Risk Factors Associated with Lifespan in Pet Dogs Evaluated in Primary Care Veterinary Hospitals

Abstract

The objective of this population-based retrospective cohort study was to identify factors associated with lifespan in pet dogs evaluated at primary care veterinary hospitals. Dogs ≥3 mo of age that visited any of 787 US hospitals at least twice from January 1, 2010, through December 31, 2012, were included. Survival curves were constructed for dogs by reproductive status, breed, body size, and purebreed (versus mixed-breed) status. Multivariate Cox proportional hazard regression was performed to identify factors associated with lifespan. There were 2,370,078 dogs included in the study, of whom 179,466 (7.6%) died during the study period. Mixed-breed dogs lived significantly longer than purebred dogs, and this difference was more pronounced as body size increased. Controlling for other factors, dogs of either sex had a greater hazard of death over the study follow-up period if sexually intact rather than gonadectomized. For dogs who lived to 2 yr of age, the hazard of death decreased with increasing frequency of dental scaling. Our findings support previous reports of the impact of body size and gonadectomy on lifespan and provide new evidence in support of ultrasonic dental scaling and mixed breeding.

Full text

ORIGINAL STUDIES
Risk Factors Associated with Lifespan in Pet Dogs
Evaluated in Primary Care Veterinary Hospitals
Silvan R. Urfer, DVM, Mansen Wang, PhD, MS, Mingyin Yang, BVMS, MS, Elizabeth M. Lund, DVM, MPH, PhD,
Sandra L. Lefebvre, DVM, PhD
ABSTRACT
The objective of this population-based retrospective cohort study was to identify factors associated with lifespan in pet
dogs evaluated at primary care veterinary hospitals. Dogs $3 mo of age that visited any of 787 US hospitals at least twice
from January 1, 2010, through December 31, 2012, were included. Survival curves were constructed for dogs by repro-
ductive status, breed, body size, and purebreed (versus mixed-breed) status. Multivariate Cox proportional hazard re-
gression was performed to identify factors associated with lifespan. There were 2,370,078 dogs included in the study, of
whom 179,466 (7.6%) died during the study period. Mixed-breed dogs lived significantly longer than purebred dogs, and
this difference was more pronounced as body size increased. Controlling for other factors, dogs of either sex had a greater
hazard of death over the study follow-up period if sexually intact rather than gonadectomized. For dogs who lived to 2 yr of
age, the hazard of death decreased with increasing frequency of dental scaling. Our findings support previous reports of
the impact of body size and gonadectomy on lifespan and provide new evidence in support of ultrasonic dental scaling
and mixed breeding. (J Am Anim Hosp Assoc 2019; 55:---–---. DOI 10.5326/JAAHA-MS-6763)
Introduction
Over the past 150 yr, selective breeding of dogs derived from traditional
foundation stock has led to the modern breeds, which are genetically
isolated and fairly inbred.
1
An adverse effect of this practice has been
the emergence of a wide range of breed-specific genetic diseases and
disease predispositions that are as diverse among breeds as are other
aspects of their phenotype.
2
Some diseases are less breed-specificbut
appear to be associated with certain body sizes and shapes; examples of
these include hip dysplasia or gastric dilation and volvulus, which are
more common in large-breed dogs than in smaller breeds.
3,4
It is not
surprising, consequently, that breed can also influence a dog’saging
process and, therefore, life expectancy. The general belief is that large-
breed dogs have shorter lives than small-breed dogs, with body weight
being a better predictor of life expectancy than height at the withers.
5,6
Prior studies of lifespan in dogs have typically relied on data
from dead animals and the calculation of life expectancy on the basis
of age at death.
7–9
This approach results in an underestimation of life-
span as a result of right censoring; that is, all dogs in a birth cohort who
are still alive at the end of the study period will not be included as they
should be. Such bias is obvious in longitudinal studies of individuals
after birth but becomes obscured in cross-sectional studies of dead in-
dividuals, which constitute the majority of studies on lifespan performed
to date.
10
Given that the degree of bias due to right-censored data can
vary considerably among different study populations, a comparison of
findings among studies becomes difficult, if not impossible.
11
Statistical methods exist that omit bias attributable to right-
censored data, including Kaplan-Meier estimation (product limit esti-
mator) and Cox proportional hazards regression.
