Epidemiologically, primary tuberculosis is defined as tuberculosis resulting from a single infection acquired within the past 5 years.
Endogenous reactivation tuberculosis is defined as tuberculosis resulting from a single infection acquired more than 5 years earlier.
Exogenous re-infection tuberculosis is defined as tuberculosis resulting from a second (or more) infection acquired at any time following the first infection.
These three component contributors to morbidity are exhaustive (summing up to 100%).
To what extent the different components contribute to the morbidity in the community, depends on the prevailing risk of infection and the age of the population segment concerned as shown here in a model developed based on data from the Netherlands. The example shown here is limited to males in two age groups.
Top panel: Among males aged 15 to 19 years in the early 1950ies, a case of tuberculosis emerging was most likely primary tuberculosis. Over the 20 years considered here, the risk of infection (and thus also re-infection) decrease sharply in the Netherlands. Thus, the already small proportion of exogenous re-infection tuberculosis dwindled further and primary tuberculosis increased even more. Endogenous reactivation tuberculosis remained a small and barely changing contributor over time.
Bottom panel: Among males aged 60 to 64 years in the early 1950ies, a case of tuberculosis emerging was, in contrast, extremely unlikely to be primary tuberculosis. These patients had been born at a time when there was little chance to escape tuberculous infection. The risk of infection these patients lived through made it even very likely that they had been infected more than once, hence the proportion of cases attributable to exogenous infection was large. Some may had indeed acquired a single infection, now reactivating at old age. With the passage of time, persons in the same age group had lived through a period with a smaller and diminishing risk of infection, thus disease due to exogenous re-infection must have diminished. Conversely, the proportion of cases attributable to endogenous reactivation must have increased. While the 20 years that had elapsed most likely allowed more people to escape infection, primary tuberculosis must also have increased, however, to a very small extent only.
This statistical model based on information on risk of infection and morbidity in the Netherlands provides a conceptual framework to understand the different contributors to tuberculosis morbidity in a community. The validity of this concept is nowadays increasingly confirmed through molecular DNA fingerprinting techniques, enabling a better characterization of strains that circulate in a community. |
