The social epidemiology of tuberculosis in South Africa: A multilevel analysis
Introduction
Tuberculosis is the world's leading curable cause of infectious disease mortality, with a disproportionate burden of disease falling on low- and middle-income countries (World Health Organisation, 1999). Globally the incidence of tuberculosis is increasing, fueled in part by the concurrent epidemic of HIV/AIDS, particularly in sub-Saharan Africa. Tuberculosis is traditionally regarded as a disease of poverty and many aspects of low socioeconomic status (SES), for example overcrowding and malnutrition, are accepted individual- and household-level risk factors for the disease. The distribution of tuberculosis within populations has also been associated historically with group-level socioeconomic conditions (Dubos & Dubos, 1992; Gandy & Zumla, 2003; McKeown, 1976).
South Africa has a long history of high tuberculosis burden. In 2005, the estimated national incidence of tuberculosis was 600 cases per 100,000 persons per annum, the third highest rate in the world (World Health Organisation, 2007). As in other countries, the tuberculosis epidemic in South Africa can be traced in part to the poor working and living conditions that surrounded early industrialization. Many of these conditions persist to this day, and along with the HIV/AIDS epidemic, have fueled the current high levels of tuberculosis disease in this country (Packard, 1989).
Despite the acknowledgment that tuberculosis has strong socioeconomic determinants, there has been surprisingly little epidemiological research into the pathways through which SES might increase the risk of tuberculosis. Ecologic studies conducted in the United States and Britain have found crude associations between tuberculosis rates across areas and low levels of education, high levels of poverty, less government social support, social deprivation and income inequality (Krieger, Waterman, Chen, Soobader, & Subramanian 2003; Parslow, El-Shimy, Cundall, & McKinney, 2001; Spence, Hotchkiss, Williams, & Davies, 1993; Tocque et al., 1999). Evidence from ecologic studies in Brazil and South Africa supports the existence of this relationship in middle-income countries (Munch et al., 2003; Souza et al., 2000). Finally, individual-level studies of the link between low SES and high risk of tuberculosis have found associations in poorer, high tuberculosis-burden settings, such as southern India and Guinea-Bissau (Gustafson et al., 2004; Shetty, Shemko, Vaz, & D’Souza, 2006), although not in Malawi (Glynn et al., 2000).
Recent decades have seen increasing attention in public health paid to quantitative study of social and economic factors as determinants of health outcomes. This field of social epidemiology recognizes the importance of both individual- and group-level socioeconomic risk factors in determining an individual's health (Berkman & Kawachi, 2000). In particular, it differentiates between two classes of mechanisms by which observed group-level associations may operate causally: a compositional effect occurs when the group-level outcome is due solely to the aggregation of individual-level characteristics of members, while a contextual effect arises when the nature of the environment in which group members live determines their health outcomes (MacIntyre, Ellaway, & Cummins, 2002). Examples of the former include the average number of years of education and mean income in an area; examples of the latter include the level of public services provided by government and the distribution of wealth in an area. Research has tended to focus on compositional effects, perhaps because these can be calculated directly through the aggregation of individual-level data, something not possible for contextual phenomena which are fundamentally properties of groups. In particular, studies of health involving income-based community-level effects more frequently consider poverty in absolute than in relative terms.
Existing epidemiological insights into the relationships between SES and tuberculosis risk come almost exclusively from either individual- or group-level (ecological) studies, yet such studies are unable to distinguish the compositional and contextual effects of socioeconomic factors on tuberculosis risk. A comprehensive investigation of the relationship between SES and tuberculosis needs to jointly consider the effect of both individual- and community-level measures of SES. Such an approach requires a multilevel analysis, incorporating variables at different levels of aggregation, to differentiate between compositional and contextual health effects (Diez-Roux & Aiello, 2005). To better understand the potential compositional and contextual influences of SES on tuberculosis risk, we conducted a multilevel analysis of the relative importance of demographic, behavioral and socioeconomic individual-level risk factors, alongside group-level measures of SES, in determining tuberculosis outcomes in South Africa. Our primary hypothesis was that group-level income inequality is an important determinant of tuberculosis disease, independent of typically used measures of SES.
Section snippets
Methods
This secondary analysis combined data from two cross-sectional surveys, the 1998 South African Demographic and Health Survey (SADHS) and the 1996 South African national census. Demographic and Health Surveys (DHS) are an international series of nationally representative surveys conducted in middle- and lower-income countries (Fisher & Way, 1988). The 1998 SADHS was the first DHS conducted in South Africa and sampling was adjusted to ensure statistically meaningful estimates could be reached for
Results
The study sample consisted of the 13,043 respondents (94.3% of the SADHS population) with information on all variables of interest (Table 1). The sample was 59% female, 75% African, 11% Colored, 10% White and 4% Asian.
In this sample 69 individuals (0.5%) reported having been told they had tuberculosis in the past 12 months. After adjusting for the survey design, the population incidence rate was 422 cases per 100,000 per annum (95% CI: 300–543). A total of 369 respondents (2.8%) reported having
Discussion
This analysis of a nationally representative sample of the South African population found the prevalence of self-reported tuberculosis disease to be associated with lower individual-, household- and community-level SES. A low level of personal education, unemployment and a low level of household wealth were associated with increased odds of tuberculosis. Community-level employment and wealth did not appear to have an independent effect on tuberculosis occurrence once individual and household
Conclusion
This study presents novel insights into the associations between socioeconomic conditions and risk of tuberculosis disease. The use of a social epidemiology framework to analyze risk factors for tuberculosis, particularly in sub-Saharan Africa, is a young endeavor. These findings require further investigation, but highlight the importance of including measures of community-level SES, particularly emergent characteristics such as income inequality, in future research on tuberculosis. Despite
Acknowledgments
The authors would like to thank Dr. Debbie Bradshaw of the South African Medical Research Council and Piet Alberts of Statistics South Africa for their assistance in accessing the South African Demographic and Health Survey and South African Census data, respectively. They would also like to thank Katie Doolan for her advice on data sources, methodology and her comments on a draft of the paper.
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