Age patterns of death are informative both within and across breeds. This paper presents comprehensive information on mortality by including individual breed patterns of death for overall mortality and by category-specific causes. Survival estimates are clinically useful information within a breed or for contrasting across breeds. Finally, multivariable models quantify the relative rate between breeds adjusted for effects of age and gender for different causes of death. Most of the age effects are, presumably, largely biological, however the model also adjusts for potential differences in age distributions between breeds that might be due to differences in enrolment in insurance or other factors. Although this study presents appropriate quantitative measures, the output can be somewhat overwhelming. Exhaustive presentation of relative rates of death for every breed, both genders and at all ages would be beyond the scope of a research publication. Therefore, estimates and standard errors from the modelling procedures have been presented to allow readers to calculate specific MRRs for any gender-age-breed combination in which they are interested. Graphic figures have been included to provide a more obvious display of some relationships across breeds.
In addition to differences between breeds, it was shown in the companion paper that for some conditions there was a statistically significant effect of gender . For example, males tended to have higher MRs for trauma and females had higher rates of cancer, except in the Bernese mountain dog, where males had twice the rate of tumours. Such complex relationships can be examined using statistical modelling. As the goal of this study was to produce clinically relevant statistics on death in dogs, a practical and parsimonious approach to modelling was taken.
Cox regression was used for the survival analysis as an appropriate technique to model time to death. Multivariable Cox regression models were initially attempted but proved unmanageable in terms of computer and time constraints, therefore Poisson regression was used for the calculation of MRRs, allowing inclusion of a primary baseline category of all other breeds.
Exploring the fit of the models several large residuals were found, mainly for low predicted values, however this was not related to low age categories. Also, the model fit was worst in the total and diagnostic mortality models (data not shown), likely partly because these contained larger numerators, and accordingly by chance some more deaths will be ill-fitting (and more deaths will be well-fitting). Further, all types of deaths were included in the outcomes and, as only a few variables were available to model, it is likely that important predictors of deaths were not included, thus limiting the fit of the model.
Unfortunately over-dispersion could not be evaluated because multiple records belonged to each dog, as the dogs moved through the age categories. However, in previous work over-dispersion has never been a problem working with the Agria insurance database (, in press, unpublished observations).
Age patterns and survival
Different breeds age at different speed, however in many breeds the mortality starts to increase after about six to seven years of age, similar to findings of increased morbidity and mortality seen in previous work by these authors . Superficially, this may be seen as supporting anecdotal suggestions that 'geriatric profiling' be started on all breeds at seven years of age. However, this increase must be considered together with the magnitude of mortality at specific ages in individual breeds of dogs. There is no accepted mortality level at which we designate dogs to be geriatric, or even senior. However, considering that for some breeds over 50% of dogs are dead by eight years of age (e.g. Irish wolfhounds, St. Bernard, great Dane) whereas in other breeds less than 25% have died (Labrador and golden retrievers, poodle, miniature dachshund), it is inappropriate to consider them as having equivalent biological age or similar risk of age-related diseases that might be detected by screening programs.
For tumours, the mortality rate starts to increase after six or seven years in Labrador retrievers and mongrels, although it is still below 200 deaths per 10,000 DYAR. In Bernese mountain dogs tumour deaths are common starting by four years of age and account for over 2,000 deaths per 10,000 DYAR by eight years of age. For boxers there is a sharp increase to very high levels after the age of six years. These findings highlight the probability that some previously published breed risk estimates are misleading. Proportional estimates of death from data sources with no population at risk (e.g. postmortem registries) have likely made erroneous conclusions about cancer risks across breeds .
The age patterns are reflected in the survival statistics and highlight differences across breeds. For Cavalier King Charles spaniels there is a very low early mortality with a sharp rise after five years of age. Only 7% of Cavalier King Charles spaniels are dead by five years of age, but 48% are dead by 10 years. This is in contrast to a breed like the German shepherd, where the early mortality is high (20% mortality by five years) but by 10 years of age the survival (49%) is quite similar to the Cavalier King Charles spaniel. Boxers have relatively low early mortality – one in nine will die before five years of age. One in five will die between five and eight and of those alive at eight, one-third will die by 10.
Information on the average survival pattern for different breeds is useful clinical information. Prospective owners should understand that in getting, for example, either a Bernese mountain dogs or a golden retriever, the likelihood of the dog living past 10 years of age is very different. Most owners, once their dog becomes seriously ill or dies, are keen to know if the condition is rare or common, in general or in their breed. Many owners are especially concerned if they feel their dog has died prematurely, that is at a younger than average age. Veterinarians and their clients can also use survival statistics as part of informed decision-making regarding expensive veterinary care and for conditions in which quality of life is an issue. For example, an Irish wolfhound that has survived to eight years has a 76% chance of being dead before 10, whereas an eight-year old golden retriever has a 90% chance of surviving to 10.
