Molecular epidemiology study of exogenous reinfection in an area with a low incidence of tuberculosis

In geographical areas with a low incidence of tuberculosis, recurrent tuberculosis is generally due to reactivation of the disease. However, the relative contribution of tuberculosis reinfection increases in parallel with the incidence of disease and is likely to depend on the epidemiological context: factors such as the spread of multidrug resistance, human immunodeficiency virus (HIV) infection, and immigration from developing countries could modify disease transmission in areas at low risk for tuberculosis. A molecular epidemiology study was performed in Lombardy, Northern Italy, where the incidence of tuberculosis is 17.5 cases per 100,000 persons. A total of 2,452 cases of culture-confirmed tuberculosis in 2,127 patients were studied. A group of 32 patients (1.5%), each of whom had two episodes of tuberculosis with cure as the outcome of the first episode and with more than 6 months between the two episodes, were studied by means of restriction fragment length polymorphism DNA fingerprinting analysis. For 5 of the 32 patients (16%), the DNA fingerprinting patterns of Mycobacterium tuberculosis strains responsible for the second episode did not match those of the corresponding isolates of the first episode, indicating exogenous reinfection. Two of these patients developed multidrug-resistant tuberculosis during the second episode, and in three cases the isolates belonged to clusters of M. tuberculosis strains spreading in the community. A fourfold-increased risk for reinfection was observed in immigrant patients compared to Italian subjects. In contrast, a higher risk of relapse rather than reinfection was evidenced in HIV-positive subjects and in patients infected with multidrug-resistant tuberculosis. Episodes of tuberculosis reinfection in areas with a low incidence of tuberculosis are rare compared to those in high-incidence geographical regions. In populations that have immigrated from high-risk areas, reinfection may represent a considerable contributor to the rate of recurrent tuberculosis. This finding emphasizes the importance of containing the spread of epidemic strains in close communities, in order to prevent changes in global tuberculosis trends for developed countries.