carrier bitch to a tested clear male in the hopes of getting a non-carrier quality puppy in the litter. That did not happen. So, she took the best puppy and bred her to a tested clear male, again hoping. Again, no luck. The third generation did finally turn up a quality clear female with which to carry on her bloodline.
Some have raised the question of why breed carriers at all, and have stated publicly that they would not do so. Others have decided to help those who simply had the bad luck to have carriers in their bloodline. In the end, the goal MUST be to get a clear pup and stop the breeding of carriers. When doing this kind of breeding, it is imperative that all pups be tested to determine who are carriers and who are not – and that they be sold on a non-breeding contract.
I left the original article as it was because while it is no longer applicable to PRA, it is applicable to any other genetic issue which we discover and do not have a test for. I hope that you have found it helpful.
Patrick Ormos