Prospective owners may wish to consider breed-specific patterns of death or disease before choosing a breed. Owner preferences and expectations about, for example, longevity, may influence their satisfaction with a chosen pet . For health management at the individual dog level, it is also useful for veterinarians to have quantified estimates of the increased risk certain breeds have for certain conditions. These findings inform diagnostic decision-making and facilitate communication with owners.
The MRRs highlight both the relative magnitude and breed differences for total and diagnostic mortality. Differences between the total and diagnostic mortality rates within breeds were discussed in the companion paper . Elective euthanasia for behaviour problems or dog-owner mismatch does not get reimbursed. Total mortality statistics probably best reflect overall mortality across breeds. However, for examining disease conditions as causes of death, the diagnostic mortality statistics are more informative, even though these are only available for the subset of dogs for which claims have been submitted.
Risk of tumour, in general was higher in males (MRR < 1), however the interaction term (age-gender, > 1) tells us that older females were at higher risk of tumours. This is likely due to the high rate of mammary tumours in females ( in press). Most interesting in the tumour model was the interaction term for Bernese mountain dogs. Overall this breed is at a much greater risk of dying from tumour than the baseline breeds and the interaction term tells us that the effect at older ages is the opposite in the Bernese mountain dog compared to other breeds, i.e. at older ages males have a higher risk of death from tumours than females. Note that confidence intervals are not included in figures 3, 4, 5.
Dachshunds (both breed groups) and poodles were at decreased risk of death due to tumours, as were Cavalier King Charles spaniels. One explanation might be a lower biological age, for example, in poodles. As only one quarter of poodles die before 10, poodles in this study population must be considered to have not even reached 'middle age'.
Traumatic deaths were more common in males and at younger ages (although there was no age-gender interaction). As was discussed in the companion paper, drevers had a high rate of death due to traumatic causes, and that was reflected in the multivariable models, where they were at 4.5 times increased risk than the baseline group. The risk for dachshunds and greyhounds were also high, although from different causes, i.e. mainly car accidents for the former and a great deal of fractures in the latter (data not shown). Interestingly German shepherds, Cavalier King Charles spaniels, Labradors, springer spaniels and golden retrievers had a significantly reduced risk of death due to trauma compared to baseline breeds. Reasons for this are unknown, but could relate to training and use of these animals and behavioural characteristics of the dogs or their owners.
Golden retrievers were at low risk for mortality in this study – only 22% died before 10 years. Golden retrievers were significantly less likely to die of trauma and heart disease and were in the baseline (average) risk group for neurological and tumour causes of death. They were at increased risk in the first age category for locomotor problems, but this effect waned with age as demonstrated by a negative age-breed interaction.
Overall, males were at increased risk and risk increased with age for locomotor causes of death. The interaction term indicates that the difference between males and females was more pronounced at higher ages. Many breeds were at increased risk of death from locomotor problems, but springer spaniels, mongrels and poodles had a significantly decreased risk. In five breeds (table 3) there were negative age-breed interactions. In four of these the risk in these breeds were pronounced at early age, but the risk waned with age. However, in the miniature dachshund, the risk was similar to baseline at low ages and less than baseline in older ages. Females were at reduced risk of dying from heart diseases compared to males. Risk increased quite sharply with increasing age. Several breeds had markedly increased risk of death due to heart causes, for example, Irish wolfhounds and great Danes were at 29 and 21 times increased risk, respectively, compared to baseline breeds. The Cavalier King Charles spaniel had a low MRR of 3, but also a positive age-breed interaction. This made the MRR in the first age category low, 1.7 and 3.0 for females and males respectively, but high, 446 and 816 in the last age category (fig 5). The miniature breeds (poodle and dachshund), the two retriever breeds and mongrels were at decreased risk.
The risk of death due to neurologic causes increased with age and males were 1.4 times more likely to die from neurologic causes compared to females. There were significant age-breed interactions in two breeds, boxers and German shepherds. Both were positive. For example, in boxers the risk in the lowest age category was not significantly different from baseline, but with age the MRR increased to 13.5 and 19 in females and males respectively (using the formulas from the part of materials and methods). In baseline breeds these MRR were 1.6 and 2.3 and females and males respectively. Further, St. Bernards and great Danes were 3.5 to five times more likely to die from neurological causes, and both regular and miniature dachshunds were over three times less likely to die of neurologic causes than other breeds (baseline). Labradors were also significantly less likely to die of neurologic causes than baseline breeds. As was seen and discussed in the companion paper, a large proportion of neurologic deaths were due to epileptic disease.