WHAT IS KNOWN
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Dog ownership has been associated with decreased cardiovascular risk. A series of studies has suggested associations of dog ownership with lower blood pressure levels, improved lipid profile, and diminished sympathetic responses to stress.
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The evidence regarding dog ownership and mortality has yielded conflicting results. Whereas the association between dog ownership and mortality has been explored since the 1980 s, living in a home with a dog has been associated with improved survival in some studies with others arguing a neutral effect .
WHAT THE STUDY ADDS
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Pooling the data of 3 837 005 participants, dog ownership was associated with a 24% risk reduction for all-cause mortality as compared to nonownership (relative risk, 0. 76; 95 % CI, 0. 67 – 0. 86).
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In analyses of studies evaluating cardiovascular mortality, dog ownership conferred a 31% risk reduction for cardiovascular death (relative risk, 0. 69; 95 % CI, 0. 67 – 0. 71;I(2) *****************, 5.1%).
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Dog ownership is associated with lower risk of death over the long term, which is possibly driven by a reduction in cardiovascular mortality. These results hold implications for future studies on lifestyle interventions.
Introduction
See Editorial byKazi
Animal companionship, in particular canine, has become increasingly common. Half of the households in the United Kingdom own pets,1and the most recent National Pet Owners Survey2demonstrated that 68% of US households own a pet, with 48% owning dogs1 dogs. The recognized health benefits of dog ownership include reduced risk of asthma and allergic rhinitis in children exposed to pets during early ages,(3)improvement in wellbeing and alleviation of social isolation in elderly individuals,(4)and increased physical activity.(5) , (6)Notably the main positive impact of dog ownership seems to be in relation to cardiovascular risk as a series of studies in recent years have suggested an association of dog ownership with lower blood pressure levels,(7)improved lipid profile,(8)and diminished sympathetic responses to stress.(9)The recognition of potential benefits of the animal -human interaction in cardiovascular disease has been reflected in a scientific statement by the American Heart Association in 2013 which concluded that “dog ownership is probably associated with decreased cardiovascular risk.” ”10
Conversely, the evidence for impact of dog ownership on mortality has been conflicting. Whereas the association between dog ownership and mortality has been explored since the 1980 s, living in a home with a dog has been associated with longer survival in some studies,11 – 13whereas others have reported a neutral effect.,15Because there is a lack of randomized clinical trials evaluating the impact of dog ownership for cardiovascular disease prevention, a robust evaluation of observational data is necessary. Thus, we performed a systematic review and meta-analysis to evaluate the association of dog ownership with all-cause mortality and cardiovascular mortality.
Methods
This systematic review and meta-analysis is reported in accordance with the Meta-analysis of Observational Studies in Epidemiology16guidelines and was registered at International Prospective Register of Systematic Reviews. The data that support the findings of this study are available from the corresponding author on reasonable request.
Search Strategy and Selection Criteria
To perform a systematic review of the published literature, we selected relevant studies published between 1950 and May 24, 2019, by searching Embase and PubMed. We used the combined terms “Dog ownership” or “Pet ownership.” The complete search used for PubMed was (“Dog ownership” OR “Pet ownership” [text]). All potentially eligible studies were considered for review, regardless of the primary outcome or language. A manual search was also performed, using references of key articles published in English (Figure 1).
(Figure 1. Flow diagram of the literature search to identify observational studies evaluating dog ownership and all-cause / cardiovascular mortality.
For this meta-analysis of observational studies evaluating the impact of dog ownership on in cident deaths from all-cause or deaths from cardiovascular disease, studies were considered eligible for inclusion if they (1) were conducted in adults aged ≥ 18 years; (2) presented original data of prospective observational studies; (3) evaluated dog ownership at baseline; and (4) reported all-cause mortality or cardiovascular mortality. We compared the number of deaths in dog owners (exposed group) to the number of deaths in individuals who do not own dogs (control group). When studies presented data on pet ownership, individuals who do not own pets were considered the control group. Studies reporting data for pet ownership without presenting data for exclusive dog ownership were included if the majority of pet owners consisted of dog owners. Exclusion criteria were as follows: (1) retrospective studies; (2) studies that did not provide any source of absolute number of events in each group; and (3) studies that only reported composite outcome that included cardiovascular nonfatal events.
