In industrialized countries the majority of tuberculosis (TB) cases are linked to immigration. In Finland, most cases are still Finnish born but the number of foreign born cases is steadily increasing. In this 4-year population based study, the TB situation in Finland was characterized by a genotypic analysis of Mycobacterium tuberculosis isolates. A total of 1048 M. tuberculosis isolates (representing 99.4% of all culture positive cases) were analyzed by spoligotyping and MIRU. Spoligotype lineages belonging to the Euro-American family were predominant among the Finnish isolates, particularly T (n=346, 33.0%) and Haarlem (n=237, 22.6%) strains. The lineage signature was unknown for 130 (12.4%) isolates. Out of the 17 multi-drug resistant TB strains, 10 (58.8%) belonged to the Beijing lineage. In total, 23 new SIT designations were given and 51 orphan strains were found, of which 58 patterns were unique to Finland. Phylogeographical TB mapping as compared to neighboring countries showed that the population structure in Finland most closely resembled that observed in Sweden. By combining spoligotyping and MIRU results, 98 clusters comprising 355 isolates (33.9%) were found. Only 10 clusters contained both Finnish and foreign born cases. In conclusion, a large proportion of the M. tuberculosis isolates were from Finnish born elderly patients. Moreover, many previously unidentified spoligotype profiles and isolates belonging to unknown lineages were encountered.
The Mycobacterium tuberculosis genotypes obtained from elderly Finns were assessed and compared with those obtained from younger Finns to comprehend the epidemiology of tuberculosis (TB) in Finland. From 2008 to 2011, a total of 1021 M. tuberculosis isolates were characterized by spoligotyping and 15-locus mycobacterial interspersed repetitive units-variable number tandem repeat typing. In total, 733 Finnish-born cases were included in the study, of which 466 (64%) were born before 1945 (older Finns). Of these, 63 (14%) shared an M. tuberculosis genotype with foreign-born or younger Finnish cases (born after 1945), and 59 (13%) shared a genotype with older Finnish cases. Eighty-five per cent had a unique genotypic profile while 70% belonged to T or Haarlem families, suggesting that ongoing transmission is infrequent among young and elderly Finns. Simultaneous reactivation of TB among older Finns was the most likely cause for clustering. As most isolates belonged to Haarlem or T, Finland was most likely affected by a similar TB epidemic at the beginning of the twentieth century as that seen in Sweden and Norway. Younger Finns were significantly more likely to be clustered (56% versus 27%, p<0.001), have pulmonary TB (87% versus 71%, p<0.001) and to be sputum smear positive (57% versus 48%, p<0.05) indicating that the risk of TB transmission from younger Finns is likely to be larger than from older Finns. The M. tuberculosis isolates from elderly Finns were associated with dominant lineages of the early twentieth century and differed from the heterogeneous lineages found among younger TB patients. Additionally, younger TB patients were more likely to transmit TB than elderly Finns.
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