Smura et al.
Recent establishment of tick-borne encephalitis foci with distinct viral lineages in the Helsinki area, Finland
Emerg. Microbes Infect. 2019; 8: 675-683
The epidemiology of TBE has changed during the last decades in Finland. In the 1990s, only the outer island of Isosaari in the Helsinki area has been known to be an endemic area. Since 2008, new foci annually appeared in Finland, but no additional TBE foci emerged in the Helsinki area until 2011. In whole Finland as well as in the Helsinki area, the incidence increased from 0.72 to 1.11 in the time span from 2012 to 2016. 2.1% of ticks collected from various locations around Helsinki and Espoo in 2016/17 were positive for TBE. Strains isolated from Espoo formed a monophyletic group together with strains from Kuutsalo, Kotka, which is about 140 km away from Espoo. Strains isolated from Sipoo (5.6% prevalence in the garden of an infected patient) formed another monophyletic group and clustered with strains from Latvia. Strains isolated from Isosaari clustered with isolates from Slovenia, Czech Republic and Germany. Thus, there seems to be an independent introduction of TBE virus lineages to restricted geographical regions in Finland and it is discussed that theses strains (all were of European subtype) have been introduced decades ago by migratory birds into the Helsinki area.