10
Other methodologic
approaches have been used and various canine populations studied to
From the Department of Medicine Pathology, University of Washington,
Seattle, Washington (S.R.U.); Providence Health & Services, Renton,
Washington (M.W.); Banfield Pet Hospital, Vancouver, Washington
(M.Y.); DataDogs, LLC, Lake Oswego, Oregon (E.M.L.); and Ameri-
can Veterinary Medical Association, Schaumburg, Illinois (S.L.L.).
Correspondence:elizabeth.lund@datadogs.us (E.M.L.)
CI (confidence interval)
Accepted for publication: October 3, 2017.
ª2019 by American Animal Hospital Association JAAHA.ORG 1

estimate lifespan and elucidate causes of death, each with limitations
unique to the populations used such as insured populations or primary
care versus tertiary care patients.
12–15
The purpose of the present study was to use a primary care
practice population database of pet health records to investigate our
hypothesis that the lifespan in pet dogs in the United States would
be associated with breed status (pure versus mixed), breed, body
size, mean adult body weight (because body weight has been
proposed to be a better predictor of lifespan in dogs than height,
breed, or breed group), and reproductive status.
6
Another objective
was to use data collected on live dogs as recorded in their health
records to estimate survival curves for the most common dog
breeds and other dog groupings. In addition, because the medical
records also included information on veterinary care received, and
becauselittleevidenceexistsregardinganyinfluence of that care
onlifespan,wealsosoughttoinvestigateourhypothesisthat
lifespan would be associated with the frequency of certain veter-
inary services for which full, consistently recorded data was
available, namely veterinary visits and ultrasonic dental scaling
(under general anesthesia).
Materials and Methods
Banfield Pet Hospital represents a national network of primary care
veterinary hospitals in the United States, joined by a common
proprietary electronic medical records system. These hospitals are
visited annually by w4% of the American Veterinary Medical
Association–reported pet dog population in the United States.
16
All
patient data entered into this system is uploaded on a daily basis to
a central database, which contains the medical histories of several
million patients including information on age, breed, reproductive
status, and death date. Wellness care is a large focus of the veterinary
service provided in these hospitals (prepaid medical care plans that
include preventive care and unlimited hospital visits), intended to
promote regular visits and health monitoring.
Data Collection
Dogs considered for inclusion in this retrospective cohort study were
those evaluated between January 1, 2010, and December 31, 2012, at
any Banfield Pet Hospital. To be included in the study, dogs were
required to have had at least two hospital visits during the study
period. Dogs who died or were euthanized during this period were
required to have survived initially until at least 3 mo of age to avoid
the potential influence of death as a result of congenital problems.
Dogs were excluded when their birth date, breed, or gonadectomy
status was unknown. Dogs with an age exceeding 30 yr at euthanasia,
death, or last visit were considered to have had their birth date
inaccurately reported and were also excluded.
Data was extracted or calculated from the electronic medical
records for each dog from the point of first visit on record (which, in
many situations, was prior to January 1, 2010) through December 31,
2013 (which provided at least 1 yr of follow-up for dogs who might
have had their first visit near the end of the inclusion period on
December 31, 2012). This data included signalment variables, go-
nadectomy status, breed, body size category, mean adult body weight,
and age at last visit, euthanasia, or death as well as the veterinary service
variables frequency of veterinary visits, frequency of ultrasonic dental
scalings performed (under general anesthesia), and frequency of anal
gland expression. Mean, rather than median, body weight was chosen
because of the desire to capture the full range of body weights on
record, which the median would not have reflected. Frequencies of
veterinary services represented rates over time and were calculated as
the number of each service recorded divided by the time elapsed from
the first recorded visit to study exit. Frequency of anal gland ex-
pression, which we hypothesized a priori would not be associated with
lifespan because of a lack of biological plausibility, was included with
the intention that if it was found through multivariate analysis to have
an association with lifespan, then that finding would suggest the
presence of a source of bias in the data that had not been accounted for.
Mean adult body weight was calculated on an individual basis
from weights on record from 2 yr of age (by which point dogs of any
breed were presumed to have stopped growing on the basis of un-
published growth curve data for the hospital population) to the end
of the study period; thus, to have a value for this variable, dogs were
required to have lived for $2 yr. Because there is no peer-reviewed
standard for canine body size classification, we used the classifica-
tion system used in a lay publication involving the same patient
population.
