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

Does immigration from a high-prevalence area contribute to an increased ris k of tuberculosis in a low-incidence country? The tuberculosis incidence in Somalia is among the highest ever registered. Due to civil war and starvat ion, 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 an d Danes, we analyzed DNA fingerprint patterns of isolates collected in Denm ark 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 iden tical DNA fingerprint patterns; 74.9% of these were most likely infected be fore their arrival in Denmark, 23.3% were most likely infected in Denmark b y other Somalis, and 1.8% were most likely infected by Danes. In the same p eriod, only 0.9% of all Danish tuberculosis patients were most likely infec ted by Somalis. The Somalian immigrants in Denmark could be distributed int o 35 different clusters with possible active transmission, of which 18 were retrieved among Somalis in the Netherlands. This indicated the existence o f some internationally predominant Somalian strains causing clustering less likely to represent recent transmission. In conclusion, M. tuberculosis tr ansmission among Somalis in Denmark is limited, and transmission between So malis and Danes is nearly nonexistent. The higher transmission rates betwee n nationalities found in the Netherlands do not apply to the situation in D enmark and not necessarily elsewhere, since many different factors may infl uence the magnitude of active transmission.