Another year has gone by and although from my point of view it has been a
"bumpy ride", breeder effort to deal with Boxer aortic stenosis (AS) is
proving successful. The number of dogs tested now stands at 3000 and
reports from the vet schools are consistently indicating that the incidence
of referred heart cases in Boxers is declining. Boxers no longer
appear to be at the top of the list of "heart breeds"
The Cardiologists have justifiably been delighted with the perceived
improvement and we can indeed, all congratulate ourselves upon this success.
But there is another problem. If the incidence of severe cases of AS
is on the decline, the incidence of murmur free dogs and those with only
minor murmurs should be on the increase. But this does not appear to
be true. The reverse would almost appear to be the case and this does
not make sense. Fortunately, some answers have come from analysis of
the computer database of records.
Several complications have come to light: first, as I have indicated before,
the severity of the grading has become much more rigorous since the start of
heart testing; second, a variation in scoring among cardiologists was
indicated; and third, the grading itself has proved in some cases to be
quite variable, even with single cardiologists. It is therefore,
hardly that many of us have been disappointed when dogs have not bred as
well as might have been expected.
It has amazed me that when the
evidence was presented to the cardiologists, rather than being offended by
apparent criticism, they have taken it as a challenge to their expertise and
attempted to resolve each of the difficulties. Hence, the planned new
initiative on heart testing which was described in Boxer '96, and ultimately
put into operation this year. Two cardiology testing sessions have been
held, the first as Towcester in the spring and the second in Coventry this
autumn.
The Towcester findings were
based on over 40 dogs, each of which had been graded previously. They
included animals that had been given the same scores on different occasions
and others which had obtained different scores. Eleven cardiologists were
present and each independently graded all the dogs, some of them twice. A
significant variation between cardiologists in the severity of their grading
was found, confirming the conclusions from the database but it was also
established that individual dogs could also vary when scored while
individual cardiologists were listening.
The variation between
cardiologists was expected to be the most easy to resolve but the variation
within dogs was thought to be a much more' difficult issue. I don't think it
is yet entirely clear with such dogs whether the higher or the lower score
is the more valid. This may only be resolved with post-mortem examinations
of hearts but, other variable factors can affect blood flow and, hence,
loudness of murmurs over short periods. For example, if there are long
intervals between heart beats, as .in hyper-fit animals, more blood is
pumped with each beat so that any murmurs are amplified. Conversely, in less
fit animals, there may be smaller intervals between beats so that no murmurs
may be heard.
Excitement itself may also affect murmurs as it appears that constriction of
the blood vessels may be induced. This again increases blood flow to amplify
murmurs. A further factor is that dogs' hearts do not beat regularly like
ours, so that blood velocity and hence any murmurs present may automatically
vary. The complications seem enormous. However, according to Miss Fuentes,
normal dogs of non-aortic stenosis breeds do not develop murmurs under any
of these "normal" conditions. This suggests that all Boxer murmurs are
indicative of heart abnormality and that maximum grades rather than minimum
grades may be the most valid.
Both the variation between
cardiologists and the variation in murmurs within individual dogs were
tackled at the second meeting held this past November in Coventry. Here, 15
dogs were repeatedly examined by each of 14 cardiologists and with much
discussion and the use of multiple headed stethoscopes the following
criteria were laid down for grading:
1. Grading should be carried out
in relationship to other heart sounds to minimise hearing or stethoscope
differences;
2. When a point of maximal
murmur intensity is localised with a lower score heard elsewhere, the grade
of the localised murmur should be given;
3. When the murmurs vary
markedly with respiration and/or sinus arrhythmia, the average grade should
be allocated;
4. Where a decision between two
grades is not easily made, the higher grade should be given if there is a
marked radiation/spreading out of the murmur and/or the murmur is of more
than brief duration;
5. When a murmur increases in
intensity with minor levels of excitement, the higher grade
should be given.
Such detail may be of little
relevance to most of us, but it does give some idea of the complexity of the
grading with minor (grade 1-2) murmurs and shows that grading involves more
than an assessment of loudness.
The issue of establishing an
expert panel for Boxer heart testing was further discussed at Coventry.
Although the details have not yet been finalised, it seems likely that the
panel will comprise those cardiologists who have attended the Towcester
and/or Coventry heart testing sessions. This applies specifically to the
standard stethoscopic testing. Therefore, it will be important for all of us
to recognise henceforth that it is only the "panel cardiologists" that can
be used for the Breed Council Control Scheme and Heart List purposes. The
list of panel cardiologists will be published as soon as available and will
be included in future Heart Lists
Another issue that was raised at
both meetings and which is highly pertinent to assessment of aortic stenosis
is the use of Doppler echocardiography. Edinburgh has led the world with the
use of Doppler and the experience gained has suggested that it may take up
to 18 months of regular use to obtain consistent Doppler readings. Without
such experience, Doppler scores may be meaningless. A need was, therefore,
seen for a panel of appropriately experienced vets, specifically qualified
cardiologists, to conduct Doppler testing for the Boxer heart scheme. Again,
a list of cardiologists having the necessary experience is to be drawn up.
Finally, there is the problem
of the more rigorous heart testing that we all now face. This appears to
have exclusively affected the proportions of dogs with the minor murmurs,
Thus, when serious grading first started in 1991, the proportions of Grade
Os, 1 sand 2s was 45%, 280;0 and 20%. Last year the equivalent figures were
19%, 43% and 30%. This, in effect means that with the longer and more
careful listening, murmurs have been found in dogs that would previously
have been considered murmur-free (Grade Os, and some Grade 1 s have been
pushed up to Grade 2s.
This shift in the grading has
two consequences. It means that parents and progeny have been scored
differently and, for me, this enormously complicates genetic analyses. More
importantly for all of us, it means that a higher proportion of clinically
normal dogs fail the heart testing, and to take 30% of dogs out of the
breeding population is too harsh for any control scheme. It was for this
reason that I have wondered whether a change in the breeding recommendations
might be warranted. However, given that about half of Grade 2s may gain
Grade 1 s on retest or pass on Doppler, the level of selection (150;0 plus
the more serious Grade 3s and worse) perhaps remains acceptable.
In summary, issues concerning
aortic stenosis in Boxers in 1997 have revolved around reappraisal of the
cardiologists' scoring systems. The vets involved have done much to
eliminate individual variation in the scoring and to adapt the scoring
procedures to cope with the additional complications recognised. An expert
panel for standard stethoscopic testing and another for Doppler use are
being set up and this should ensure yet more consistent scoring. It is
therefore up to us again to exploit the improved scoring system.
But there is one other major
drawback; we still do not have the post-mortem evidence in Boxers to show
that all the variations in murmurs discussed are caused by specific heart
abnormalities. This information is essential to verify the basis for our
selective breeding programme. Please think about donating the hearts of
previously heart tested dogs for research when they die, no matter what the
cause of death. I know from personal experience that this is very difficult
to do. It may be better arranged in advance. Please contact me for details
if you are willing to help with this research.
|
