a First Nations and Inuit Health Branch, Health Canada.
b Health Secretariat, Assembly of First Nations.
Michael Clark, Tuberculosis Program Officer, First Nations and Inuit Health Branch, Health Canada, PL 1920D, Tunneys Pasture, Ottawa, Ontario, K1A 0L3, Canada. E-mail: Michael_Clark{at}hc-sc.gc.ca
Abstract
Background First Nations communities in Canada experience disproportionately high levels of overcrowded housing, degree of isolation, and rates of tuberculosis (TB). A study was done to assess the association between housing density, isolation, and the occurrence of TB in First Nations communities.
Methods Average persons per room (ppr), isolation type, average household income, population, and TB cases (19971999) at the community level were entered into a database. Tuberculosis notification rates and 95% CI were calculated for different strata of ppr and isolation. Two multiple logistic regression models were developed to examine the association of ppr, isolation, income, and population with the occurrence of 1, or
2, TB cases in a community.
Results The rate was 18.9 per 100 000 (95% CI: 13.324.6) in communities with an average of 0.40.6 ppr, while communities with 1.01.2 ppr had a rate of 113.0 per 100 000 (95% CI: 95.4130.5). An increase of 0.1 ppr in a community was associated with a 40% increase in risk of 2 TB cases occurring, while an increase of $10 000 in community household income was associated with 0.25 the risk, and being an isolated community increased risk by 2.5 times.
Conclusions This study shows a significant association between housing density, isolation, income levels, and TB. Overcrowded housing has the potential to increase exposure of susceptible individuals to infectious TB cases, and isolation from health services may increase the likelihood of TB.
Keywords Tuberculosis, housing, overcrowding, isolation, Aboriginal, Canada
Accepted 25 February 2002
In the early decades of the 20th century, the death rate from tuberculosis (TB) in the First Nations population of Canada was as high as 700 per 100 000. A combination of malnutrition, confinement on crowded reservations with poor sanitation, and lack of immunity to the TB bacillus contributed substantially to this epidemic.1 In 1999, the TB notification rate among First Nations people living on reserves in Canada was 10 times that of the overall Canadian rate in 1997.2
Overcrowded housing conditions can increase exposure of susceptible people to those with infectious respiratory disease, and in doing so may increase the probability of transmission. The association between overcrowded housing and TB incidence, paediatric TB, and TB mortality has long been recognized.35 Major housing problems have been identified in First Nations communities in Canada, and analyses have shown TB incidence is higher in communities isolated from health services.2,6 It has been widely stated that social conditions drive TB rates in Aboriginal communities upwards, but evidence for this relationship has been largely anecdotal and assumed to exist. This has led to a recommendation that this area be researched further.79 The purpose of this study is to examine the association between community housing density, isolation and the incidence and occurrence of TB in Canadian First Nations communities.
Methodology
Tuberculosis data by on-reserve community were provided by the seven First Nations and Inuit Health Branch (FNIHB) Regional TB programmes, which are responsible for TB control on reserves, for the period 19971999. The case definition for TB disease in Canada is standardized at the federal level, and used by each Regional programme. All forms of TB disease, including both pulmonary and extrapulmonary cases, were included in the analyses. The Canadian federal case definition is: (1) cases with Mycobacterium tuberculosis complex (i.e. M. tuberculosis, M. bovis [excluding BCG strain] or M. africanum) demonstrated by microscopy and/or on culture; or (2) in the absence of bacteriological proof, cases clinically compatible with active TB. The latter may include: chest X-ray changes compatible with active TB including idiopathic pleurisy with effusion; active extrapulmonary TB (meningeal, bone, kidney, peripheral lymph nodes, etc.); or pathological or post-mortem evidence of active TB.10 Population estimates and isolation types for on-reserve communities were obtained from the Community Workload Increase System (CWIS), a database used to estimate resource, funding and health programme needs in First Nations communities. Population figures were available for 1997. Isolation types are remote isolated (no scheduled flights, minimal telephone and radio, no road access); isolated (flights, good telephone service, no road access); semi-isolated (flights, good telephone service, road access >90 km to physician services); or non-isolated (flights, good telephone service, road access <90 km to physician services). Housing density and household income data by on-reserve community were available from the 1996 census, and were provided by Indian and Northern Affairs Canada (INAC). Housing density is expressed as the average number of persons per room (ppr) in a community, and household income is expressed in Canadian dollars. The ppr indicator for housing density has been used in other studies on the socioeconomic determinants of TB.3,5 All of these data were entered into a common database containing 602 community records. All associations analysed in the study were ecologic in nature, and not intended to establish causal links.
Tuberculosis incidence rates and 95% CI were calculated for different strata of community housing density and categories of isolation type. Confidence intervals for incidence were estimated using the formula:
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Bivariate relationships between socioeconomic determinants were assessed using correlation analyses and t-tests. Correlation statistics were calculated for pairs of continuous variables (e.g. ppr and income), while t-tests were done to examine the difference in means of isolated versus non-isolated communities. The three isolation types other than non-isolated were combined into a single isolated category for t-tests and logistic regression analyses, because the number of people living in non-isolated communities is far greater than the number living in all other types combined.
Multiple logistic regression analyses were done to assess the association between potential predictors of TB in a community and the occurrence of 1 case, and
2 cases. The combination of TB, population, isolation, housing, and income data was available for 298 First Nations, on-reserve communities. A forward selection procedure was used, in which variables were entered into the model if P < 0.15, and taken out if P > 0.15. Hosmer and Lemeshow goodness-of-fit tests were done to assess the overall fit of the models to the data. All analyses were done using SAS 6.12 statistical software (SAS Institute, Cary, NC, USA).
