|To Test or Not To Test|
|George A. Padgett, D.V.M.|
Professor of Pathology
College of Veterinary Medicine
Michigan State University
This article is copyright to the author. Federal law may prohibit the copying or unauthorized use of the article without express permission of the author.
The term "Test" is a confusing one because we use it in varying contexts which give the word various meanings. Therefore, I shall begin by defining some of the contexts and thus meaning of the word. We will later discuss several aspects of these contexts.
The problem with the word "test" is that many times, if not most of the times, you hear the word you cannot determine for sure the context in which it is used. For example, someone tells you their dogs has been "tested" clear for C.M.O. Do they mean their dog has had its jaw palpated and the owner or their vet found no bumps or swellings and their puppy shows no pain? Do they mean their dog was radiographed at 10 months of age and no evidence of C.M.O. was present? Do they mean their dog was bred to a mate that had C.M.O., produced 4 pups all normal, and they now know their dog is 93% sure to be normal genetically for the C.M.O. gene? Do they mean their dog was bred to 4 (or 8) different mates (with no knowledge about their phenotype or genotype), produced 12 (or 24) puppies, nobody told them they had a puppy with C.M.O. and, therefore, their dog has to be free of C.M.O.? Do they mean no one has told them they have ever had a case of C.M.O. in their family or line? There are clearly different meanings and different levels of precision which are involved in the potential interpretations of the statement "my dog has been tested clear for C.M.O.".
Now let us look a little more closely at what is involved in the various definitions of "test" or "testing" that were described above and how they relate to controlling genetic disease. But before you bother to read any further, you need to make some personal and professional (as a breeder) decisions about the following:
IF THE ANSWER TO ANY OF THESE QUESTIONS IS NO, STOP! THROW THIS ARTICLE AWAY IMMEDIATELY, BECAUSE THERE IS AN OUTSIDE CHANCE IT WILL CONTAMINATE YOU.
Test mating is a type of "test": Test mating is a technique that is embraced by some, vilified by others, and not understood by most. There are ground rules for a good test mating program, and if they are followed, it will not be, cannot be, the "downfall of the breed" as predicted by so many of its detractors. Test mating is a prospective technique; that is, you are trying to determine what disease will occur or will not occur as a result of a particular mating. In other words, you are looking at the future. An essential requirement of a test mating is that you must know the genotype of one member of the mating which makes the opposite member the test mate (i.e., the one whose genotype you are trying to determine, and it can be either a dog or a bitch). In addition, there is a breakpoint in the test mating. The breakpoint is determined by the number of puppies produced in a litter. The breakpoint is about 5 puppies per litter: that is, if you average 5 or more puppies per litter, you can use either an affected or a carrier animal on your test mate and be effective. This difference is designed to allow (particularly in a bitch) reproductive time to produce the normal litter(s) which is your goal (given that an average number of litters for a quality bitch is 4). Further, to genetically clear a dog to the 99% levels using a dilution system, you also produce an identical number of carriers. If you just randomly mate your dogs, the production of carriers goes on forever.
Ground Rules for Test Mating:
Retrospective "test" mating:
The discussion of test mating applies equally here; the only difference being that the actual mating took place before you knew the genotype of one of the animals involved in the test mating. This subject will be covered much more extensively in the future article.
Random "test" mating:
In order to use random test mating, you need to have a reasonably extensive knowledge of the incidence or prevalence of genetic disease within a breed. Once you have that knowledge, you can then calculate the number of carriers in the general population. Once you know the number of carriers, you can calculate the number of random matings which would be needed to "test mate" a dog or bitch to a given degree of risk of having a gene for any of the diseases which occur in the breed. At the present time, to my knowledge, there is sufficient information on C.M.O., Legg-Perthes, and hip dysplasia in Westies to use this technique. In Cairn Terriers and Newfoundlands there is sufficient information on about 30 disorders in each breed to use this technique. This is particularly useful for matadors. I will discuss random test mating much more extensively in a future article. It should be clear that random test mating evaluates matings that have already taken place and from this standpoint it is like retrospective test mating. In order to use this effectively, one needs to keep track of as many offspring as possible.
Screening tests are generally used to evaluate a dog for diseases that have a high incidence or prevalence in the breed, or a disease in which all dogs are at risk. They may be inherited or not inherited. An example of a non-inherited disease is brucellosis and both the stud and bitch should be screened for this disorder prior to breeding, particularly is the proposed mate is an "outside" animal. Evidence that their dogs have been recently "tested" for this disease should be made available by the owners of both the sire and the dam.
For a genetic disease, the ground rules are a little more complicated. Every major stud (every matador), including those dogs which have the potential or likelihood to become a matador, should be "tested" (screened, examined) for the disease for which tests are available. The reason, of course, is that matadors become matadors because they are used mainly in outcrosses. Outcrosses, unlike inbreeding, spread a genetic disease throughout a breed.
Examples of tests that should be used are annual C.E.R.F. exams for eyes, G.D.C. or O.F.A. radiographs for Legg-Perthes (and on the same radiograph, hip dysplasia), and C.M.O., as well as V.W.D. and thyroid testing. It is a little different with bitches because they would rarely be classed as matadors in any breed. Nevertheless, if they are quality animals, they may influence a significant number of descendants as the maternal influence.
As a person who is looking to breed a Cairn (or any other breed for that matter), you might think, "Hey Doc, that's nice for you researchers to decide how I should spend my money, but I'm only going to breed this bitch once", and that the foregoing is a waste of money. If you are talking about the type of dog that should only be bred once (if at all) you are probably right. But, what I am talking about for screening tests are significant animals: those dogs and bitches that have, or are likely to have, a significant genetic impact on the breed.
Aside from the obvious information screening tests provide for the animal tested, screening tests also provide information that may well be useful later on for retrospective or random test matings. Remember, screening tests only give genetic information on the animal tested if they are positive. If they are negative (i.e., the animal is clinically normal), they do not tell you if the animal is a carrier or not.
Phenotype tests or clinical tests:
Phenotype or clinical tests should obviously be used on any animal displaying symptoms that need to be diagnosed as one disease or another. They are obviously needed for both genetic and non-genetic diseases. Aside from animals that are displaying symptoms of a disease, clinical tests in regard to genetic diseases are most useful when the results are entered in a registry and those results are made available, whether they are positive or negative, to people who are considering the dogs for breeding. If we wish to control genetic disease in dogs, we not only need to "test" our dogs, but we need to register them in an open registry in order make the information available for proper use.