Potential years of life lost at ages 25 to 74 among Status Indians, 1991 to 2001
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Compared with other Canadians, First Nations peoples experience a disproportionate burden of illness and disease,1-3 which is reflected in shorter life expectancy. For Status Indians, life expectancy at birth is 8 years less for men and 7 years less for women.4 Life expectancy, however, tends to be dominated by deaths at older ages. A complementary way of examining mortality is to focus on premature mortality, specifically, potential years of life lost (PYLL) before age 75. PYLL sums the additional years people would have lived if they had had a full lifespan.5
The rate of premature death, and by extension, PYLL, is higher for Status Indians than for other Canadians.6-9 Possible reasons include differences in broad social determinants of health, such as income, education, and housing quality10 that are experienced over a lifetime.11 Despite their importance,12,13 these factors have not usually been included in analyses of mortality differences between Aboriginal and non-Aboriginal populations.13 However, with the 1991 to 2001 Canadian census mortality follow-up study, it is possible to examine the effect of socio-economic variables on the disparity in premature death between Status Indian and non-Aboriginal adults.
This article presents estimates of PYLL at ages 25 to 74 for Status Indians living on or off reserve, identifies the causes of death for which disparities between Status Indians and non-Aboriginal Canadians were greatest, and examines the effects of socio-economic factors on those differences.
The 1991 to 2001 Canadian census mortality follow-up study is a probabilistically linked cohort consisting of a 15% sample (n = 2,735,152) of the non-institutional population aged 25 or older, all of whom were enumerated via the 1991 census long-form questionnaire.14 This cohort was tracked for mortality from June 4, 1991 through December 31, 2001.
Because names were not captured on the census database, but were needed for linkage to the mortality database, creation of the cohort required two probabilistic linkages. First, eligible census respondents were linked to a nominal list (name) file (abstracted from 1990 and 1991 tax-filer data and then encrypted) using common variables such as date of birth, postal code, and spousal date of birth (if applicable); 80% of eligible respondents were successfully matched. Second, the census plus encrypted names were matched to the Canadian Mortality Database.15 Based on 1991 deaths, which could be identified independently in the Canadian Mortality Database and/or the name file, ascertainment of deaths among the cohort overall from 1991 to 2001 was estimated at 97%, and 95% to 96% among cohort members reporting any Aboriginal ancestry, Registered Indian status, or membership in an Indian Band or First Nation.
People enumerated by the 1991 census long-form questionnaire who had reached age 25 by census day were eligible to be part of the cohort. The long form, which was usually given to 1 in 5 households, was administered to all residents of Indian reserves, many remote and northern communities, and non-institutional collective dwellings. However, 78 Indian reserves, representing about 38,000 people, were either not enumerated or incompletely enumerated,16 and thus, were not part of the cohort. As well, data quality reports found that the 1991 census missed 3.4% of Canadian residents; these individuals were more likely to be young, mobile, low income, of Aboriginal ancestry,17 or homeless.
Because it was necessary to obtain encrypted names from taxation data, only tax-filers could be followed for mortality. Under Section 87 of the Indian Act, Status Indians are entitled to an exemption for income earned or considered to be earned on a reserve.18
Owing to the exclusion of institutional residents and non-tax-filers, life expectancy of the cohort at age 25 was 1 year longer for men and 2 years longer for women, compared with 1995 to 1997 life tables for all Canada. This bias would apply equally to Aboriginal and non-Aboriginal cohort members and should not appreciably affect relative differences between the two groups.
The cohort was divided into ten one-year follow-up periods (June 4, 1991 to June 3, 1992; June 4, 1992 to June 3, 1993; and so on) and one seven-month period (June 4, 2001 to December 31, 2001). Age was transformed from age at baseline (June 4, 1991) to age at the beginning of each year of follow-up. Deaths and person-years at risk were calculated separately for each follow-up period, and then pooled by five-year age group (determined at the beginning of each year of follow-up).
Deaths before age 75 were considered premature. The number of potential years of life lost (PYLL) was calculated by multiplying the number of deaths in each age group by the mean number of potential years of life lost for the same age group. For example, the death of someone aged 25 to 29 would contribute 47.5 potential years of life lost before age 75.