Data Extraction and Quality Assessment
Two independent investigators (Dr Kramer and S. Mehmood) reviewed study titles and abstracts, and studies that satisfied the inclusion criteria were retrieved for full-text evaluation. Studies selected for detailed analysis were analyzed by 2 investigators with an agreement value (κ) of 96 .5%, and data extraction was also performed in duplicate. Disagreements were resolved by a third investigator (R. Suen). We extracted the following data from each selected study: total number of participants, mean age, study duration as median or mean follow-up duration as described by each study, percentage of dog owners within the pet owners’ population (Table 1),11 – 15,– 21and number of deaths from any reason and cardiovascular deaths in dog owners and non – dog owners. Unadjusted estimates were extracted, enabling inclusion of the maximum number of studies.
| Author, Year | Country | Sample Size | Population | Age (y) | Duration of Follow-Up (y) | % Dog Owners Among Pet Owners / Overall | Mortality Outcome |
Male (%) |
|---|---|---|---|---|---|---|---|---|
| Friedmann et al,111980 | United States | 92 | Participants postacute coronary syndrome | 100 / 43 4 | All-cause (14) | 69 .5 | ||
| Friedmann et al,121995 | United States | 396 | Participants postmyocardial infarction enrolled in cardiac arrhythmia trials | 62 8 | 100 / 21 .9 | 1. | ||
| Gillum et al,142010 | United States | 5903 | Participants of the NHANES III study older than 40 | >40 | 100 / (9) (All-cause) 1624) | 46 | ||
| Friedmann et al,172011 | Australia, Canada, New Zealand, and United States | 460 | Participants postmyocardial infarction enrolled in a trial evaluating the impact of automa ted external defibrillator | 61 .0 | 2.8 * | 60 / 34 .6 | All-cause (17) | 85 .2 |
| Ogechi et al,182016 | United States | 3964 | Participants of the NHANES III study without major illness | 63 .1 | 12 8† | 100 / 22 | 47 .5 | |
| Chowdhury et al,132017 | 6083 | 71 6 | 10 .9‡ | 60 / 21 | 49 .5 | |||
| Swedish residents | 57 | 12 .0* | 100 / 13 1 | 47 .7 | ||||
| Torske et al,202017 | 53 418 | Participants of the HUNT study | (3) | 18 .5‡ | 100 / 20 | 53 .9 | ||
| Ding et al,152018 | 59 352 | English National Death Registry | 46 .5 | 11 5† | 100 / 28 .8 | 5) | ||
| Sorensen et al,212018 | 275 184 | Danish National Registry | 76 .6 | 22* | 100 / 8 | All-cause (45 864) | 49 .8 |
The Newcastle-Ot tawa Scale for assessing quality of observational studies in meta-analysis was applied (Table 2).22The Newcastle-Ottawa Scale conta ins 8 items, categorized into 3 domains including selection, comparability, and exposure. For each item, a series of response options is provided. A star system is used to enable semiquantitative assessment of study quality, such that the highest quality studies are awarded a maximum of 1 star for each item, with the exception of the comparability domain which allows the assignment of 2 stars. As such, the Newcastle-Ottawa Scale ranges between 0 to 9 stars.23
| Study Reference (Author, Year) | Selection (0–4 Stars) | Comparability (0–2 Stars) | Outcome (0–3 Stars) |
|---|---|---|---|
| Friedmann et al,111980 | * | *** | |
| Friedmann et al,121995 | *** | * | *** |
| Gillum et al,142010 | **** | * | *** |
| Fried mann et al,172011 | *** | * | |
| Ogechi et al,182016 | **** | * | *** |
| Chowdhury et al,132017 | **** | * | *** |
| Mubanga et al,192017 | **** | * | *** |
| Torske et al,202017 | **** | * | *** |
| Ding et al,152018 | **** | * | *** |
| Sorensen et al,212018 | **** | * | *** |
Data Synthesis and Analysis
An overall relative risk (RR) was calculated to assess the impact of dog ownership in all-cause mortality and cardiovascular mortality. We calculated pooled estimates of the relative risk by using a random-effects model (DerSimonian-Laird method) to adequately account for the additional uncertainty associated with study-to-study variability where each study weight are estimated based on individual study sample sizes and between -study variability. The CochranQtest was used to evaluate heterogeneity between studies, with threshold (P0.1 indicating statistically significant heterogeneity. We also evaluated the magnitude of the heterogeneity between studies usingI(2) testing, with values>50% considered indicative of high heterogeneity.24We explored heterogeneity between studies using 2 strategies. First, we reran the meta-analysis removing each study one at a time to determine whether a particular study accounted for the heterogeneity. Second, we performed sensitivity analyzes to assess subgroups of studies most likely to yield valid estimates of the exposure based on clinically relevant study characteristics such as characteristics of study population (Figure 2)11 – 15,17,19 – 21and duration of follow-up (Figure 3).13,15,19 – 21In addition, recognizing that dog ownership may impact incident death by mainly affecting cardiovascular death, we performed analyses that were restricted to studies reporting data for fatal cardiovascular events ( (Figure 4) .13,15,18,19
Figure 2.Meta-analysis of the association between dog ownership and the risk of all-cause mortality.11 – 15, 17 –I2is a measure of heterogeneity between studies, whereI2>50% indicates significant heterogeneity. RR indicates relative risk.
