Age pattern of mortality in eight breeds of insured dogs in Sweden

The objective of this study was to use several methods to describe the age patterns for risk of death in selected breeds of dogs insured for life in a Swedish animal-insurance company in 1996. Data on eight breeds were analyzed for age at death (including euthanasia). If dogs left the insurance for reasons other than death, they were regarded as censored. Dogs were only insured up to 10 years of age. Four analytical approaches were used. First, descriptive statistics of age distributions (e.g. breed-specific median ages at death, breed- and age-specific mortality risks) were computed. Second, age-specific estimates of survival were calculated using the formula: survival=(1-risk(age<1 year))(1-risk(age 1<2 year))... (1-risk(age 9<1 0 year)). Third, Cox regression (proportional-hazards model) was used to estimate survival and hazard functions. Finally, hierarchically coded Poisson regression was used to determine age-specific cut-points in the risk of death. The hazards from Cox and the incidence-density rates from the hierarchically coded models were transformed to estimates of risk: risk=1-exp¿-(hazard)¿ or 1-exp¿-(incidence-density rate)¿. The breeds studied were Beagle, Bernese mountain dog, Boxer, Cavalier King Charles spaniel, Drever, German shepherd dog, Mongrel and Poodle, together representing over 50000 dogs each year. The yearly breed-specific mortality risk varied between 1.7% (Poodle) and 6.5% (Bernese mountain dog). In all breeds, the risk of death increased with age but the pattern varied by breed. The probability of survival at 5 years of age varied between 94% (Cavalier King Charles spaniel and Poodle) and 83% (Bernese mountain dog, Drever, German shepherd dog) and the survival at 10 years between 83% (Poodle) and 30% (Bernese mountain dog). The survival estimates from Cox and those derived using the combined-risk formula were similar. The cut-point risk estimates provided a simplified picture of when the risk of death changed significantly compared to previous age categories. As anticipated, breeds differed widely in survival up to 10 years of age and there were marked differences in age patterns of mortality. The implications of these findings should be considered in multivariable analyses, where the confounding effect of age is often controlled for using a single age variable common to several breeds.