Boxer Heart Testing - I Think We Are Winning
By Dr Bruce Cattanach BSc, PhD, DSc, FRS
Boxer 2001
(pages 114 - 115)

In last year's Boxer '99 I was able, for the first time, to present some positive evidence on heart testing for Boxer aortic stenosis. After ten years of worsening yearly heart murmur scores attributable to increasingly rigorous scoring there was an indication of an improvement. That this was evident in both the show and private testing data suggested that this was not just due to chance. In addition, I was able to provide the first scientifically convincing evidence that progeny grade is dependent upon parental grade. Heart murmurs, as an indication of aortic stenosis, clearly have a genetic basis. The only bad news was that the numbers of dogs tested each year had steadily dropped since its peak year in 1994, when some 500 dogs were screened at shows. In 1999 only 277 dogs were so tested and there were approximately 350 in each of the preceding two years. Some disillusionment with the system appeared to be growing and, given all the grading problems identified over the years, perhaps this is not surprising.

So! Was last year's improvement meaningful or simply due to chance. The short answer is that I think improvement is now definite. This year's data again look good, but there are complications. Interpretation is not straightforward.

The data for the past three years, together with those for this millennium year, are shown in the accompanying table. Consider first the frequencies of grade 3 and louder murmurs. These are lower than in all previous years, and there are several reasons for believing this to be a real meaningful result rather than just due to chance. First, the shift is statistically highly significant.

Table of results - Murmur grades of dogs tested at shows

Second, almost half the dogs remaining in these categories are Irish, and heart testing in Ireland only started very recently. Remove these, and the result would be even better. And, third, several of the remaining dogs are related to each other or to other high scoring dogs of previous years; they appear to be relics of the past wobbly scoring and, as a group, should soon disappear. Add to this the oft-repeated reports from the veterinary groups that the incidence of seriously affected dogs has diminished sharply in recent years and we can claim that the main objective of the heart scheme is being achieved.

Turning now to the "pass" grades (0 and 1), it can be seen from the table of results that the combined frequencies have been maintained at about the same level as last year, just over 60 per cent. But why has the frequency of grade as decreased and the frequency of grade 1 s increased? This seeming worsening of the situation among these most mildly affected dogs contrasts with the improvement at the other end of the scale. This does not make sense. It is either due to chance, or some other factor is operating.

I believe the interpretation is that we have a true improvement at the "bad" end of the scale, and this brings about a compensating slight increase at the "better" end, perhaps among the grade 2s and grade 1 s. But there is also the very real possibility that the grading has inadvertently been tightened up even further. I hate to say this, but it does accord with the psychology of the cardiologists' testing. They are trying to be as accurate as possible and I recognise their professional desire to ensure that grade a dogs really do not have even the suggestion of murmurs. I would doubt that the grade 1-2 border is also involved. If the grade 0/1 shift is real, it may be a further irritation for breeders but, since it does not toughen attainment of the "pass level", it should not add further difficulties to Boxer breeding.

I can't prove that the above is the correct interpretation of the grade 0-1 data, but time will tell. However, on the basis of the decline in numbers of the grade 3-6 groups, it may be expected that the frequency of grade 2s should also soon start to decline - if selection of the "best" dogs for breeding is maintained (or increased). This further improvement might occur quite rapidly, as it will be enhanced by the oldest and least rigorously tested dogs leaving the breeding population.

Could we ever get down to a majority grade a situation? This will, in part, depend upon the toughness of selection, but it will also depend upon whether or not we have any really genuine aortic stenosis-free dogs within the population. Here, I am afraid that my call for breeders to rigorously test some of their grade a dogs for this research purpose has fallen on deaf ears. Only a very few breeders have shown interest. The opportunity for obtaining breeding data for genetic analysis is therefore minimal. I also have not had any response whatever on the issue of collecting litter size and puppy survival data from matings involving dogs with different murmur grades. We, therefore, proceed "blindly", not knowing when, if ever, we may be able to scale down or terminate the control scheme.

It is also disappointing that the total number of show tested dogs for the year (297) remains well down on earlier years, but I hope the new results indicating improvement will help rectify this situation. A cardiologist with experience in Boxer heart testing is now established in the North-East, and this will offer better opportunity for show testing in the North. Another is working in Dublin and should advance testing in Ireland. Their names/addresses are given in the latest Heart List booklet.