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

Abstract

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.

Prevalence and Genotyping of Tick-Borne Encephalitis Virus in QuestingIxodes ricinusTicks in a New Endemic Area in Western Switzerland

“…The presence of TBEV in questing ticks could be conÞrmed in one endemic region (region A, Plaine de lÕOrbe), situated West of the Lake of Neuchâtel, with a global MIR of 0.1 per 100 tested ticks (6/6120). This prevalence is in accordance with the study of Burri et al (2011) carried out in a known endemic region located near Bern (Switzerland) and is also in line with TBEV prevalence recorded in Europe (Durmisi et al 2011, DÕAgaro et al 2009, Gäumann et al 2010, Han et al 2001.…”

Diagnosis of Tick-Borne Encephalitis by a μ-Capture Immunoglobulin M-Enzyme Immunoassay Based on Secreted Recombinant Antigen Produced in Insect Cells

“…The European subtype of TBEV is the only subtype endemic in Finland (11), so in our material we did not have tick-borne encephalitis patients infected with the Far Eastern or Siberian subtypes. However, it is most likely that an antigen based on one of the TBEV subtypes recognizes the antibodies formed against any TBEV strain, as suggested by the facts that the formaldehyde-inactivated vaccine based on the European subtype protects from a disease caused by any of the three TBEV subtypes (4,15,16,21,34,41) and that mice immunized with the western subtype and then challenged with different western and Far Eastern strains show no significant difference in crossprotection experiments (16).…”

Prevalence and genetic variability of tick-borne encephalitis virus in host-seeking Ixodes ricinus in northern Italy

“…In particular, two possible evolutionary constraints affecting the genetic diversity of TBEV are the combination of the tick's long lifespan and its infrequent feeding (only three blood meals per generation), which imply a limited viral replication rate in ticks per tick generation, and the necessity for TBEV replication in two very distantly related biological systems (Marin et al, 1995;Weaver, 2006;Gould & Solomon, 2008). Previous molecular epidemiological studies suggest that TBEV has evolved slowly while spreading towards the north and west (Zanotto et al, 1996;Gould et al, 2003), and within-subtype observations suggest that the European subtype in particular has levels of low genetic diversity (Han et al, 2001;Haglund et al, 2003;D'Agaro et al, 2009). The majority of phylogenetic studies of TBEV have analysed the envelope glycoprotein (E) (Haglund et al, 2003;Melik et al, 2007; