What failure to screen says (or dosen’t) say about frequency of genetic disease
by C. A. Sharp
First published in Double Helix Network News, Summer 2007
“My dogs have never had that! That isn’t a skeleton in my closet, so why should I waste time and money checking for it?”
Breeders sometimes express sentiments like the above when asked if they’ve done a particular health screening test. While personal experience and knowledge of one’s line is important, particularly among breeders with a long history in a breed, is this a realistic attitude to take?
A number of different tests are available to breeders. Hip and eye screening are all but standard in many breeds. One can also screen elbows, patellas, hearts and thyroid gland function. Add on the growing number of DNA tests and doing everything available for a particular breed can add up to a big chunk of change per dog.
Pretty much everyone who is seriously involved in Australian Shepherds does the hips and eyes on all their breeding stock. They usually do the eyes more than once. But does a conscientious Aussie breeder need to do other tests if she hasn’t encountered those diseases in her own dogs or heard of related dogs that were affected?
Consider elbow dysplasia. Aussie breeders in the US and Canada screen elbows with much less frequency than they do hips. Nobody talks much about elbow dysplasia. Is checking elbows really necessary?
I regularly provide genetic counseling to owners and breeders, mostly of Aussies but sometimes other breeds. Up until the late 1990s, I only recommended elbow screening for Aussies if a dog was exhibiting constant or intermittent front-end lameness that couldn’t be attributed to other causes. At that time I was only aware of a relative few cases of elbow dysplasia in the breed. However, when I began obtaining screening results from the open health registries and breed club reports in various European countries, and was surprised at the frequency of elbow dysplasia reported, particularly in data I received from the Swedish Kennel Club’s on-line database.
The SKC has long maintained an open health registry. Hip and elbow checks are not only expected, they are required for most breeds and the results are public. Almost the same number of Aussies was listed by SKC as having elbow dysplasia as hip dysplasia, which we know to be a common problem in the breed. The Swedes have a long-standing and internationally-respected program. Their results had to be taken seriously.
My first thought was that the result might have been an artifact of population dynamics. When a breed is introduced to a new country, the lines present may differ markedly from those in the home country, so frequency of hereditary diseases may be quite different from one country to another, depending on what dogs they happened to have. I researched the pedigrees of the listed dogs and found that they represented a mix not terribly different in make-up from what once finds in the US. They were mostly show-bred dogs with a much smaller number of working-line dogs. In short, not terribly different, proportionally, from what we have here in the US. Many of the dogs were only a generation or two removed from US breeding.
As a result of this discovery, I started recommending that all Aussies intended for breeding first be screened for elbow dysplasia. When Aussies became one of the breeds enrolled in the CanineHealthInformationCenter (CHIC,) I was pleased to see that elbow screening was one of the three mandatory exams (along with hips and eyes) required for a dog to receive a CHIC certificate.
But what if a breeder’s dogs have never had elbow dysplasia? Maybe they haven’t, but perhaps they have and the breeder wasn’t aware of it. It is possible with elbow dysplasia for an affected dog to be sub-clinical. Such dogs may not show lameness. The degree to which those with visible lameness are affected varies. Most people don’t think much about an occasional minor limp in a stockdog or a dog that regularly competes in demanding performance events like agility or flyball or even if it is just a very active animal. Assuming a minor limp is nothing, particularly if it recurs, could be preventing a breeder from identifying a genetic skeleton rattling quietly in the back of his kennel’s closet.
A dog’s relatives that are not under the breeder’s direct control may or may not be clear of any given hereditary issue. If no one who owns the related dogs has ever mentioned having a particular inherited problem, that is no guarantee it hasn’t happened. Other breeders may not know the status of dogs they bred but do not own: Puppies that don’t’ go into breeding homes generally don’t get screened beyond a puppy eye check or the breeder may not diligently follow up on health status of sold dogs. They may have different habits when it comes to screening: Eye exams may not be done beyond the puppy exam or screening for other common traits, like MDR1 (multi-drug reactivity) in various “collie-type” breeds with a high frequency of the mutation. It’s easy to miss things if you aren’t actively looking for them.
There is also the issue of that other species of skeleton in the closet – the things people know but don’t share. More than one breeder has found herself saddled with a genetic health issue only to be told, after the fact, “Oh yeah, so-and-so produced a couple of cases of that once.”
Most breeders would agree that it is vital for the health of our breed that we take every reasonable step within our power to prevent the production of puppies afflicted with serious inherited diseases. The fly in the ointment is the definition of “reasonable.”
For Aussies, hip and eye exams have achieved a high level of acceptance as a necessary screening measure for breeding stock. Even those who don’t bother or are haphazard about hip and eye screening will either lie or present some sort of justification for why they have not tested when questioned directly. If someone wants to be considered a serious breeder of Australian Shepherds, they cannot evade the expectation that hips and eyes will be done.
But what about the other things we can screen for? When is it reasonable to expect that other tests be done? Frequency of the gene mutations that cause a particular disease should be a major consideration. The MDR1 gene provides an excellent example. There is a DNA screening test for the mutation that causes some dogs to react to particular drugs. The research which lead to the test revealed that this mutation was extremely common in Aussies and Mini-Aussies. Roughly a third of Aussies and almost half of Minis have at least one copy of the mutation. The high frequency combined with the fact that drug reactions can be serious or even lethal should make this test mandatory for every Aussie and Mini-Aussie, regardless of whether it is of known pedigree or not.
Fortunately, most mutations are not this common. Mutation frequency for Collie Eye Anomaly (CEA) and Progressive Rod Cone Degeneration (PRCD, a form of PRA) in the breed is much lower. Less than 10% of Aussies carry at least one copy of the CEA mutation and around 5% carry PRCD. There are DNA tests for both diseases, so a breeder needs to determine whether or not it is necessary to use them. In cases like these, only relatives of identified affected or carrier dogs need to be screened. The breeder should review her dogs’ family history to make a determination on whether to test; always keeping in mind that one can never be entirely sure how accurate information on other people’s dogs may be.
Each breeder needs to perform due diligence so a problem present in his line doesn’t slip by his notice. In the case of CEA and PRA, the former is present at birth and the latter comes on when the dog is a few years old. the breeder should check all puppies he produces between 5 and 7 weeks of age. (Inner eye pigment can mask the presence of CEA lesions if the exam is put off until later.) Annual exams are required for PRA (as well as some other diseases, like cataracts.) If a dog only gets a puppy check or is checked rarely PRA might be missed unless someone notes that the dog is having trouble seeing and takes it to a vet for diagnosis.
Sometimes there aren’t any research or broad-based health registry statistics available to guide us. For the less-reported things like luxated patellas or cardiac issues, the best we can do is rely on what pedigree information we have, have our vets check any signs we may see in our own dogs.
Regular follow-up with puppy-buyers, at least annually, can help a breeder determine whether some of the less-used screening tests are necessary or not. Puppy owners who are not active in the breed may not be aware that some types of health information are important to the breeder.
Ideally, every breed should have a detailed health screening protocol for hereditary diseased that have been identified in the breed and for which there are screening tests. The protocol should take into account both the frequency of the causative genes in the breed population and how seriously each disease impacts health. It should advise breeders on when and how often to test, as well as make breeding recommendations for carrier and affected dogs.
Fortunately, Aussies have such a protocol already. The Australian Shepherd Health and Genetics Institute developed one in 2005. The tools are available to enable breeders to keep those skeletons from rattling around in their closets, so long as we don’t allow ourselves to be set in old habits or disregard helpful information that is readily available about health concerns in our breed.