Prevalence of tick-borne encephalitis virus in Ixodes ricinus ticks in Finland

Approximately 20 cases of tick-borne encephalitis (TBE) occur annually in Finland. The known endemic areas are situated mainly in the archipelago and coastal regions of Finland, with highest incidence in Aland islands. Ixodes ricinus panels collected in 1996-1997 from two endemic areas were screened for the presence of RNA. Two distinct RT-PCR methods were applied, and were shown to have an approximate detection limit of 10 focus forming doses (FFD)/100 microl. One out of 20 pools (a total of 139 ticks) from Helsinki Isosaari Island and one out of 48 pools (a total of 450 ticks) from Aland were positive with both methods, whereas the remaining pools were negative. The observed overall frequency (0.34%) in ticks in endemic areas of Finland, was similar to the low incidence found by virus isolation in mice in the 1960s (0.5%). Viral RNA was detectable in a diluted sample representing 0.005% of a positive pool of ten nymphs suggesting that the viral RNA load within an infected tick pool was approximately equivalent to 20,000-200,000 FFD. Sequence analysis did not show geographical clustering of the Finnish strains, suggesting an independent emergence of different TBE foci from the south. TBE virus RNA positive ticks were not found in I. ricinus panels consisting of 130 pools (726 ticks) from Helsinki city parks or 41 pools (197 ticks) from Võrmsi Island in Estonia.