To the Editors:
Janssens and Rieder 1 recently reported an inverse linear association between tuberculosis (TB) incidence and per capita gross domestic product, after analysing data from 171 World Health Organization (WHO) member states with paired data. We are in full agreement with the “take home” message that poverty alleviation must be an integral component of global TB control efforts. However, from an epidemiological perspective, additional factors may need to be considered to better understand the complex host–pathogen interplay at the population level.
Poverty facilitates the transmission of Mycobacterium tuberculosis, primarily through 1) its influence on living conditions, such as people living in overcrowded and poorly ventilated homes, 2) prolonged diagnostic delay and 3) increased vulnerability due to malnutrition and/or HIV infection 2, 3. Although these ecological associations are widely recognised, it is frequently not supported by observations from individual studies. A potential explanation for this discrepancy is that beyond a certain threshold, the exact level of poverty has little influence on transmission risk, since living conditions remain conducive. The importance of living conditions rather than the exact level of poverty is supported by the rural/urban discrepancy in TB incidence rates reported in Norway by Heimbeck 4 in 1928 and reviewed elsewhere 5. Additional support for this “threshold effect” is provided by differences in the calculated annual risk of infection observed in two adjacent study communities, where socioeconomic conditions in the area with reduced transmission are sufficiently improved to affect general living conditions 6.
Therefore, from a TB transmission perspective it seems less informative to stratify extreme levels of poverty, and more informative to evaluate whether the available epidemiological evidence supports the existence of a threshold effect. Determination of a threshold effect will guide epidemiological thinking and potentially assist in identifying global poverty alleviation targets. Measuring child TB incidence rates as an epidemiological marker of ongoing transmission could be considered in future analyses, once more reliable WHO data becomes available. Myers et al. 7 have found lower income, ethnicity and immigration status to be significantly associated with child TB incidence in California, USA.
Effective public health strategies remain critically important, even in the poorest of settings. However, the global distribution of the tuberculosis disease burden bears witness to the fact that extreme inequalities in wealth remain one of the key factors sustaining the tuberculosis epidemic.
Statement of interest
None declared.
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