Molecular epidemiology of Mycobacterium tuberculosis in western Sweden

The genetic diversity of Mycobacterium tuberculosis isolates among patients from Sweden was determined by a combination of two PCR-based techniques (spoligotyping and variable number of tandem repeats analysis). It resulted in a clustering of 23.6% of the isolates and a rate of recent transmission of 14.1%. The clustered isolates mainly belonged to the Haarlem family (23.2%), followed by the Beijing (9.8%), Latin American and Mediterranean (LAM; 8%), and East African-Indian (EAI; 6.2%) families. A comparison of the spoligotypes with those in the international spoligotyping database showed that 62.5% of the clustered isolates and 36.6% of all isolates typed were grouped into six major shared types. A comparison of the spoligotypes with those in databases for Scandinavian countries showed that 33% of the isolates belonged to an ill-defined T family, followed by the EAI (22%), Haarlem (20%), LAM (11%), Central Asian (5%), X (5%), and Beijing (4%) families. Both the highest number of cases and the proportion of clustered cases were observed in patients ages 15 to 39 years. Nearly 10% of the isolates were resistant to one or more drugs (essentially limited to isoniazid monoresistance). However, none of the strains were multidrug resistant. Data on the geographic origins of the patients showed that more than two-thirds of the clustered patients with tuberculosis were foreign-born individuals or refugees. These results are explained on the basis of both the historical links within specific countries and recently imported cases of tuberculosis into Sweden.