Molecular epidemiological approaches to study the epidemiology of tuberculosis in low-incidence settings receiving immigrants.

Although in most wealthy western countries the total incidence of tuberculosis (TB) steadily decreased in the last decades, the proportion of cases in immigrants from high-burden TB countries generally increased and to date a large proportion of all new active cases, often exceeding 50%, occurs among foreign-born individuals. In this context, molecular typing of Mycobacterium tuberculosis isolates has significantly contributed to understand the epidemiology of TB, allowing an objective evaluation of the dynamics and the risk of TB importation and transmission within low-incidence host countries. Many molecular studies have shown that a large part of active TB cases in immigrants yield unique, unclustered DNA fingerprints; this finding reflects reactivation of remote latent infections, rather than recent TB infections, and, at the same time, indicates a low rate of disease transmission within the host country. Some studies however express more concern on the basis of higher rates of transmission within the host country with foreign-born index cases. Molecular analysis of isolates also showed that TB infections in migrants may be acquired after arrival in the host country and that TB transmission between foreign-born and autochthonous individuals may occur in both directions. Molecular typing of isolates has been also profitably used to evaluate the diffusion of M. tuberculosis strain families across different geographic areas and human populations and to monitor the diffusion of threatening strains, such as Beijing and/or (multi)drug-resistant strains. To date, based on the contribution of molecular epidemiology, it is possible to adapt or design appropriate strategies for a rational control of TB in low-incidence countries.