Figure 1. The Pathogenesis of Tuberculosis in the Infected Host
(Adapted from the 6th Edition of the Canadian TB Standards)
This figure describes potential outcomes for the untreated, infected host including hypersensitivity reactions that can occur shortly after the initial infection.
Approximately 5% of persons who become infected with M. tuberculosis will develop TB disease relatively soon afterward (primary TB disease). The probability of primary TB disease is much greater in those with severe immune-compromising conditions such as HIV/AIDS, and children under 5 years of age. Those who do not develop primary disease will be left with latent TB infection (LTBI). A small proportion of persons with LTBI, on balance about 5% of those infected, will later develop TB disease (reactivation TB disease). Pulmonary forms of TB disease can lead to new infections. About 90% of persons infected with M. tuberculosis, who do not have immune-compromising conditions such as HIV/AIDS, will never develop TB disease.
Figure 2. Framework for proximate risk factors and upstream determinants of TB
This figure by Lönnroth et al depicts a framework that identifies entry points for interventions to address TB risk factors and social determinants of TB. The framework is organized into an upper section, labeled “upstream determinants” and a lower section, labeled “proximate risk factors”.
Upstream determinants of TB are described as weak and inequitable economic, social and environmental policy, and globalization, migration, urbanization, and demographic transition. The framework suggests these conditions can result in:
In turn, these outcomes impact the cycle of TB (exposure, infection, active disease) by influencing proximate risk factors, described in the framework as:
Proximate risk factors are drivers of TB in that they facilitate exposure to and transmission of M. tuberculosis, and increase the likelihood of progression from TB infection to TB disease in the infected.
The framework implies that interventions to reduce the incidence of TB disease in a community or population ultimately serve as protective factors against future cases by interrupting the perpetuation of consequences of TB disease, such as poverty, low socioeconomic status and low education level. The framework also implies that sustained reduction in TB incidence will not be possible without attention to these upstream determinants as well as the resulting proximate risk factors.