To calculate rates of PYLL, the number of person-years at risk (up to age 75) was determined for each five-year age group, and the rates were age-standardized to the Aboriginal population. The Aboriginal age distribution was based on those in the cohort who indicated an Aboriginal ancestry, registration under the Indian Act of Canada, or membership in an Indian Band or First Nation. Confidence intervals for the age-standardized rates were produced from variances derived using the Spiegelman method.19
Premature mortality (Cox models)
For each cohort member, person-days of follow-up were calculated from baseline (June 4, 1991) to the date of death, emigration (known only for 1991), end of study (December 31, 2001), or until the person reached age 75. Because exact date of birth was not available on the analysis file, age in completed years (as of June 4 of each follow-up year) was used to derive age at death and person-years of follow-up.
Cox proportional mortality hazard ratios were used to estimate the effect of socio-economic factors on the disparity in premature mortality between Status Indians and non-Aboriginal adults. All models were sex-specific and were run separately for Status Indians on and off reserve. The base model (Model 1) controlled only for age. Models 2 to 7 controlled for age and one other socio-economic factor. The full model (Model 8) controlled for age and all socio-economic factors simultaneously. Differences in excess mortality (1 minus the hazard ratio) comparing the full model to the base model were interpreted as estimates of the effect of the socio-economic variables on the disparities. The variables controlled for were age, marital status (married/common-law, not married), single parent (yes, no), educational attainment (less than secondary graduation, secondary graduation, postsecondary diploma, university degree), income adequacy quintile, labour force status (in, not in), crowding (more than one person per room; yes, no); home ownership (yes, no), dwelling in need of major repairs (yes, no), and urban population size (1 million or more; 500,000 to 999,999; 100,000 to 499,999; 10,000 to 99,999; less than 10,000).
Cause of death
The underlying cause of death of those who died during the study period had been previously coded to the World Health Organization's International Classification of Diseases, Ninth Revision (ICD-9)20 for deaths occurring from 1991 through 1999, and to the Tenth Revision (ICD-10)21 for deaths occurring in 2000 or 2001. Deaths were also grouped by the Global Burden of Disease categories, which underscore human development rather than the body system,22 and by risk factors, namely, smoking-related,23 alcohol-related23 and drug-related diseases24 or premature deaths that are potentially amenable to medical intervention.23
Registered Indian status was determined by a direct census question: "Is this person a Registered Indian as defined by the Indian Act of Canada?" (yes, no). A respondent answering "yes" was considered a Status Indian.
Place of residence was determined for June 4, 1991; subsequent mobility was not tracked. Indian reserves were defined to include the following types of census subdivisions: Indian government district; Indian reserve; Indian settlement; Terres réservées; Village Cri; Village Naskapi; Village nordique. All other areas were classified as off reserve.
The demographic and socio-economic characteristics of Status Indian cohort members differed from those of non-Aboriginal members and also varied by on- or off-reserve residence (Appendix Table A). Compared with non-Aboriginal adults, Status Indians were younger and less likely to be legally married. Status Indians, particularly those living on reserves, were less likely to have completed secondary school, to be employed, and to be homeowners, and were more likely to be in the two lowest income adequacy quintiles and to live in crowded conditions and in dwellings needing major repairs.
Status Indians tended to die earlier than did non-Aboriginal people. Of all deaths of Status Indian cohort members that occurred at ages 25 to 74, 28% were at ages 65 to 74, compared with more than 50% of the non-Aboriginal deaths (Appendix Table B).
Causes of death
Among Status Indians overall, non-communicable diseases accounted for the highest percentage of total potential years of life lost (PYLL) (53% for men, 69% for women), followed by injuries (38% and 21%) (Appendix Table C). Noteworthy contributors to total PYLL were cardiovascular diseases (19% and 14%), malignant neoplasms (13% and 25%), digestive diseases (6% and 9%), unintentional injuries (26% and 14%) such as road traffic accidents, and intentional injuries (11% and 7%) such as suicide.
Table C Cohort members, deaths ascertained, age-standardized rates of potential years of life lost (PYLL) and distribution of PYLL by cause of death at ages 25 to 74 for Status Indians living on and off reserve, compared with non-Aboriginal men and women, non-institutional cohort members, Canada, 1991 to 2001
The percentage distribution of total PYLL by major cause of death among Status Indians was generally similar whether they resided on or off reserve. However, the percentage of PYLL due to intentional injuries (suicide, homicide) was twice as high for Status Indian women living on than off reserve (8% versus 4%). Malignant neoplasms accounted for a larger share of total PYLL for Status Indian men living off than on reserve (17% versus 12%).
PYLL was also classified as being due to deaths caused by smoking-, alcohol- and drug-related diseases or to diseases that are potentially amenable to medical intervention (for example, cerebrovascular diseases, hypertension, breast cancer, pneumonia/influenza). For Status Indians, the percentages of PYLL attributable to deaths in these categories were: smoking-related (6% for both sexes), alcohol-related (8% for men and 7% for women), drug-related (2% and 5%), and amenable to medical intervention (8% and 20%). The percentages were similar for Status Indians living on and off reserve.
