It proves useful to utilize a model to understand the epidemiology of tuberculosis. The model should be as simple as possible, yet sufficiently detailed to allow presentation of all the major components that determine the dynamics of tuberculosis in a population. The model selected here follows the pathogenesis of tuberculosis, with exposure, infection, disease, and death.
Exposure is defined as occurring in a person who breathes in an environment that contains tubercle bacilli. It is difficult to quantify this exactly, as basically all humans breathe air that contains tubercle bacilli. A pragmatic definition thus might be contact with a person at a distance that allows talking when outdoors. Indoors, it might be defined as a room in which a tuberculosis patient has been within the past few hours.
Infection, or more precisely, latent, sub-clinical infection with M. tuberculosis, is defined as a person harboring viable tubercle bacilli but without having any clinical, bacteriologic or radiographic signs or symptoms of disease. Henceforth, this will be named "tuberculous infection".
Tuberculosis is the term used to denote the disease that M. tuberculosis is causing. A dichotomization is made here to distinguish between infectious and non-infectious tuberculosis. Infectious tuberculosis is the form of disease that allows potentially transmission of tubercle bacilli to another human while non-infectious tuberculosis does not. At this point in time, these two epidemiologically important forms are not further specified.
Death from tuberculosis is the final step in the pathogenesis of tuberculosis and needs no further definition.
For each of these steps in the pathogenesis, there are risk factors that can be identified: risk factors for exposure, risk factors for infection given that there is exposure, risk factors for tuberculosis given that tuberculous infection has been acquired, and risk factors for dying of tuberculosis in patients who have tuberculosis. It is etiologic epidemiology that is concerned with the identification of such factors.
We are also concerned at identifying the magnitude of the problem. To determine the magnitude of the problem, a tool to measure it is required. If it is available, we are interested in how much infection, disease, and death there is that is caused by M. tuberculosis. This is the task of descriptive epidemiology. We observe that certain populations or population segments have a higher incidence (newly occurring during a specified time period) or prevalence (currently existing burden) of infection or disease. Such an observed increased incidence or prevalence might be attributable to a higher underlying prevalence of the risk factor (leading to increased incidence in the subsequent step in the pathogenesis). Some people in such a population segment have, however, no risk at all. Here, we prefer thus to speak of risk groups (rather than risk factors as in etiologic epidemiology).
Finally, we would like to know about the likely future course of the epidemic. This is the task of predictive epidemiology. It uses modeling techniques to predict the likely future course of tuberculous infection, disease, and death from observations made in the past. |
