Virginia Luis Fuentes and Anne French as appeared in the Boxer Heart List 2000

THE BOXER HEART TESTING SCHEME
CURRENT PERSPECTIVES
 

Screening for aortic stenosis by stethoscope was adopted early on by the British Boxer Breed Council as the method of choice because it was a practical, economical system which was accessible to the majority of boxer breeders. Doppler echocardiography has been used in selected cases for screening. Doppler is expensive and limited in availability.  In 1994 a study was carried out at the University of Edinburgh looking at phonocardiography, the recording of heart sounds by microphone onto a machine, as an alternative screening method. This method was also shown to have limitations. No test is perfect and any method of screening will have its limitations.

Screening by stethoscope
Having looked at other tests it has been decided that auscultation is still the most practical test available to us for routine screening for heart problems. Over the past 12 months the Veterinary Cardiologists routinely involved in the screening of Boxers have met together to discuss the problems encountered in the use of auscultation and to look at ways of improving this test further. Two recent studies at Towcester and at Coventry have confirmed what had previously been suspected that there can be variation in the same dogs at different times and that there can be some variation between cardiologists in interpretation of low grade murmurs. So what can be done to improve the reliability of this test?

Different cardiologists
The grading system currently used is a universal system which has been used for many years by veterinary cardiologists. The human ear is a subjective measuring instrument and there is bound to be slight variation between cardiologists. The number of cardiologists available for examining boxers has increased considerably in the past four years. To ensure the minimum variation between cardiologists, a panel of recommended cardiologists has now been selected for the control scheme. This panel has been selected from cardiologists who have considerable experience with Boxers. The panel has worked closely together to draw tip auscultation guidelines that will ensure minimal variation between cardiologists.

Variation within the same dog
We now know conclusively that some boxers have murmurs which vary from moment to moment, whilst other boxers have relatively constant murmurs.

Relying on one test result alone increases the chance of error. However multiple testing will NOT improve accuracy if one can continue testing repeatedly, until by chance one low score is given and this is the score allowed. It is for this reason that a limitation three tests has been agreed. One of the reasons for allowing a dog to be listed as their best result is because we believe that a boxer with a murmur which can become quieter at times is probably less likely to be severely affected than a boxer which has a constant loud murmur.

Progression of aortic stenosis
The vast majority of boxers with aortic stenosis do not appear to show any progression in severity after reaching maturity. This means that a grading given after 12 months of age can generally be expected to remain the same thereafter. However, we have found a very small number of boxers which do appear to have shown progression between the ages of 1-4 Years. For the vast majority of boxers, testing between one and two years of age should be all that is required. However this system might allow a very occasional dog to be listed as suitable for breeding, even when it has gone on to show progression after this age.

What happens with severe aortic stenosis?
A recent retrospective survey at the University of Edinburgh has shown that boxer dogs with murmurs grade 4 to grade 6 (scale grade 0-6) and with aortic velocities on Doppler examination of greater than 4 metres/sec are at high risk of acute death or heart failure at an early age. It is important to continue screening so that we can prevent breeding from affected animals.

Is the scheme working?

A review of the prevalence of severe aortic stenosis in Boxers from the University of Edinburgh is very encouraging. A wider survey is presently being planned involving all the clinical departments of the university and the cardiology referral centres.

Results from Doppler studies in Boxers at the University of Edinburgh
 

These figures show that whilst we are still seeing aortic stenosis in boxers, that these are now mainly mild cases. The number of severe cases is decreasing.

Screening by Doppler Echocardiography
Should this be part of the scheme? We feel that it should remain part of the scheme for selected cases. As with auscultation it is important to be aware of the limitations of this test. It is not a black and white test. The cut off point of aortic velocity < 2 metres/sec will allow some dogs with mild aortic stenosis to be used for breeding. A lower cut off would ensure that no dogs with aortic stenosis are used for breeding but this would mean taking more dogs out of the gene pool. As the number of Veterinary cardiologists grows, a recent change in the Doppler screening is the recommendation of a limited panel of Veterinary Cardiologists with extensive experience in Doppler echocardiography to undertake any Doppler screening.

Recent change in the Royal College of Veterinary Surgeons (RCVS,) regulations
The RCVS has recently requested that all results of screening programs should be reported back to the Veterinary Surgeon who usually attends the animal. The implications of this request for the control scheme are being investigated.

Value of post-mortem
There are still many unanswered questions about mild aortic stenosis especially the significance of low grade variable murmurs. Post mortem examination of aortic valves would help significantly in answering some of these questions. It is for this reason that arrangements have now been made at Edinburgh for post-mortem examination of boxer hearts with provision for a small reimbursement to the Veterinary Surgeon involved which hopefully may be claimed from the Boxer Breed Council (request sought). In particular we are interested in hearts from dogs with low grade murmurs or without heart murmurs. We will make every endeavor to report results back to owners and their veterinary surgeons as soon as possible. It is never an easy time when a loved pet dies or has to be put to sleep and it can be difficult to make the decision to allow post-mortem. Support of this request would be very appreciated.

The future
The aortic stenosis breeding scheme has had many teething problems. Despite these problems the results would suggest that already after a short time of several years the number of severely affected dogs has decreased. It is important that the scheme continues with the incredible support it has had over the past seven years.

 

Anne French

 Address for sending hearts and contact number for obtaining information.
Anne French MVB DVC MRCVS
Lecturer in Veterinary Cardiology, University of Edinburgh
Summerhall, Edinburgh. EH9 IQH. Tel - 0131 6506061 Fax - 0131 650657