PROGRESS IN DEALING WITH AORTIC STENOSIS
By Bruce Cattanach
Boxer '93
(pages 19-21)

New Boxer owners and non-UK readers may wonder about all the attention recently given to the subject of Boxer hearts. A brief recap is therefore necessary before updating the findings.

Breeder concern in the 1980s over the numbers of young Boxers dying of heart related disease led to ths British Boxer Club heart survey at their summer championship show in 1990. There, two Edinburgh cardiologists, Miss Virginia Luis Fuentes and Mike Martin, spent the day examining with their stethescopes the 125 Boxers that were presented before them. Their findings were subsequently published in the club's newsletter. A high percentage of the dogs had heart murmurs, indicating that all was not well in the breed, and this was confirmed at an equivalent survey held by the Scottish Boxer Club later that year. We were to learn from the cardiologists that not only was the the Boxer recognised by veterinarians internationally as a "heart breed" but that the incidence of UK Boxers being referred to the veterinary schools with heart problems had risen rapidly over the preceding 10 or so years; the numbers were now as great as for all other breeds together. Something had to be done.

Miss Fuentes and colleagues presented a report (published in Boxer 1991) describing the range of heart conditions that occur in dogs, but indicating that the specific heart condition that affects UK Boxers is aortic stenosis (AS). The stenosis (narrowing) of the aorta limits blood flow resulting in fainting, collapse or even death. The high velocity of blood passing through the narrowed valve (detected by Doppler echocardiography) is one cause of the sounds heard as heart murmurs, but other abnormalities (thickening of the heart wall, development of fibrous nodules etc) variably associated with aortic stenosis and part of the same defect also cause such sounds and can be picked up with the stethescope even in the absence of stenosis. Heart murmurs are graded on a scale of 1 to 6 for minor to very loud, with murmur-free normal dogs being graded as 0. A quite different heart condition, known as cardiomyopathy, appears to be more of a problem in American Boxers. Odd cases of cardiomyopathy have been found in the UK, as also yet another heart condition, pulmonic stenosis.

In the same year the Boxer Breed Council set up a "Heart Panel" which brought out a set of guidelines for breeding and testing for AS (published in Boxer 1992). The basic recommendations were that 1) all adult breeding stock should be tested, 2) that grade 0 and grade 1 dogs should be considered the most desirable for breeding purposes, and 3) that dogs with louder murmurs should be further investigated. A list of grade 0 and grade 1 dogs was brought out and circulated around all UK clubs for distribution to their members. This list, as presented here, has since been regularly updated.

Heart testing clinics have become a feature of many breed club shows and have increased in number each year, 7 in 1991, 8 in 1992 and 14 in 1993. The result has been that almost 1300 dogs, representing the top section of the breed, have now been examined and the means to breed selectively away from AS is now possible. The grass roots level of support within the breed for dealing with AS is quite phenomenal.

Two issues came to the fore in 1993. One issue was the recognition that the cardiologists were now scoring more rigourously than during the initial surveys, with the result that the recommended breeding policy was out of balance with the scale of the problem; the newer testing was generating too high a proportion of dogs with grade 2 murmurs (30% compared with 5%in the surveys). To counterbalance this, the policy of retesting dogs was emphasised. Retesting, not only gives a more objective score, but with the practice of accepting the best grade achieved and the natural "wobble" in the in the testing procedure, a higher "pass" rate could be expected. The second issue pertained to Doppler testing. As already indicated, Doppler detects and measures a different aspect of the condition from the stethescopic screening;

Doppler measures the degree of stenosis (narrowing), the aspect of clinical significance, the stethescope detects abnormality leading to sound (stenosis and other abnormality), the aspect advised by cardiologist to be of greater overall significance.

There is still debate at Breed Council level over the use of Doppler for the breeding control scheme, but Doppler usage has in any case almost ceased since the subject was aired in the dog press. By contrast, the practice of retesting has risen, with the result that a significant number of dogs initially scored as having grade 2 murmurs have obtained lower scores. The custom of testing dogs under one year of age is also increasing. Although puppy grade 0s and grade 1s do not qualify for inclusion on the heart list, a provisional result is clearly of use to breeders; it also provides important information on the development of AS.

Turning now to the recent findings. The distribution of dogs with the various grades of murmurs is currently as follows:-

                   DOGS           BITCHES

Grade 0    42%             43%

Grade 1    30%             31%

Grade 2    21%             20%

Grade 3     5%               5%

Grade 4     1%             <1%

Grade 5     <1%           <1%

Grade 6       0%             0%

These results take note of all retesting and represent the best overall picture we have at this time of the state of the breed in the UK with regard to AS. They also establish that heart murmurs are no more common in one sex than the other, which is an important finding. It should be noted that the above results derive exclusively from the Breed Council testing scheme which focuses on show dogs, almost all of which are likely to be clinically normal. Results from the veterinary hospitals, which in the main deal with clinically affected animals, obviously present a very different picture with all dogs having the louder murmurs and diagnosed by Doppler as having AS.

The first results from family studies were published in the Spring/Summer issue of Boxer Magazine 1993 and in the Summer issue of the British Boxer Club's Newsletter. These gave the first indications on the genetics of AS in Boxers, and suggested a relatively simple mode of inheritance. When the most informative crosses were considered a clear pattern was seen to be emerging. The risks of heart murmurs among the progeny of murmur-free dogs was lower than amongst those of dogs with murmurs; and the worse the murmurs in the parents, the greater the risks among the progeny.

The data (see table) are now substantially larger and this has allowed further sub-divisions of the crosses to be made. Although the progeny scores are affected by the more rigourous grading that has recently been imposed, the overall picture continues to be one of the "best" parents producing the "best" progeny. Only the results from the higher grade crosses shown on the bottom line do not appear to fit the pattern, but there are only 8 dogs in this group. Perhaps coincidentally, almost all of these had one grade 0 parent.

Otherwise the exceptions to the rule now stand out, and these merit further investigation. Pulmonic stenosis has been found to be responsible for the heart murmurs in at least one of the exceptional cases and the others are marked down for study.

 

This is where Doppler is proving its worth as it can clearly distinguish the two forms of stenosis.

Overall, the findings suggest that there should have no difficulty in breeding away from AS. The only problem is the high incidence of dogs with significant murmurs as these limit the scope for selection. Nevertheless, careful breeding should effectively remove Boxers from the list of AS breeds.

Shown below is a complete heart list updated to December 1993. Also provided is a copy of a handout sheet on AS which breeders can duplicate and give to puppy purchasers. The main purpose of this document is to limit the difficulties for breeders that can occur when new owners take their puppies for their first veterinary examinations. Some vets have become unduly concerned over what have ultimately proved to be transitory flow murmurs.

Bruce M Cattanach

Boxer, 1993, publ. 1994