Continued Low Rates of Transmission of Mycobacterium tuberculosis in Norway

Abstract

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.Globally, the number of tuberculosis (TB) cases is currently increasing at 2% per year (8). Programs to keep TB from rising in low-incidence countries are challenged by international traveling and immigration from high-burden countries (2-5, 14, 15, 18, 22). In Norway, patients with a foreign background contribute substantially to the TB incidence (4, 5, 11).In recent years, DNA fingerprinting of Mycobacterium tuberculosis based on restriction fragment length polymorphism (RFLP) analysis with IS6110 as a probe has given new insight into the nature of TB transmission. It has also become an indispensable tool for quality assurance of the processing and culturing of patient samples (1, 4, 5, 7, 9, 13-16, 18, 21, 22). Since 1994, IS6110 RFLP analysis has been performed on basically all strains of M. tuberculosis isolated in Norway (4, 5). However, isolates of M. tuberculosis that possess few copies of IS6110 do not generate sufficient polymorphisms to be readily distinguished by this technique. Another method, spoligotyping, involves the simultaneous detection of the presence or absence of 43 DNA spacer sequences and is superior to IS6110 RFLP analysis in discriminating isolates with fewer than five IS6110 copies (12,13,20). Spoligotyping is therefore generally used to analyze strains with fewer than five copies of IS6110 (12,13,18,20) and has been performed on such strains isolated in...

Analysis of Changes in Recent Tuberculosis Transmission Patterns after a Sharp Increase in Immigration

“…Immigration from countries with high rates of TB to countries with lower rates is a phenomenon which has affected different European and North American countries for quite some time (6,8,12,(16)(17)(18)22). This obviously has an impact on the epidemiological features of TB of the host countries, and therefore the recent transmission patterns of TB are expected to be affected.…”

Migrant tuberculosis screening in the EU/EEA: yield, coverage and limitations

“…Assessment and improvement of cost-effectiveness TB screening programmes are currently the subject of debate because of their costs and public health impact [11,51,[76][77][78][79]. Recent analysis of the cost-effectiveness of screening programmes concluded that current radiographic screening programmes have minimal impact and are not cost-effective [81].…”

“…2) Recent TB infection or reinfection due to travel to the home country [7][8][9]. 3) Recent infection or reinfection within the new country that is related to local exposure and social mixing [6,[10][11][12].…”

Clustering of Mycobacterium tuberculosis strains from foreign-born patients in Korea

“…Interestingly, we suspect that in some incidences, native Korean TB patients may be the source of infection for foreign-born TB patients, as K family strains endemic in Korea (Kim et al, 2001;Park et al, 2000) were identified in foreign-born TB patients. A previous study in Norway also found that outbreaks of TB involving foreign-born individuals were often determined to be of Norwegian origin (Dahle et al, 2003). The origin of strains uncommon in Korea was less clear; some strains, especially those with high RFLP copy numbers, resembled a modern Beijing family type that may have potential for global transmission as they have been identified in multiple countries surrounding Korea (Kang et al, 2010).…”