17
That system had been derived from cutoffs identified
through internet searches of breed association and pet food man-
ufacturer websites as follows: small, ,9.1 kg (20 lb); medium, 9.1
to ,22.7 kg (20 to ,50 lb); large, 22.7 to ,40.9 kg (50 to ,90 lb);
and giant $40.9 kg (90 lb). Then, an internal standard was devel-
oped using data from all canine patients in our entire database (no
time restriction; excluding duplicate visits, dogs ,2 yr of age, and
dogs with recorded weights on file ,1lbor.300 lb), by which each
purebred breed in the entire database (485 total breeds recorded)
was assigned to one of the aforementioned body size categories on
the basis of the median population body weight for the breed.
For analyses comparing purebred and mixed-breed dogs within
body size categories, dogs whom veterinary personnel had flagged in
the record as being a mixed version of a particular breed (e.g., dogs
for whom personnel had entered a specific breed but had also
checked a mixed-breed flag option) were grouped into body size
categories corresponding with the indicated breed. For example, a
dog for whom the recorded breed was Labrador retriever (a breed
2JAAHA | 55:3 May/Jun 2019

classified as large) but with a “mixed-breed”flag in its record was
treated as a mixed-breed large dog rather than a purebred large dog.
All other mixed-breed dogs (i.e., those lacking any mention of a
pure breed designation) were excluded from these analyses.
Statistical Analysis
All statistics were computed with the aid of statistical software
a
.
Values for signalment variables such as breed, breed status (pure
versus mixed), and reproductive status are summarized as counts
and percentages. Values for veterinary service variables, such as
frequency of veterinary visits, dental scaling, or anal gland expres-
sion, are summarized as median (interquartile range), and corre-
lations among these variables were evaluated by calculation of the
Pearson correlation coefficient (r).
To provide lifespan estimates for dogs in various signalment
groupings, without controlling for other factors, Kaplan-Meier
survival curves were constructed.
18
The outcome of interest was
age at euthanasia or reported death. The endpoint of the follow-up
period was December 31, 2013. Dogs who survived to the final visit
within the study period were included in the analysis and censored
at the point of the final visit. Dogs who had visits beyond the study
endpoint were censored at December 31, 2013. Lifespan estimates
are reported as median and 95% confidence interval (CI) of the
median.
To simultaneously evaluate associations of signalment and
veterinary service variables with the hazard (or risk) of death over the
study follow-up period, multivariate Cox proportional hazards re-
gression was used.
18
Because of the large size of the data set and the
recognition that high power can result in significant but spurious
findings, as well as balancing complexity of modeling and relevance
of results, a single interaction term was included in the model be-
tween purebred (versus mixed-breed) and body size.
Model building was performed manually (rather than through
a stepwise selection process) to identify candidate models.
19
The
proportional hazard assumption was tested for each candidate
model by examination of the log-cumulative hazard plot for parallel
lines for the groups for each model, and the goodness-of-fit test was
performed.
20
The final model was chosen by comparing values of
the Akaike information criterion among candidate models. Because
of the large sample sizes and the high likelihood that a significant
effect would be detected by chance alone, only values of P,.001
were considered significant.
21
Results
Animals
A total of 2,504,518 dogs visited a Banfield Pet Hospital at least twice
between January 1, 2010, and December 31, 2012. Of these, 2,370,078
(94.6%) dogs met the inclusion criteria, including 85,630 (3.6%)
dogs who were reported as having died and 93,836 (4.0%) who were
euthanized on or before December 31, 2013, the end of the follow-up
period. A total of 787 hospitals in 43 states were represented. States
without hospitals included Hawaii, Alaska, Wyoming, North Dakota,
West Virginia, Vermont, and Maine.