Figure 3.Meta-analysis of the association between dog ownership and the risk of all-cause mortality.13, 15, 19 – 21I2is a measure of heterogeneity between studies, where (I)(2)>50% indicates significant heterogeneity. RR indicates relative risk.
Figure 4.Meta- analysis of the association between dog ownership and the risk of cardiovascular mortality.13, 15, 18, 19I2is a measure of heterogeneity between studies, where (I) (2)>50 % indicates significant heterogeneity. RR indicates relative risk.
Publication bias was assessed by funnel plot of effect size against SE for every study. Funnel plot asymmetry was tested using Harbord-Egger test, with significant publication bias defined byP (Data Supplement). All analyzes were performed using Stata 11 .0 (Stata Corp, College Station, TX).
Role of Funding Source
The study was supported by intramural funds, with no commercial entity involved. The funding source had no role in study design, data collection, data analysis, data interpretation, or writing of the report. The corresponding author (Dr Kramer) had full access to all the data in the study and had final responsibility for the decision to submit for publication.
We identified 1155 studies through electronic searches and 1 study by manual search. Of these, 1141 were excluded on the basis of title and abstract, leaving 15 studies for f urther assessment Figure 1) ). Ten studies met our inclusion criteria11 – 15,17 – 21and were included in the analysis, providing data on 3 837 (participants) 530 515 deaths; mean duration of follow-up ranging from 1 to 22 years).Table 1summarizes the included studies, which were published from 1980 to 2018. The mean study-specific duration of follow-up varied from 1.0 to 22 .0 years. (Table 2) demonstrates study quality. In accordance with the Newcastle-Ottawa Scale quality assessment scale for cohort studies, the majority of studies achieved at least 7 stars, indicative of overall good quality. However, these observational studies received 1 star for comparability as the study population was not controlled for additional confounders.
In the main analyzes (n=3) 041 participants), dog ownership was associated with a 24% risk reduction for all-cause mortality as compared to non – dog ownership (RR, 0. 76; 95 % CI, 0. 67 – 0. 86; Figure 2).11 – 15,17,19 – 21This association was significant in the context of population-based studies (RR, 0. 78; 95 % CI, 0. 68 – 0. 88) as well as in the context of individuals with prior coronary events ( RR, 0. 35; 95 % CI, 0. 17 – 0. 69; Figure 2).11 – 15,17,19 – 21In the overall analyzes, 6 studies demonstrated significant decrease in the risk of death associated with dog ownership. However, there was significant heterogeneity in the individual estimates of the magnitude of the association (I(2)=6%,PP=0. 94; Figure I in theData Supplement). In an attempt to explore the heterogeneity between studies, we reran the meta-analysis removing each study one at a time but found that no particular study accounted for the heterogeneity. In addition, subgroup analyzes excluding the studies with the shortest duration of follow-up (13,15,19 – 21Noteworthy, the high heterogeneity was eliminated in the analyses restricted to studies evaluating individuals with prior cardiovascular disease ( (I)(2)=0%,P=0. 52;Figure 2)11 – 15,17,19 – 21and in the meta-analyses of cardiovascular events described below.
To evaluate the impact of dog ownership on the risk of death in the setting of cardiovascular disease, we performed analyzes including only the 4 studies13,15,18,19that reported data on fatal cardiovascular events and demonstrated that dog ownership conferred a 31% risk reduction for death due to cardiovascular disease as compared to non – dog ownership (RR, 0. 69; 95 % CI, 0. 67 – 0. 71) with no significant heterogeneityI(2) 5.1%;Figure 4)13,15,18,19
Discussion
In this meta-analysis involving over 3 million participants, we demonstrate that dog ownership is associated with a 24% risk reduction in overall mortality over long-term follow-up. Importantly, the risk reduction of mortality conferred by dog companionship is possibly driven by a reduction in cardiovascular death as observed in our results restricted to participants postacute coronary syndrome (65% risk reduction) and data exclusive to cardiovascular mortality (31% risk reduction).
