Drivers of tuberculosis epidemics: The role of risk factors and social determinants

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Abstract

The main thrust of the World Health Organization's global tuberculosis (TB) control strategy is to ensure effective and equitable delivery of quality assured diagnosis and treatment of TB. Options for including preventive efforts have not yet been fully considered. This paper presents a narrative review of the historical and recent progress in TB control and the role of TB risk factors and social determinants. The review was conducted with a view to assess the prospects of effectively controlling TB under the current strategy, and the potential to increase epidemiological impact through additional preventive interventions. The review suggests that, while the current strategy is effective in curing patients and saving lives, the epidemiological impact has so far been less than predicted. In order to reach long-term epidemiological targets for global TB control, additional interventions to reduce peoples' vulnerability for TB may therefore be required. Risk factors that seem to be of importance at the population level include poor living and working conditions associated with high risk of TB transmission, and factors that impair the host's defence against TB infection and disease, such as HIV infection, malnutrition, smoking, diabetes, alcohol abuse, and indoor air pollution. Preventive interventions may target these factors directly or via their underlying social determinants. The identification of risk groups also helps to target strategies for early detection of people in need of TB treatment. More research is needed on the suitability, feasibility and cost-effectiveness of these intervention options.

Introduction

The Commission on Social Determinants for Health (CSDH), set up by the World Health Organization (WHO) in 2005, has attempted to revitalize the debate and actions to improve health through addressing the “causes of the causes” of ill health. The commission has developed action frameworks in several fields to address the social determinants of health including early childhood development, globalization, urbanisation, employment conditions, social exclusion, etc. A key message from the CSDH is that public health achievements will largely depend on actions outside the health care sector (CSDH, 2008).

The work of the commission builds on a long social medicine tradition, from Virchow and the public health oriented social and environmental interventions that he and others inspired from the mid-19th century onwards (Rosen, 1974), through the social model of health adopted by WHO in the late 1940s (Chisholm, 1948), to the Health for All concept in the 1970s (WHO, 1978). It has not always been easy to keep the social medicine concepts and action frameworks alive (Amrith, 2002, Rosen, 1974). Several countries have tried consistently to pursue social policies aimed to promote equity in health, some with a great deal of success (CSDH, 2008). However, in most countries, little of the social model of health advocated by WHO and others was translated into practice. As a consequence, much preventable ill health and health inequity prevails today (CSDH, 2008).

The last 2–3 decades have seen reinvigorated attempts to fight diseases with medical technologies alone. Key concepts in today's discourse on disease control include: evidence-based medicine; health systems strengthening; and community involvement. The latter two focuses mainly on the capacity to deliver medical technologies effectively. Meanwhile, social medicine has often been reduced to actions to identify risk groups that need to be targeted with medical interventions and attempts to influence individuals' risk behaviour (Porter, 2006).

The current global tuberculosis (TB) control paradigm mainly focuses on cutting transmission through early case detection and effective treatment. Medical interventions are at the core of the global strategy (Lönnroth and Raviglione, 2008, WHO, 2006). However, historically, TB has been used as a prime example of a “social disease”, the control of which requires social, economic and environmental interventions. Furthermore, the need to control TB has often been used as an argument for improving living conditions and reduce inequity (Chisholm, 1948, Rosen, 1974). After the second World War, a medically oriented TB control model emerged (Amrith, 2002). Much hope was initially placed on mass vaccination with BCG (Brimnes, 2007), hopes which were later shattered when it was demonstrated that the protective effect and epidemiological impact were limited (Rieder, 1999). When effective chemotherapy for TB became available in the end of the 1940 and beginning of 1950s, the control model switched to an almost completely curative focus. “Prevention starts with cure” became a slogan for the global role out of effective treatments. It was predicted that good coverage of effective treatment would result in a rapid decline in TB incidence (Styblo & Bumgarner, 1991). However, some recent observations have indicated that the impact of the present TB control strategy has been less than expected (WHO, 2008a). If these observations are confirmed, there may be a need to again broaden the strategy to include more preventive efforts (Farmer, 1997, Jaramillo, 1999). One avenue for improved TB prevention is the ongoing quest for better TB vaccines and better chemotherapy for preventive treatment. Another avenue leads through actions to address the social determinants of TB, as well as the more proximate risk factors (the physical and biomedical factors that directly influence the mechanisms that govern exposure to tuberculosis, risk of acquiring tuberculosis infection, and risk of progression from tuberculosis infection to active tuberculosis disease).

In order to assess the relevance and potential benefits of the latter, a review was conducted to identify key social determinants of TB and the possible causal pathways through which they operate. We also reviewed the historical and recent progress in global TB control with a view to assess the prospects of effectively controlling TB in the future with the current global strategy, and the potential to increase epidemiological impact through adding actions on social determinants and risk factors.

Section snippets

Methods

A literature review was undertaken to identify historic and recent trends in TB burden; social determinants of TB; and the most important proximate TB risk factors. We searched PubMed, the send-list “TB-Related News and Journal Items Weekly Update” (Centre for Disease Control and Prevention, Atlanta, USA), and a private data-base on tuberculosis publications from the past 40 years (created by Dr Hans Rieder). The analysis was carried out by the Stop TB Department at WHO, in collaboration with

History of TB epidemiology and social change in industrialized countries

Epidemiological TB data before the 20th century is imprecise. However, some broad trends have been identified with a reasonable level of certainty. The conventional wisdom is that the incidence of TB increased in industrialized countries in the 17th to 18th centuries, peaking at different times in different places from the middle of the 1700s in Great Britain to the beginning of the 1900s in Japan. From these trends, a temporal association has been suggested between increased TB incidence and

Discussion

This review suggests that, in order to reach long-term TB control targets, the current TB control strategy needs to be complemented with efforts to address TB risk factors and social determinants. Fig. 4 presents a framework for identifying entry points for such interventions. In principle, reducing the prevalence of the proximate risk factors will reduce TB incidence. This may be achieved by tackling these risk factors directly, or the social determinants – on the individual, community,

Competing interests

The authors are staff members of the World Health Organization. The authors alone are responsible for the views expressed in this publication and they do not necessarily represent the decisions or policies of the World Health Organization. No external funding was provided for this research.

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