The age-standardized rate of PYLL was about two and half times as high for Status Indians as for non-Aboriginal adults, reflecting higher rate ratios for most causes of death (Tables 1 and 2). For all causes combined, the relative inequality was greater among Status Indian men living on than off reserve, but similar for Status Indian women on and off reserve.
Table 1 Age-standardized rate ratios (RR) for potential years of life lost at ages 25 to 74 for Status Indian men living on and off reserve, compared with non-Aboriginal men, by cause of death, non-institutional cohort members, Canada, 1991 to 2001
Table 2 Age-standardized rate ratios (RR) for potential years of life lost at ages 25 to 74 for Status Indian women living on and off reserve, compared with non-Aboriginal women, by cause of death, non-institutional cohort members, Canada, 1991 to 2001
Rate ratios for most communicable and non-communicable diseases were elevated—substantially for some causes. Among Status Indian men, these causes included alcohol use disorders, genitourinary diseases, respiratory infections, diabetes mellitus, and cirrhosis of the liver. Among Status Indian women, rate ratios were particularly high for alcohol use disorders, cirrhosis of the liver, respiratory infections, diabetes mellitus, and infectious and parasitic diseases.
Rate ratios were also high for deaths due to injuries, particularly drowning, violence, fires, road traffic accidents, and poisoning. The magnitude of these relative inequalities was greater among women living on reserve, notably so for suicides (self-inflicted injuries) and violence.
Rate ratios were high for alcohol-related deaths among Status Indians of both sexes, and for drug-related deaths among Status Indian women. However, rate ratios were not statistically elevated for smoking-related deaths, and modestly elevated for deaths amenable to medical intervention.
Among Status Indian men, the overall rate difference, or "excess PYLL," was 8,692 years per 100,000 person-years at risk (9,976 years on reserve; 5,293 years off reserve) (Table 3). Among Status Indian women, excess PYLL was 5,128 years per 100,000 person-years at risk (5,386 years on reserve; 4,561 years off reserve) (Table 4).
Table 3 Age-standardized rate differences (RD) for potential years of life lost at ages 25 to 74 for Status Indian men living on and off reserve, compared with non-Aboriginal men, by cause of death, non-institutional cohort members, Canada, 1991 to 2001
Table 4 Age-standardized rate differences (RD) for potential years of life lost at ages 25 to 74 for Status Indian women living on and off reserve, compared with non-Aboriginal women, by cause of death, non-institutional cohort members, Canada, 1991 to 2001
More than half (57%) of excess PYLL among Status Indian men was due to injuries, followed by non-communicable diseases (31%) and communicable diseases (2%) (percentages not shown). Percentages were similar for those on and off reserve.
Results differed for Status Indian women, among whom non-communicable diseases contributed the largest share (53%) of excess PYLL, followed by injuries (35%) and communicable diseases (7%) (percentages not shown). The percentage due to injuries for Status Indian women living on reserve was 39%, compared with 23% for those off reserve.
For Status Indian men, road traffic accidents and suicides were large contributors to excess PYLL. Suicide was a larger contributor for those living on reserve, and drowning, a larger contributor for those living off reserve. For Status Indian women, road traffic accidents and poisonings were large contributors to excess PYLL. Intentional injuries such as suicide and homicide were large contributors to excess PYLL for Status Indian women living on, but not off reserve.
The non-communicable diseases that were particularly large contributors to excess PYLL among Status Indian men were cardiovascular diseases, alcohol use disorders, and cirrhosis of the liver. The percentage of excess PYLL due to cardiovascular diseases was greater for Status Indian men off reserve than for those on reserve (percentages not shown). Among Status Indian women, rate differences were elevated for cardiovascular diseases, malignant neoplasms, and cirrhosis of the liver. The percentage of excess PYLL due to malignant neoplasms was higher for those living off reserve than for those on reserve (percentages not shown).
Alcohol-related deaths contributed about 10% of total excess PYLL for Status Indians of both sexes (percentages not shown). The percentages of total excess PYLL attributable to drug-related deaths and to deaths amenable to medical intervention were significantly elevated for Status Indian women.
The magnitude of the difference between Status Indians and non-Aboriginal adults in the risk of dying before age 75 varied by residence on or off reserve and by socio-economic factors (Table 5).
