Tuberculosis recurrence: Exogenous or endogenous?
With the advent of molecular epidemiology, perhaps the most significant finding has been that a recurrent episode of tuberculosis (TB) can occur by exogenous reinfection. This raises several questions: Does prior infection confer protective immunity? Do mixed infections occur, and does exogenous reinfection play a significant role in high-incidence settings, where individuals could be exposed to multiple infecting doses during their lifetime? Combining data from the literature with a simple mathematical model we illustrate that while disease due to exogenous reinfection after cure constitutes a significant proportion of recurrent TB, it represents a very small portion of the total number of TB cases. Although the true contribution of exogenous reinfection cannot be quantified, its association with recurrence serves as a barometer of its contribution to disease progression in previously untreated individuals. One of the most important functions of any new antituberculosis drug is the ability to prevent endogenous reactivation of disease. Molecular epidemiology now enables us to differentiate endogenous reactivation and exogenous reinfection. In this paper we discuss the clinical relevance of exogenous reinfection, questions surrounding its role in TB disease and its importance in the accurate evaluation of new drugs. Copyright © 2011 S. Karger AG, Basel.