Severe cases of hip dysplasia can cause lameness, so breeding programs aimed at eradicating the condition just makes sense, writes Dr Peter Higgins.
Hip dysplasia is sometimes, incorrectly, called congenital dislocation of the hip. It is actually a laxity of the hip joint, resulting from a shallow acetabulum (the ‘socket’ of the ‘ball and socket’ joint) or a small, misshapen head on the femur (the ‘ball’), or both. In severe cases, affected animals may be lame.
This disorder is a classic example of a multifactorial trait – one which has several causes or is caused by several genes. It is definitely inherited, but is definitely not due to a single gene. Most estimates place the chance of inheriting the condition when a parent is affected at 25 to 40 per cent.
In order to devise a sensible strategy for minimising the condition, we must first distinguish between the trait that we wish to improve (the breeding objective) and the trait that is actually measured (called the selection criterion).
In the case of hip dysplasia, the breeding objective is the clinical expression of hip dysplasia, such as lameness. However, this clinical expression is very difficult to measure and may not be expressed at an early age.
Therefore, the measurable selection criterion used in most hip dysplasia control programs is ‘radiographic hip dysplasia’ (RHD), which is the appearance of hip dysplasia on an x-ray and is readily assessable at an early age.
RHD is assessed by a specialist on a case-by-case basis, so scores may be thought of as subjective. This does not detract from the usefulness of RHD as a selection criterion. All that is required is that RHD be measured on an arbitrary scale each time, and that this scale matches up generally with instances of clinical hip dysplasia.
Available evidence indicates that the correllation between measured RHD and clinical hip dysplasia is sufficiently high to justify the use of RHD as an indication of the risk of developing hip dysplasia in breeding programs.
Breeding to minimise the condition is more effective if the RHD is assessed to calculate an Estimated Breeding Value (EBV) for each animal. This provides the best possible prediction of the RHD score that can be expected in the offspring of that animal.
One of the best scoring schemes currently in use is the one devised for the British Veterinary Association and the German Shepherd League in 1989. This is now the standard scheme for almost all breeds in the UK, New Zealand and Australia.
It involves assessing nine different radiographic features of each hip, and giving a score to each of the nine on a scale from zero (ideal) to six (worst). The 18 scores between zero and six are then totalled, giving a total potential range of scores from 0 to 108.
With such a large range of possible scores, selection based solely on RHD is not a black and white issue. It is very rare to find a dog whose score indicates a clear ‘yes’ or ‘no’ as to whether it should be used in breeding programs.
Rather, the advantage to breeders is that it is possible to use these scores to continuously improve the average scores of the breed as a whole, thereby decreasing the incidence and severity of hip dysplasia in the longer term.
Dr Peter Higgins is a Veterinary Surgeon & Honorary Associate of the Faculty of Veterinary Science at the University of Sydney.