How long will my dog live? – cautionary comments by Dr Brenda Bonnett

I am grateful to Dr Brenda Bonnett for the following comments on my blog post “How long will my dog live?“. Brenda is an epidemiologist and CEO of the International Partnership for Dogs.

Thanks for an informative overview of this paper.  As an epidemiologist, however, I think a few cautions should be added.  Owner-reported mortality frequently lists ‘old-age’ as a cause of death, whereas this is essentially never listed by veterinarians or clinical data.  To me, what that designation means is that the dog achieved an age that approximated the owner’s expectation, and of course that would vary across breeds and owners.  Where breeds have a lower median age at death, owners might be more likely to pursue veterinary care, thereby resulting in a diagnosis.  This could confuse a comparison across breeds.  Not to discount the information, but just to suggest a little caution in its interpretation. 

A more important caution is in the use of proportional mortality.  As this value does not account for the actual rate of death or the base population, using it to compare across breeds is very risky. Let me give you one example from Swedish insurance statistics on deaths before 10 years of age in a population of millions of dog-years-at-risk, and reflecting almost 40% of all dogs in Sweden.  Bernese Mountain Dogs, Flat-coated Retrievers and Golden retrievers all had as their #1 cause of death Lymphosarcoma (a type of cancer).  So, if one had looked at proportional mortality, that was the highest cause.  On that basis, on might be tempted to say that these breeds were similar for this cause of death.  However, when one looks at the rates of death, and compares them as relative risk, the picture is very different as seen in the graphic below.  Essentially, very few Goldens died before 10 years of age, but among those who did, lymposarcoma was the most likely cause. It is fairly obvious that the importance of cancer as a cause of death before 10 years of age is very different for these breeds.  So – proportional mortality can be useful within a breed, but it is very dangerous to use it to compare across breeds.

Again – not to detract from the useful information in the paper, just to caution us all on how we apply it.

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