Table 5 Hazard ratios for death before age 75 among Status Indians living on and off reserve, compared with non-Aboriginal cohort members, controlling for selected demographic, economic, housing and geographic factors, by sex, non-institutional cohort members aged 25 to 74, Canada, 1991 to 2001
Compared with non-Aboriginal men, the age-adjusted hazard ratios for Status Indian men were 1.92 and 1.58, respectively, for those living on and off reserve (Model 1). Models 2 to 7 each adjusted for age plus a single socio-economic factor. Except for Models 6 (for Status Indians on reserve) and 7, which controlled for housing and geographic variables, respectively, the hazard ratios were attenuated, suggesting that each factor had an effect on the disparity. In the full model (Model 8), which controlled for all socio-economic factors simultaneously, the hazard ratios were reduced to 1.41 for Status Indian men on reserve, and to 1.09 for those living off reserve.
Results were similar for Status Indian women: in Model 1, the age-adjusted hazard ratios were 2.37 (on reserve) and 2.27 (off reserve), but in the full model, the hazard ratios were reduced to 1.92 and 1.70, respectively.
This study emphasizes the burden of premature deaths among Status Indians of working-age. In other studies of PYLL, the effect of infant and child deaths tended to mask patterns among adults.
The rate of PYLL among Status Indians aged 25 to 74 was approximately two and a half times that of non-Aboriginal adults, and slightly higher for Status Indians living on reserve. Although not directly comparable, the results are consistent with two other studies of PYLL among Status Indians.6,7
As was found in other research,6,25,26PYLL rates for injury-related deaths were high for Status Indians. In absolute terms, unintentional and intentional injuries were large contributors to excess PYLL among Status Indian men and women.
Even so, the results indicate that chronic diseases are a growing cause of mortality among Status Indians, reflecting an epidemiological transition from infectious to non-communicable diseases.10 Earlier studies too, have reported that in Aboriginal populations, the prevalence of diabetes is high and continues to increase,27 and that cardiovascular diseases28,29 and some cancers8,30-33 are more common.
Differences between Status Indians residing on and off reserve were not large, although the overall PYLL rate was slightly higher among those on reserve. A Manitoba study found that disparities between Status Indians and other residents were greater in southern than in northern areas of the province.7 Because the present analysis did not track mobility, the movement of Status Indians between reserves and other locations was not known. Had such information been available, the geographic differences reported here might have been either reduced or accentuated.
This study demonstrated that socio-economic factors (education, income, housing, and labour force status) were important contributors to disparities in PYLL between Aboriginal and non-Aboriginal people. The results are consistent with other population-based research demonstrating that socio-economic status was an important contributor to health inequalities—specifically, chronic conditions, self-rated health and mortality.8,34
Several limitations of this analysis must be acknowledged. Eligibility for the cohort was limited to people enumerated by the 1991 census long form. Because of systematic census long-form over-sampling of residents of Indian reserves and remote and northern communities, the cohort had an over-representation of the on-reserve and territorial populations. On the other hand, the 1991 census missed about 3.4% of the population, including residents of 78 Indian reserves (about 38,000 people).
As well, the cohort consists of census respondents who filed taxes in 1990 or 1991. Previous analysis demonstrated that this cohort is longer-lived than the total Canadian population. However, this should have little impact on estimates of relative inequality, because the healthy cohort effect would apply to both Status Indian and non-Aboriginal members. And despite the exclusion of non-tax filers, the socio-economic characteristics of those eligible to be linked and those actually linked to the name file were similar.
The results apply to the non-institutional population on June 4, 1991, not the population as a whole. Status Indians may be over-represented in the institutional population.
Ascertainment of deaths among Aboriginal cohort members is estimated to be slightly lower than for the cohort as a whole. Consequently, a small downward bias in calculated mortality rates for Status Indians is expected, and the true extent of disparities could be somewhat larger than indicated in this study.
Some suicides may have been misclassified as another cause of death such as drowning, poisoning or other injury. The reporting of suicides may also differ by jurisdiction (that is, reserves, towns, cities).
Rates of PYLL were significantly higher for Status Indians compared with non-Aboriginal adults. Non-communicable (chronic) diseases such as cardiovascular diseases and cancers were the largest contributors to total PYLL. However, injuries, especially unintentional injuries, were a major contributor to the disparities, highlighting the importance of injury prevention programs. Many of these health disparities are related to indicators of socio-economic status.
Funding for this study was provided by the First Nations and Inuit Health Branch at Health Canada. The 1991 to 2001 Canadian census mortality follow-up study was developed with funding from the Canadian Population Health Initiative, part of the Canadian Institute for Health Information.
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