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Journal of Clinical Microbiology, July 2003, p. 2968-2973, Vol. 41, No. 7
0095-1137/03/$08.00+0     DOI: 10.1128/JCM.41.7.2968-2973.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Continued Low Rates of Transmission of Mycobacterium tuberculosis in Norway{dagger}

Ulf R. Dahle,* Per Sandven, Einar Heldal, and Dominique A. Caugant

Norwegian Institute of Public Health, Nydalen, N-0403 Oslo, Norway

Received 2 December 2002/ Returned for modification 23 January 2003/ Accepted 30 March 2003

In this study, we determined the genetic diversity of Mycobacterium tuberculosis isolated in Norway from 1999 to 2001. The results were compared to those for strains isolated from 1994 to 1998. A total of 818 patients were diagnosed with tuberculosis (TB) during the last 3-year period. Of these cases, 576 (70%) were verified by culturing, and strains from 551 patients (96%) were analyzed by the IS6110 restriction fragment length polymorphism (RFLP) method. We excluded 13 strains (2.4%) from the analyses, since they were found to represent false-positive samples. A total of 67 strains (12%) that carried fewer than five copies of IS6110 were analyzed by spoligotyping. The strains were from 157 patients (29%) of Norwegian origin and 381 patients (71%) of foreign origin. The rate of diversity among all of the strains was 90%, while in 1994 to 1998 it was 87%. Clusters were assumed to have arisen from recent transmission; the degree of such transmission was 10% in 1999 to 2001, while for the whole 8-year period (1994 to 2001), it was 11%. Of the 109 patients diagnosed as being part of a cluster in 1999 to 2001, 89 were infected with a strain that carried more than four copies of IS6110. Among these 89 patients, 52 (58%) were infected with a strain that had already been identified in 1994 to 1998. The results indicated that most cases of TB in Norway were due to the import of new strains rather than to transmission within the country. This finding demonstrates that screening of immigrants for TB upon arrival in Norway needs to be improved. Outbreaks, however, were caused mainly by strains that have been circulating in Norway for many years.

* Corresponding author. Mailing address: Division of Infectious Disease Control, Norwegian Institute of Public Health, P.O. Box 4404, Nydalen, N-0403 Oslo, Norway. Phone: (47) 22 04 23 65. Fax: (47) 22 04 25 18. E-mail: ulf.dahle{at}fhi.no.

{dagger} This article was written as part of European Union Concerted Action on TB project QLK2-CT-2000-00630.

Journal of Clinical Microbiology, July 2003, p. 2968-2973, Vol. 41, No. 7
0095-1137/03/$08.00+0     DOI: 10.1128/JCM.41.7.2968-2973.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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