Distributions of dogs by reproductive status were as follows:
sexually intact female, 230,974 (9.8%); sexually intact male, 322,958
(13.6%); spayed female, 906,252 (38.2%); and neutered male,
909,894 (38.4%). Small-sized breeds made up the largest proportion
of the study sample for which a specific breed (other than mixed) was
recorded (1,208,085; 52.8%), followed by medium-sized breeds
(545,219; 23.8%), large-sized breeds (493,539; 21.6%), and giant-
sized breeds (42,882; 1.9%). Approximately one-third (706,000;
29.8%) of dogs were recorded as mixed-breed. For these dogs, either
the breed entered in the record by veterinary personnel was mixed
(n ¼80,353) or the personnel had entered a breed other than mixed
but also selected a mixed-breed flag (n ¼625,647). The median
number of dental scalings and anal gland expressions per year for all
dogs was 0 (interquartile range, 0.2) and 0 (interquartile range, 0.4),
respectively. The median number of visits per year was 2.7 (inter-
quartile range, 3.1). The mean adult body weight was calculated for
2,289,725 (96.6%) dogs.
A total of 1,848,824 (78.0%) included dogs had been enrolled in
a wellness plan at some point in the past, and 521,254 (22.0%) dogs
had never been enrolled in such a plan. Because wellness plan en-
rollment (duration of enrollment or whether a dog had ever been
enrolled) was strongly correlated (r.0.80) with the frequencies of
dental scaling, anal gland expression, and veterinary visits, that
variable was not evaluated further. These frequency variables had a
minimal degree of correlation (r¼0.01–0.21) with each other.
Survival Estimates
Kaplan-Meier survival analysis revealed that, without controlling for
other factors, survival rates differed significantly (P,.001) by body
size category for all dogs for whom a body size could be determined
(i.e., excluding dogs for whom the recorded breed was mixed), with
giant-sized dogs having the lowest survival rate (Figure 1). The
median (95% CI) estimated lifespan of dogs in the giant, large,
medium, and small categories was 11.11 yr (11.01–11.25 yr),
13.38 yr (13.35–13.40 yr), 13.86 yr (13.82–13.89 yr), and 14.95 yr
(14.92–14.98 yr), respectively, and all estimates differed significantly
(P,.001). Mixed-breed dogs had a significantly (P,.001) higher
survival rate than purebred dogs. The median (95% CI) estimated
lifespan of mixed-breed dogs was 14.45 yr (14.42–14.49 yr) and
differed significantly (P,.001) from that of reportedly purebred
dogs, which was 14.14 yr (14.12–14.15 yr).
Factors Related to Lifespan in Pet Dogs
JAAHA.ORG 3

Gonadectomized dogs also had a significantly (P,.001) greater
survival rate than sexually intact dogs up to 15 yr of age (Figure 2).
However, beyond that point, the difference between the groups was
no longer significant (P$.07). The median (95% CI) estimated
lifespan of sexually intact females, sexually intact males, neutered
males, and spayed females was 13.77 yr (13.68–13.86 yr), 14.09 yr
(14.03–14.16 yr), 14.15 yr (14.13–14.18 yr), and 14.35 yr (14.33–
14.37 yr), respectively. For both sexes, survival rates and estimated
lifespans were significantly (P,.001) higher for gonadectomized
versus sexually intact dogs. Kaplan-Meier survival curves and
lifespan estimates were also generated for the five most common
breeds within each body size category (Figure 3, Table 1).
Multivariate Hazard Analysis
Pearson correlation analysis revealed that none of the service fre-
quency variables evaluated were strongly correlated with each other
(all values of r#0.21), so the relationship of all frequency variables
on lifespan was evaluated via multivariate proportional hazard re-
gression, controlling simultaneously for the other variables already
identified as influential (purebred status, body size, mean adult body
FIGURE 1 Survival curves and
95% CIs (shaded regions) for dogs
evaluated at US primary care veter-
inary hospitals by body size category.
Note that small standard errors result
in small (and poorly visible) CIs. CI,
confidence interval.
FIGURE 2 Survival curves and
95% CIs (shaded regions) for dogs
evaluated at US primary care veter-
inary hospitals by reproductive status.
CI, confidence interval.
4JAAHA | 55:3 May/Jun 2019

weight, and reproductive status) as well as for the interaction be-
tween purebred status and body size. The Spearman correlation
coefficient between mean body weight and body size was 0.85 and
significant (P,.001). Because of this high degree of collinearity,
model building required a choice between the two variables. Body
size, rather than mean weight, was included in the fitted model as
model fit criterion values were comparable, and body size would
lend itself to more clear interpretation and communication of re-
sults, especially for the interaction between purebred status and
body size.
