Phylogeography of Francisella tularensis subspecies holarctica in Finland, 1993–2011

Background: Finland repeatedly reports some of the highest incidences of tularaemia worldwide. To determine genetic diversity of the aetiologic agent of tularaemia, Francisella tularensis, a total of 76 samples from humans (n = 15) and animals (n = 61) were analysed. Methods: We used CanSNPs and canINDEL hydrolysis or TaqMan MGB probes for the analyses, either directly from the clinical tissue samples (n = 21) or from bacterial isolates (n = 55). Results: The genotypes of the strains were assigned to three previously described basal subspecies holarctica clades. The majority of strains (n = 67) were assigned to B.12, a clade reported to dominate in Scandinavia and Eastern Europe. A single strain was assigned to clade B.4, previously reported from North America, Europe and China. The remaining strains (n = 8) were members of clade B.6. Importantly, new diversity was discovered in clade B.6. We describe two newly designed TaqMan MGB probe assays for this new B.6 subclade B.70, and its previously identified sister clade B.11, a clade dominantly found in Western Europe. Conclusions: The high genetic diversity of F. tularensis subspecies holarctica present in Finland is consistent with previous findings in Sweden. The results suggest a northern and southern division of the B.6 subclade B.10, where B.11 predominates in Western and Central Europe and B.70 is found in Fennoscandia. Further research is required to define whether the vast diversity of genotypes found is related to different habitats or reservoir species, their different postglacial immigration routes to Fennoscandia, or dynamics of the reservoir species.