The results of our meta-analyses are consistent with previous reports demonstrating an association between dog ownership and lower cardiovascular risk.(7-9)Specifically, an Australian epidemiological study of 5741 participants found that dog owners had lower systolic blood pressure, reaching 2 mm Hg difference, and plasma triglycerides as compared to nondog owners.(8)Notably, this association was independent of smoking status, diet, body mass index, or socioeconomic status.(8)
Similar association was demonstrated in the Rancho Bernardo Study , a community-based cohort study, although the significance of this association did not persist after adjustment for age, sex, body mass index, beta-blocker use, exercise, and use of antihypertensives.(7)In another report, a cross-sectional survey of 916 individuals, dog owners who walked their dogs were less likely to report dyslipidemia and diabetes mellitus than non – dog owners.25In parallel to these findings, previous reports also demonstrated a beneficial association between pet ownership and autonomic function or cardiovascular reactivity to stress, with lower resting heart rate and blunted blood pressure responses to stress observed in pet owners as compared to non – pet owners.9,26
Although there is a lack of randomized clinical trials evaluating the impact of dog companionship on cardiovascular risk factors and cardiovascular reactivity over long term follow-up, short-term interventional studies have yielded positive results. An interventional study of 48 hypertensive individuals randomized to acquire a pet demonstrated a diminished response to mental stress at 6 months in pet owners as compared to non – pet owners.27In addition, among dog owners, the act of petting a dog has been observed to lower blood pressure levels as compared to other activities,28supporting the positive impact of dog companionship in reducing cardiovascular reactivity. The acute effect of living with a dog on cardiovascular responses to stress may be particularly relevant for individuals post – acute ischemic heart events. In this context, the results of these previous studies are consistent with the 65% risk reduction for mortality conferred by dog ownership following myocardial infarction and 31% risk reduction of cardiovascular mortality observed in our analyzes.
A possible mechanism for the longer survival associated with dog ownership is through augmented physical activity provided by dog walking. A meta-analysis of 11 studies demonstrated that dog owners walk significantly more and are more likely to achieve the recommended level of physical activity than nonowners.(5)Reinforcing these findings, 2 reports noted an increase in self-reported physical activity after adopting a29,30In addition, having a dog was shown to support maintenance of physical activity in older English adults during poor weather.31The importance of attaining the recommended level of physical activity for overall health is highlighted by a recent worldwide report from 17 countries, including 130 843 middle-age participants without preexisting cardiovascular disease.32In that study, higher recreational and nonrecreational physical activity was associated with a lower risk of mortality and cardiovascular events, suggesting that increasing physical activity is a simple, widely applicable, low-cost global strategy that could reduce deaths and cardiovascular events.
Our meta-analysis suggests that dog ownership warrants further investigation as a lifestyle intervention given the positive association with longer survival. Randomized controlled trials evaluating the impact of dog ownership on mortality are needed. In particular, future interventional studies assessing the adoption of a dog by individuals at higher risk for cardiovascular events should be considered. Likewise, additional health benefits of dog companionship, such as positive social-psychological effects, should be taking into consideration as dog ownership can be particularly beneficial for specific populations such as single elderly individuals.(6)
A limitation of our meta-analysis is that it was not possible to perform an analysis adjusted for confounders of the association between dog ownership and the outcomes. Other factors, such as socioeconomic and overall health status, could interfere with the mortality estimates. For instance, individuals with higher socioeconomic status, better physical fitness, and healthier lifestyle (such as not smoking or drinking alcohol) could be more likely to own a dog such that the impact of dog ownership in reducing risk of death may partially be representative of an overall lifestyle profile. Finally, we recognize that publication bias and the quality limitations of individual studies may still be relevant, despite our best efforts to conduct a comprehensive search and the lack of statistical evidence of bias.
In conclusion, our findings demonstrate that dog ownership is associated with reduced all-cause mortality possibly driven by a reduction in cardiovascular mortality. Taken together, our meta-analysis suggests the need for further investigation of the potential for dog ownership as a lifestyle intervention that may offer significant health benefits, particularly in populations at high-risk for cardiovascular death.
Acknowledgments
Dr Kramer holds the Canadian Diabetes Association Clinician-Scientist award. We thank Dr Ravi Retnakaran for his critical review and Romeo Kramer, the correspondent author’s miniature Schnauzer, for inspiring this meta-analysis. Dr Kramer conceived the systematic review and analysis plan. Dr Kramer and S. Mehmood selected studies for inclusion and abstracted data. Dr Kramer performed the statistical analyzes, interpreted the data, and wrote the first draft. All authors critically revised the article for important intellectual content and approved the final draft.
Sources of Funding
This study was supported by intramural funds.
Disclosures
Dr Kramer reports grants from Boehringer Ingelheim, outside the submitted work, and is a proud dog owner. The other authors report no conflicts.
Footnotes
Data Supplement is available athttps://www.ahajournals.org/doi/suppl/ 10. 1161 / CIRCOUTCOMES. 119. 005554.
Dr Kramer reports grants from Boehringer Ingelheim, outside the submitted work, and is a proud dog owner. The other authors report no conflicts.
Caroline K. Kramer, MD, PhD, University of Toronto Leadership Sinai Center for Diabetes, Mt Sinai Hospital, 60 Murray St, Suite L5 – 210, Mailbox – 21, Toronto, ON, Canada M5T 3L9. EmailCaroline.
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