Characteristics of non-clustered tuberculosis in a low burden country

SummaryMolecular genotyping studies often focus on clustered tuberculosis and recent transmission. Less attention has been paid to non-clustered tuberculosis. However, non-clustered cases also contribute significantly to the tuberculosis burden, especially in low-incidence countries. The objective of this study is to characterize non-clustered tuberculosis cases in Denmark and point out potential implications for tuberculosis control. The study is based on nationwide IS6110-RFLP genotyping of tuberculosis cases from 1992 through 2004, corresponding to 98% of culture verified cases.Of 3988 cases, 45% were non-clustered. Both Danes and immigrants had a peak incidence of non-clustered tuberculosis at older ages, 80–89 years (4.3 cases/105 population/year) and 60–69 years (28.8 cases/105 population/year), respectively. In addition, immigrants had a peak at 20–29 years (43.2 cases/105 inhabitants/year). In Danes, the incidence of non-clustered tuberculosis decreased during the study period and was predominantly found in elderly persons, presumably reactivating infection acquired during 1910–40, when tuberculosis incidence was high. In immigrants, the incidence was high at all ages, presumably reflecting reactivation of imported infections.In the future, the number of non-clustered tuberculosis cases will decrease, as older Danes die, and as time since primary infection increases for immigrants residing in Denmark. TB control should include focus on non-clustered cases.