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Canine Hip Dysplasia (CHD) is a complicated and often misunderstood disease. We are hoping this will provide you with a basic understanding of Canine Hip Dysplasia (CHD), particularly concentrating on its causes.
The term Dysplasia itself means "improper growth". Canine Hip Dysplasia simply means improper growth or development of the canine hip joint. This improper growth is usually characterized by lax or loose hips, which allows excessive movement in the Hip joint which in turn leads to arthritis and lameness. Hip Dysplasia is common in many of the large breed dogs. CHD is not an "all-or-nothing" kind of disease, but a syndrome that can present itself in a variety of clinical forms that range from clinically undetectable to a severe crippling disease.
Dogs, just like people, appear to differ in their ability to adapt to a disease. Some dogs may have severe radiographic evidence of the disease and show very little in the way of clinical signs or symptoms, while others may only have mild radiographic changes yet show severe clinical disease signs. This may be due to differences in the ability of individual dogs to adapt to the presence of chronic pain. If a dog develops CHD it is not necessarily a death sentence, with the exception of the severest forms, symptoms can be relieved by pain medication. Yet even in its mildest forms this disease may limit the dog from obtaining his full potential.
Both heredity and environmental factors are important in the development of CHD. CHD has an estimated inheritability that ranges from 0.2 to 0.6, with 0.0 being non-inherited condition and 1.0 meaning that a condition is completely under genetic control. Therefore, demonstrating that CHD is a condition where both the environment and genetics of the dog, basically play a role in the development of this disease.
The interaction between the dog's genetic makeup and environment determine whether an individual dog will develop CHD. The genetics of the dog, to a large part, determine if a given dog has the potential to allow the environmental factors to act in such a way that CHD is produced. However, even dogs that are not genetically predisposed to developing CHD can contract the disease if they are pushed too hard when young by hyper-nutrition and excessive exercise.
As puppies, diets that are low in protein combined with low activity levels reduced the severity of CHD in animals that were already genetically predisposed to developing this condition. However, even dogs that are not at a genetic risk of developing CHD, if exercised too strongly early in life and/or are fed diets that are too high in calories and protein can develop CHD, since hyper-nutrition and excessive exercise may interfere with proper joint growth and development. It is best to prevent a puppy from jumping or undergoing constant sustained exercise until at least a year old. Also, the premium dog foods that are so often recommended may also contribute to CHD development by increasing the growth rate. A protein percentage of 22% (dry food) is generally all that is required for normal growth. However, we all want our dogs to reach their full potential, thus being why premium dog foods are often fed. Here in lies the problem - consider this information as food for thought.
In addition to the environmental influences that confound our understanding of CHD, the genetics of this condition are very complicated and not completely understood. CHD is inherited as a polygenetic trait which means there are several genes are involved. This makes sense when we consider that that the Hip joint is made up of not only of bone but also of muscles, tendons, ligaments, and a joint capsule which together hold the bones in the proper position for a normally functioning Hip. Coordinating all of these parts into a normal functioning joint requires many different genes. But it is even more complicated, if we consider that this joint during the first year of life is continuing to change and adjust to the tremendous growth of the dog. If the parts of the joint grow at different rates or are out of synchrony with each other, then this will lead to a joint that is loose which predisposes the dog to develop CHD. Additionally, other seemingly unrelated factors such as rear leg angulations could also exacerbate or alleviate CHD. Dogs that have straight rear legs have less shock absorption, while well angulated dogs would not transmit as much of the movement stress to the Hip joint, as more of the trauma is absorbed by the hock and knee. When all other factors are equal, the dog with well angulated rear legs is less likely to develop CHD than a dog with straight rear legs. Thus, it is easy to see that the genetic control of CHD is complicated and difficult to understand.
Both environmental factors and genetics determine whether an individual dog will develop CHD. Genetics alone is not the only cause of CHD. We will never be able to completely eliminate CHD from the breed by selective breeding. However, we can reduce the incidence of CHD through appropriately selecting the correct dogs to breed. Thus, only breed animals that do not have CHD and all should be fine - right? Unfortunately this is not the case. The polygenetic control of the hip joint structure greatly complicates the situation. It is entirely possible for a dog with an OFA rating of “excellent” to produce puppies that develop CHD. How can this occur?
Without going too deeply into the genetics, some basic genetic background is needed to explain how an OFA certified animal can produce off-spring that develops CHD. The following is a gross over simplification of the genetics of the CHD; but does provide a framework for understanding a problem with multiple gene control over a single trait. Dogs have two copies of each gene that controls CHD (the total number of genes that control or contribute to the development of CHD is unknown; but it is believed that many are involved). They get one gene from their mother and the other from their father. Some of the "good genes" can mask the presence of a "bad gene". In other words some of the "good genes" are dominant over the recessive "bad genes". Thus, the physical expression of the "good genes" will be result in good Hips, even though the animal is carrying a number of hidden (recessive) "bad genes". When the sperm or egg is formed the two copies of each gene are separated so that sperm and the egg end up with only one copy of each gene. In a case where an animal had a copy of one good gene and one copy of the bad gene, 50% of the eggs or sperm would get the "good gene" and 50% will get the "bad gene". Now recall that many genes are involved in the development of CHD, so this separation occurs for each of the genes that control Hip joint development. Thus, a very few of the eggs and sperm will get all of the "good genes" and a very few will get all of the "bad genes" but most will get a mixture of both "good" and "bad" genes that control CHD. The outcome of a particular mating will be determined by the match up between the genetic make-ups of the parents. If the bad recessive genes line up incorrectly then more of the puppies could have Hip Dysplasia. However, if the good dominate genes line up well then many of the pups will have good hips, but may still carry the hidden bad genes. Complicating the situation even further is that some genes may be good when combined with one set of genes and bad when combined with different set of genes. It is also possible for some genes to have a greater or lesser influence on CHD than others, adding to the complexity of the problem. So you can see that the genetics of CHD is very complicated.
Some puppies will have the same genetic combination as the parents. Some may have a more desirable genetic make-up than both parents; and others may have a less desirable genotype. Predictions of a specific CHD outcome from a particular mating, is currently very difficult, if not impossible.
It is OFA's opinion, that a dog that has “fair” hips and less than 25% of its brothers and sisters exhibiting Dysplasia is a better breeding prospect than a dog with “excellent” hips having more than 25% of its brothers and sisters affected by Hip Dysplasia. In other words, less of the bad genes are likely to be present where a smaller percentage of the litter mates have CHD. Ideally, we would want to achieve a homogeneous gene pool composition where the dogs carry two copies of the good genes for each gene that controls CHD. In other words, eliminate the bad hidden (recessive) genes from the pool. This could be achieved by careful line breeding, making good hips part of that line's type. However, to accomplish this it is important to look beyond the individual animals of the proposed mating and consider the OFA status of the brothers and sisters as well as other relatives on both sides of the mating before proceeding. It is also important to point out that a good dog is more than just good hips. Care must be taken that the breeding of dogs is not based only on one trait. Otherwise we may end up with dogs that have excellent hips but have poor temperaments, bad coats, no hunting instincts, or other genetically related health problems. The whole animal should be considered with hips status being important but not the only thing to consider.
For the latest information, please visit the (WCA) Weimaraner Club of America’s website
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