All variables in the final model were significant (P,.001; Table 2).
Independent of the other factors, frequency of ultrasonic dental
scaling had a large, positive (decreased hazard) association with
lifespan that increased with increasing number of cleanings per year.
Anal gland expression had a smaller but still significant positive
association as well. Visit frequency, on the other hand, appeared to
increase the hazard of death over the study period.
Discussion
To our knowledge, the present study included the largest sample of
dogs ever used to investigate factors associated with lifespan. Use of
data from a national, primary care veterinary practice population
with standardized recordkeeping and daily record submission to a
central database yielded results that we believe can be more easily
extrapolated to the general pet dog population than results from
referral hospital or insured pet populations. A survival analysis
approach yielded more valid results than calculations of mean or
median ages at death, which fail to account for dogs still alive at the
time of those calculations, potentially resulting in an underesti-
mation of lifespan.
10
Our methodology accounted for right-
censored data and included dogs from primary care hospitals
rather than tertiary care facilities or insured populations, which
would explain the longer lifespans identified in our study than in
previous studies.
7–9,13,14,22–24
Large dogs are generally believed to have a shorter life expec-
tancy than smaller dogs, as our findings confirmed. Scientific con-
sensus has leaned toward a somewhat shorter life expectancy in
purebred dogs.
1,12
The present study offered a novel approach to
answering this question by comparing lifespan between mixed-
breed and purebred dogs within different body size groups, show-
ing that the mixed-breed advantage in lifespan was proportional to
body size, with the difference between mixed-breed and purebred
FIGURE 3 Survival curves and 95% CIs (shaded regions) for the most common (A) giant dog breeds, (B) large dog breeds, (C) medium dog
breeds, and (D) small dog breeds evaluated at US primary care veterinary hospitals. CI, confidence interval.
Factors Related to Lifespan in Pet Dogs
JAAHA.ORG 5

dogs being most pronounced for giant-sized dogs and least pro-
nounced for small-sized dogs.
It should also be noted that body size was a more important
predictor of lifespan than purebred status; mixed-breed large and
giant dogs did not live as long as purebred small-breed dogs. In-
terestingly, when the five most common breeds in all five size cat-
egories were evaluated, considerable breed differences were identified
in Kaplan-Meier survival curves and lifespan estimates. These dif-
ferences among purebred dogs of similar size belonging to different
breeds may have been attributable to breed-specific diseases, varying
degrees of inbreeding depression among breeds, or other factors.
Overall, spayed females had the longest life expectancy of all
dogs (w30 wk longer than sexually intact female dogs), suggesting a
clear and clinically important advantage to gonadectomy for female
dogs. A lesser (w3 wk difference) but still significant positive re-
lationship with gonadectomy was also identified for male dogs,
although the clinical importance of this finding is likely low. A
beneficial association of gonadectomy with lifespan has previously
been described for female dogs.
7,25,26
The usual explanation for this
effect is that the increase in lifespan reflects a lack of pregnancy- and
birth-associated hazards as well as a significant reduction in the risk
of mammary tumors and uterine infections in spayed females. In
contrast, the situation is less clear in males, for which the literature
TABLE 1
Median (95% CI of the Median) Estimated Lifespan* of Dogs of
the Five Most Common Breeds Evaluated at US Primary Care
Veterinary Hospitals Within Various Body Size Categories
Body Size Breed No. of Dogs Median (95% CI), yr
Giant Great Pyrenees 2714 11.66 (11.35‒12.10)
Cane Corso 2103 10.66 (9.40‒11.43)
Mastiff 7387 10.19 (9.92‒10.51)
Saint Bernard 3171 10.07 (9.80‒10.59)
Great Dane 10,261 9.63 (9.48‒9.85)
Large Labrador retriever 135,179 13.27 (13.22‒13.30)
Golden retriever 46,896 12.93 (12.85‒12.98)
German shepherd dog 49,789 12.46 (12.37‒12.57)
American bulldog 12,025 11.71 (11.43‒11.95)
Rottweiler 19,116 10.98 (10.85‒11.08)
Medium American cocker spaniel 33,649 14.28 (14.17‒14.37)
Beagle 39,181 14.06 (13.93‒14.14)
American pit bull terrier 95,740 12.91 (12.73‒13.04)
Boxer 53,257 11.22 (11.14‒11.30)
English bulldog 26,144 11.16 (10.86‒11.28)
Small Dachshund 62,372 15.20 (15.10‒15.29)
Shih tzu 109,559 15.08 (14.99‒15.15)
Chihuahua 144,169 15.01 (14.89‒15.11)
Maltese 46772 14.70 (14.58‒14.88)
Yorkshire terrier 110,706 14.62 (14.51‒14.72)
* The displayed data represents the results of Kaplan-Meier survival analysis,
without controlling for other variables.