Results
Descriptive statistics for TB, housing density, income and isolation are presented in Table 1. The 3-year, age-standardized TB notification rate (19971999) is eight times higher than the 1997 Canadian rate. Average housing density is higher in the First Nations, on-reserve population (0.7 ppr) than in the non-Aboriginal Canadian population (0.4 ppr). First Nations TB rates are highest in Saskatchewan, followed by Alberta and Manitoba. These three provinces also had the highest average levels of housing density. Income levels are lowest in the maritime provinces, similar to the trend in the non-Aboriginal population. Manitoba has the highest proportion of communities considered isolated (55.6%).
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Results of the two multiple logistic regression models are shown in Tables 5 and 6. In model 1 (Table 5
), ppr and income level were not significantly related to the occurrence of
1 TB cases in a community. An increase of 100 in community population was associated with a small increase in risk for TB (P = 0.0001), and being isolated was related to a twofold increase in risk (P = 0.0354).
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Discussion
In 1999, 40% of the total cases reported in the First Nations on-reserve population occurred in five communities.2 All of these communities were located in isolated areas, with average housing densities of 8.0 ppr. Not all First Nations communities experience overcrowded housing and/or persistent TB outbreaks. This research attempts to increase understanding of what factors are associated with an increased risk in the minority of communities that did have a reported TB case between 1997 and 1999. The results suggest that TB incidence is higher in communities located in isolated areas, and in communities with a higher average housing density. The first of the two multivariate models (model 1) showed that isolation was associated with an increased risk of TB, while all predictors were significantly related to
2 TB cases in model 2. It is possible that communities with
2 cases experience higher transmission rates, and were more likely to be influenced by factors such as overcrowded housing.
Although an increase in community income was associated with a decreased risk of TB, income levels were higher in isolated communities where TB incidence is higher. This may be due to higher wages given to employees in isolated areas where the cost of living is elevated. The significant interaction between ppr and household income in model 2 may indicate that effect modification is present, and that housing density is associated with an increased risk of TB across different levels of income.
Some underlying factors that explain the occurrence of TB in certain communities were not considered in the multivariate models. For example, it is possible that communities with overcrowded housing also experience a higher prevalence of latent TB infection, and/or risk factors for progression from TB infection to disease. Such risk factors may include substance abuse,11 and undernutrition,12 which may be more prevalent in communities with socioeconomic disadvantages. Other socioeconomic variables associated with TB, such as unemployment and homelessness,13,14 were also neglected in the analysis due to lack of data. An ascertainment bias may have exaggerated the association between isolation and TB, as medical professionals in isolated communities where TB is endemic may be more likely to consider TB in their diagnoses. It is also possible that people in non-isolated communities are more likely to move off reserve, and may break down to disease in areas where FNIHB does not provide services. Despite these limitations, the data do support the conclusion that overcrowded living conditions and isolation from health services are associated with an increased risk of developing TB.
It has been hypothesized that the severity of the TB epidemic among First Nations after contact with Europeans was due more to social conditions such as crowding than the introduction of a more virulent strain of TB.15,16 In recent years, analyses have shown that clustering of culture-positive TB cases with the same DNA fingerprint is much more common in the First Nations population than in the non-Aboriginal Canadian population.17 The occurrence of these clusters, and high rates of TB infection and disease among children,2 show that First Nations likely experience an elevated risk of transmission, a risk that may be related to housing conditions. Overcrowded housing and inadequate community infrastructure have been associated with increased risk for other infectious diseases in First Nations populations, such as Shigellosis.18 The effect of overcrowding on TB epidemiology and control can be expressed in terms of its impact on the basic reproductive rate R0 of the disease over time. R0 is the average number of secondary infectious disease cases per infectious case.19 To reduce incidence of that disease and eventually achieve elimination, it is essential to bring R0 to <1 from year to year. Reducing the contact rate between infectious cases and other individuals generally has the effect of reducing R0, and contact rates decrease as socioeconomic conditions improve and overcrowding is prevented.20 The increased potential for contact due to overcrowding emphasizes the need to find and treat infectious TB cases in First Nations communities rapidly before transmission and clustering occur.
The association between poverty and TB is well documented.21,22 Barr et al. showed that neighbourhood poverty was associated with an increased risk of TB in New York City, adjusting for several other risk factors in a multivariate Poisson regression model.23 The results of this study show that increasing levels of income are associated with a reduced risk of TB in a community, and the combination of overcrowding and lower income levels may together increase risk of TB. It has been demonstrated that First Nations people with an annual income of <$10 000 are less likely to use health services than other income groups.24 Tuberculosis patient delays in presenting to health services are recognized as an important hindrance in finding and treating infectious TB cases before they spread the disease to others.25
The need to address overcrowded housing and other socioeconomic determinants of TB to eliminate TB in First Nations communities has been recognized.17 It may be beneficial to supplement traditional indicators of TB risk in a community, such as annual risk of infection and disease incidence,26 with the socioeconomic risk factors considered in this study. These indicators can be used by TB control programmes to assist community health staff in understanding risk for TB in the community, and to plan appropriate preventive actions based on this risk. The results also suggest that TB is more common in isolated communities, where unique challenges exist in transporting patients, equipment, drugs, and staff.
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Acknowledgments
The authors wish to thank Indian and Northern Affairs Canada (INAC) for providing 1996 census data, the First Nations and Inuit Health Programs (FNIHB) Regional Tuberculosis Programs for their provision of TB data and ongoing efforts to eliminate TB, and the Assembly of First Nations (AFN) Information Governance Committee for supporting the study. The authors would also like to acknowledge Dr Shauna Hudson and Dr Pam Smith from FNIHB Saskatchewan Region, for their ideas and comments.
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