Risk of Mycobacterium tuberculosis transmission in a low-incidence country due to immigration from high-incidence areas

Does immigration from a high-prevalence area contribute to an increased risk of tuberculosis in a low-incidence country? The tuberculosis incidence in Somalia is among the highest ever registered. Due to civil war and starvation, nearly half of all Somalis have been forced from their homes, causing significant migration to low-incidence countries. In Denmark, two-thirds of all tuberculosis patients are immigrants, half from Somalia. To determine the magnitude of Mycobacterium tuberculosis transmission between Somalis and Danes, we analyzed DNA fingerprint patterns of isolates collected in Denmark from 1992 to 1999, comprising >97% of all culture-positive patients (n = 3,320). Of these, 763 were Somalian immigrants, 55.2% of whom shared identical DNA fingerprint patterns; 74.9% of these were most likely infected before their arrival in Denmark, 23.3% were most likely infected in Denmark by other Somalis, and 1.8% were most likely infected by Danes. In the same period, only 0.9% of all Danish tuberculosis patients were most likely infected by Somalis. The Somalian immigrants in Denmark could be distributed into 35 different clusters with possible active transmission, of which 18 were retrieved among Somalis in the Netherlands. This indicated the existence of some internationally predominant Somalian strains causing clustering less likely to represent recent transmission. In conclusion, M. tuberculosis transmission among Somalis in Denmark is limited, and transmission between Somalis and Danes is nearly nonexistent. The higher transmission rates between nationalities found in the Netherlands do not apply to the situation in Denmark and not necessarily elsewhere, since many different factors may influence the magnitude of active transmission.