CI, confidence interval.
TABLE 2
Selected Hazard Ratios Estimated Through Multivariate Propor-
tional Hazards Regression Analysis for Dogs (n ¼345,393) Eval-
uated at US Primary Care Veterinary Hospitals
Variable HR 95% CI
Visit frequency (No. of visits/yr) 1.095 1.092–1.099
Dental scaling frequency (No. of scalings/yr) 0.817 0.794–0.841
Anal gland expression frequency (No. of expressions/yr) 0.916 0.907–0.925
Comparisons of reproductive status
F vs SF 1.468 1.388–1.552
M vs CM 1.059 1.014–1.106
CM vs SF 1.092 1.071–1.114
F vs M 1.270 1.187–1.358
Comparisons of purebred vs mixed-breed dogs by body size category
Giant 2.557 2.098–3.116
Large 1.487 1.429–1.547
Medium 1.356 1.298–1.417
Small 1.160 1.120–1.202
Comparisons of body size categories for purebred dogs
Giant vs large 3.024 2.770–3.302
Giant vs medium 4.093 3.750–4.466
Giant vs small 6.527 5.989–7.112
Large vs medium 1.353 1.310–1.398
Large vs small 2.158 2.097–2.221
Medium vs small 1.595 1.553–1.637
Comparisons of body size categories for mixed-breed dogs
Giant vs large 1.759 1.467–2.109
Giant vs medium 2.170 1.807–2.606
Giant vs small 2.961 2.468–3.552
Large vs medium 1.234 1.175–1.296
Large vs small 1.684 1.609–1.762
Medium vs small 1.364 1.298–1.434
HRs represent the hazard (or risk) of death over the study follow-up period. For
signalment variables, an HR .1or,1 with a 95% CI that excludes the value 1
represents a significantly increased or decreased hazard, respectively, for the first
stated group relative to the second stated group. For veterinary service variables,
such hazard ratios represent the increase or decrease, respectively, in the hazard
of death for each 1 unit increase in frequency. Although body size category,
purebred status, and the interaction between these two variables were included
in this model, and all three terms were significant, only the odds ratios for the
interaction terms are reported here because they were deemed more clinically
informative.
CI, confidence interval; CM, castrated male; F, sexually intact female; HR, hazard
ratio; M, sexually intact male; SF, spayed female.
6JAAHA | 55:3 May/Jun 2019

is scant. Hoffman et al. evaluated the association of gonadectomy
with canine lifespan and found a protective association for both
sexes in a teaching hospital population.
13
This was also found to be
true for gonadectomy in our primary care practice population, with
significant associations with lifespan identified in both sexes and
females having the greater advantage.
Studies of the impact of veterinary services on patient longevity
are theoretically difficult to perform in a prospective or experimental
manner, particularly in pets versus research animals, because of the
challenge in ensuring uniformity of care within the treatment group
and the ethical concerns that would arise when attempting to es-
tablish a control group that lacks such services. To our knowledge, the
influence on lifespan of veterinary visit, vaccination, prophylactic
deworming, or dental cleaning frequency has not been investigated in
pet dogs, although such practices are strongly advocated by pro-
fessional veterinary associations such as the AAHA.
27
Three interesting findings arose from the multivariate modeling
that controlled for factors associated with lifespan in dogs. The first was
that increasing visit frequency increased the hazard of death over the
study period. A supposition made when these variables were chosen
for the study was that visit frequency could be used as an indicator of
general level of care provided by owners that was independent of dental
scaling frequency. Given the increasing focus on preventive medicine
within the veterinary profession, one would expect that the more a
dog visited the veterinarian, the sooner disease could be detected
and treated, thereby extending its lifespan. However, given the results
reported here, our interpretation would be that visit frequency in-
creases with increasing health problems that could portend a decrease
in lifespan. We believe this explanation likely to be valid given that, in
our experience analyzing health data of the source population, the risk
of illness diagnosis also increases with increasing visit frequency, which
we attribute to the increased probability of disease detection.
Frequency of ultrasonic dental scaling was selected as a variable
in our analysis because the data could be more easily and reliably
extracted from the electronic medical records history than other
aspects of preventive care history such as vaccination or deworming.
In addition, the techniques and equipment used for dental scaling
and anal gland expression remained fairly constant over the study
period, whereas other services changed in both manner of recording
and manufacturers. The magnitude of the beneficial association of
dental scaling frequency with lifespan (hazard ratio ¼0.817)
identified in the study was surprising. When comparing two dogs,
all other factors being equal, a difference of one dental cleaning per
year is associated with a nearly 20% decreased risk for death.
However, this finding cannot be interpreted as indicating that dental
scaling directly increases lifespan in dogs because of the observa-
tional nature of the study design. Indeed, dental scaling may be in
whole or in part a surrogate indicator for some other factor such as
general degree of veterinary (or owner) care dogs receive, owner
attachment, or dog health status. One might also argue that dogs
who live longer have more opportunity to have their teeth cleaned
than dogs who die earlier and therefore that scaling has no associ-
ation; however, we believe this possibility was controlled for through
the use of dental scaling frequency rate (number of scalings per year
over the dog’s lifespan) rather than a simple count of scalings in the
modeling process.
Frequency of anal gland expression was chosen as a control
variable because of our a priori hypothesis that it would not be as-
sociated with lifespan. It was therefore surprising that this variable also
had a beneficial association with lifespan, although the magnitude of
the effect was far smaller (hazard ratio ¼0.916) than for dental scaling
frequency. Again, as for dental scaling, frequency of anal gland ex-
pression may have been a reflection of general care provided or other
unidentified factors. Because the correlation between frequencies of
anal gland expression and dental scaling was weak (r¼0.22), we
believe the overlap between the actual factors underlying the observed
beneficial association for both variables was likely small.
In the present study, potential for misclassification of purebred
versus mixed-breed status existed because either the dogs’owners
or the veterinary personnel assessed and recorded a specific breed
(including an option of mixed-breed) and could have assigned a
mixed-breed flag in the record if indicated. Given the high pro-
portion of study dogs who were deemed purebred by this method
(71.2%) and therefore treated as such in the study, it is likely that
some of the reportedly purebred dogs were actually of mixed
breeding. We believe this limitation should not impact generaliza-
tion of our findings to the US pet dog population; however, the
findings may be less generalizable to confirmed purebred dogs.
A unique feature of the data used in this study is that a large
number of dogs remained alive at the end of the study period, leading
to right censoring of a high percentage (92.4%) of individuals;
nonetheless, the Kaplan-Meier and Cox proportional hazards tech-
niques that we used take the bias introduced by right censoring into
account.
28
An uncommon but potential problem was the large
number of dogs, which could have led to statistically significant, but
clinically or biologically negligible, associations attributable to the
high power to detect a difference in lifespan between groups. For this
reason, we focused solely on results pertinent to tests of hypotheses
established a priori rather than report incidental findings. We believe
the mentioned limitations had minimal impact on the major study
findings that a significant lifespan advantage exists of small-sized over
large-sized, neutered female over sexually intact female, neutered
male over sexually intact male, and mixed-breed over purebred dogs
as well as dogs who undergo dental scaling or anal gland expression
Factors Related to Lifespan in Pet Dogs
JAAHA.ORG 7

over those who do not. Additionally, mixed-breed dogs have a life-
span advantage over purebred dogs that is proportional to body size.
Conclusion
Although additional research is necessary to elucidate the specific
mechanisms underlying the differences in lifespan by purebred
status, breed size, and body weight, the knowledge of these dif-
ferences as well as the association with gonadectomy and other
veterinary services can assist practitioners in counseling dog
owners about gonadectomy, pure versus mixed breeding, and life
expectancy.
FOOTNOTES
a
SAS, version 9.3; SAS Institute Inc., Cary, North Carolina
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8JAAHA | 55:3 May/Jun 2019