Vikse et al.
Geographical distribution and prevalence of tick-borne encephalitis virus in questing Ixodes ricinus ticks and phylogeographic structure of Ixodes ricinus vector in Norway
Zoonoses Public Health 2020, in press
In Norway, the first TBE case has been reported in 1998. Most cases occur in the southern parts of the country, specifically in Vest- and Aust-Agder (see also Snapshot week 15/2020), Vestfold, Telemark and Buskerud. From 1998 to 2018, a total of 169 TBE cases were reported in Norway, of which 139 have been acquired within the country.
A study has been carried out about the distribution and phylogeographic structure of Ixodes ricinus together with the prevalence of TBE virus in ticks collected along the Norwegian coastline. A total of 44,000 nymphs and 3,404 adult questing I. ricinus have been flagged from 2009 to 2016 at 62 different sites from the southeast to Nordland in the north from 1 to 155 m above sea level and mostly less than 1000 m from the coastline. Only I. ricinus ticks were found (no I. persulcatus) and the phylogenetic tree suggests that there is limited phylogeographic structure for I. ricinus in Norway and the sequences were found together in a clade with sequences from Denmark, Italy and Finland.
TBE virus was detected in nymphs and adults at the coastal counties from Østfold to Nordland. In adults, the overall mean prevalence for TBE virus was 4.3% and in nymphs 0.3% – with a strong variation between sampling sites, counties and years. Yet, TBE cases have only been reported from the southern Norwegian counties. However, it could be shown in this study that the TBE virus seems to have a greater geographic distribution than previously known, and TBE virus positive ticks were found in areas with no reported human TBE cases, such as the western, eastern and northern parts of Norway.