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Bruce M Cattanach as featured in the Boxer
Heart List 2002 pp 34 - 37
INHERITANCE OF BOXER HEART MURMURS
by Bruce Cattanach
Aortic stenosis (AS) is the heart condition
that is most prevalent in UK Boxers. ‘The genetics of the disease was
studied many years ago in Newfoundlands and a dominant-type of inheritance
was suggested. In this veterinary study diagnosis was unambiguous, being
based upon pathological studies upon dogs that had been put down solely for
this purpose. Such investigations are, of course, no longer either feasible,
or acceptable. Therefore, genetic studies upon AS in Boxers has had to use
some other method of diagnosis.
One of the simplest and economical means of detecting AS is auscultation,
the use of the stethoscope. As practiced by specialist cardiologists, the
method has been considered to be generally reliable and consistent.
Therefore when studies first started upon Boxer AS in 1990, the cardiologist
with the most expertise and interest in Boxers, Virginia Luis Fuentes
recommended auscultation as the principal method of screening our dogs for
AS. She also recommends another method, Doppler echocardiography, as a
backup test for use with ambiguous cases. Details of the screening and the
breeding control scheme adopted by Breed Council are given in this booklet.
Basically, they require that dogs should be screened for heart murmurs when
adult, and only those with the best scores (Grade 0 and 1, plus those Grade
2’s which can obtain a lower grade on retest) are recommended as suitable
for breeding purposes.
The first data
on the breeding performance of dogs with different grades of murmurs
suggested that the inheritance was simple; the best dogs clearly produced
the best progeny. But as the numbers of cardiologists increased and
information on the variability within individual dogs was produced.
Complications began to emerge and this began to confound the genetic
analysis. A major advance was standardisation of the
grading but this was achieved through a more rigorous testing regime.
Progeny were now scored much more stringently than their parents and this
added further problems for genetic analysis. However, now several years on,
the difficulties have been substantially resolved and the problem data
diluted out. A more valid genetic analysis has again become possible. Figure
1 summarises the current evidence, it may be seen that as the grading in
parental matings increases (0x0, 0xl, 1x1 etc.), so does the average grading
of the progeny’ (marked points on the graph). This is best illustrated by
the straight “line of best fit” for all the data. The vertical error bars
show the “wobbles” at each point. The range of these wobbles is dependent
both upon the variability among the progeny scores and upon the numbers of
dogs tested. These wobbles are gradually being “ironed out” as the newer
data feed in. Even as the data stand, the results from 0x0 matings are
significantly different from all other types of matings. Heart score has a
genetic basis. The graph thus presents the first evidence that Boxer heart
murmurs are inherited.
Figure 2 provides the second line of
evidence. It shows the same data but in the form of distributions of progeny
grades (0,1,2 etc.) from each type of mating. Here it can be seen that Grade
0 and 1 progeny are the most common in 0x0 matings, but that there are also
Grade 2, Grade 3 and occasionally higher grade progeny in this group. But as
the grades of the parents increases (0xl, lx1. 1x2 etc.) there is a steady
shift of the progeny grades to the right (to higher grades) with the highest
grade parents producing the fewest Grade 0 and 1 progeny and the highest
proportions of higher grade progeny.
Given that this analysis is based upon the total 10 year data with all the
changing levels of grading over this time and with dogs getting their “best”
(not necessarily correct) scores, the results are surprisingly impressive.
Could the newer data on dogs with recently tested parents be readily
separated out, the data would be expected to be yet more impressive.
The evidence that heart murmurs in Boxers have a genetic basis indicates
that selective breeding for dogs with the “best” hearts will be effective in
reducing the incidence of clinically affected dogs. From the cardiologists’
reports, as described in the SWBC’s “Blue Book”, the scheme has already had
such an impact and even the incidence of Grade 3 murmurs is declining.
However further progress will largely depend upon breeder application of the
scheme. There are currently too many untested dogs being used for breeding,
as also same with higher grades. This acts as a “drag” on progress.
Beyond this, there are still unknowns concerning the disease itself and its
distribution in the breed. The main genetic advance has been made but we
need to know:
-the relationship between heart murmurs and direct evidence of AS for each
murmur grade at the pathological level;
-if there are in fact any Boxers that are truly free of the disease;
-and. if there are such Boxers, would they “breed true” with all progeny
being AS free?
Answers to such questions require further active research, as opposed to
standard heart testing. These answers would then allow us to see how far we
might be able to go in dealing with AS. what the “costs” to the breed would
be, whether ongoing heart testing will continue to be worthwhile or only of
limited value, and whether studies at the DNA level might be possible and so
allow us to completely free the breed of AS.
The first step of establishing the genetic basis of Boxer heart murmurs has
now been made. but it will take fund-raising and active help from breeders,
from breed clubs, and from Breed Council to make further progress. I will
shortly hear if Kennel Club help with funding might be a possibility. Help
from the American Boxer Club and their Kennel Club might also be possible
were a grant application successful, but it is certain that activity will
first have to start from within the breed in the UK.
We could continue just “blindly” testing. perhaps forever! But should we not
try for something better?
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