1 Global Epidemiology and Control of Tuberculosis Christopher Dye, Martien Borgdorff 1.1 Introduction The recent discoveries that Mycobacterium tuberculosis has probably been a human pathogen for millions of years [1], and that cattle and other animals are likely to have acquired mycobacteria from humans rather than the reverse [2,3], have profound implications for the epidemiology and control of human tuberculosis (TB). Epidemiological theory and data have shown that, for directly-transmitted pathogens to persist, host population size (or density) must exceed a threshold, because the size of the host population determines the rate of production of susceptible hosts [4]. If M. tuberculosis once survived and reproduced only in small human populations, it must have evolved mechanisms for doing so. Partial immunity and latent infection are both devices that increase the chance of persistence, the former by ensuring that the number of hosts available for infection does not become too small, and the latter by spreading the risk (to the pathogen) of infection over decades. Whether or not these biological characteristics of M. tuberculosis actually did evolve to aid persistence, they are at the heart of two major, contemporary problems in TB control – the difficulty of developing an effective vaccine, and the removal of the huge reservoir of latent infection. Without an efficacious vaccine, or an effective way of removing latent infection, the dominant method of TB control at present is through the treatment of active disease, standardized as the DOTS component (based on Directly Observed Treatment and Short-course chemotherapy) of the World Health Organizations (WHO) Stop TB Strategy [5,6]. The treatment of active disease reduces the burden of illness and death, and curtails transmission. However, while curative treatment for TB is comparatively cost-effective among health interventions [7], the ultimate goal must be the prevention of this life-threatening disease. The targets for global TB control are set within the framework of the United Nations Millennium Development Goals (MDGs), and are reinforced by the addi- tional goals of the Stop TB Partnership (Table 1.1). The overarching MDG Goal 6, Handbook of Tuberculosis: Clinics, Diagnostics, Therapy and Epidemiology. Edited by Stefan H.E. Kaufmann and Paul van Helden Copyright Ó 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31888-9 j1
Global Epidemiology and Control of Tuberculosis
Aug 19, 2015
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1Global Epidemiology and Control of TuberculosisChristopher Dye, Martien Borgdorff1.1IntroductionThe recent discoveries that Mycobacterium tuberculosis has probably been a humanpathogen for millions of years [1], and that cattle and other animals are likely to haveacquired mycobacteria from humans rather than the reverse [2,3], have profoundimplications for the epidemiology and control of human tuberculosis(TB). Epidemiological theoryanddatahaveshownthat, fordirectly-transmittedpathogenstopersist, host populationsize(ordensity) must exceedathreshold,because the size of the host populationdetermines the rate of productionofsusceptible hosts [4]. If M. tuberculosis once survived and reproduced only in smallhumanpopulations, it must have evolvedmechanisms for doing so. Partial immunityandlatentinfectionarebothdevicesthatincreasethechanceofpersistence, theformer by ensuring that the number of hosts available for infection does not becometoo small, and thelatter by spreadingthe risk (to the pathogen)of infection overdecades.Whether or not these biological characteristics of M. tuberculosis actually did evolveto aid persistence, they are at the heart of two major, contemporary problems in TBcontrol the difculty of developing an effective vaccine, and the removal of the hugereservoir of latent infection. Without an efcacious vaccine, or an effective way ofremoving latent infection, the dominant method of TB control at present is throughthetreatmentofactivedisease,standardizedastheDOTScomponent(basedonDirectly Observed Treatment and Short-course chemotherapy) of the World HealthOrganizations (WHO) StopTBStrategy [5,6]. The treatment of active disease reducesthe burden of illness and death, and curtails transmission. However, while curativetreatment for TB is comparatively cost-effective among health interventions [7], theultimate goal must be the prevention of this life-threatening disease.Thetargetsforglobal TBcontrol areset withintheframeworkoftheUnitedNations Millennium Development Goals (MDGs), and are reinforced by the addi-tional goals of the Stop TB Partnership (Table 1.1). The overarching MDG Goal 6,Handbook of Tuberculosis: Clinics, Diagnostics, Therapy and Epidemiology. Edited by Stefan H.E. Kaufmann andPaul van HeldenCopyright 2008 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-31888-9j1Table 1.1The Stop TB Strategy [5,6].Vision: A world free of TBGoal: To dramatically reduce the global burden of TB by 2015 in line with the MillenniumDevelopment Goals and the Stop TB Partnership targetsObjectives:- Achieve universal access to high-quality diagnosis and patient-centered treatment- Reduce the human suffering and socioeconomic burden associated with TB- Protect poor and vulnerable populations from TB, TB/HIV and MDR-TB- Support development of new tools and enable their timely and effective useTargets:MDG 6, Target 8: Halt and begin to reverse the incidence of TB by 2015Targets linked to the MDGs and endorsed by the Stop TB Partnership:By 2005: detect at least 70 % of infectious TB cases and cure at least 85 % of these casesBy 2015: reduce TB prevalence and deaths rates by 50 % relative to 1990By 2050: eliminate TB as a public health problem (
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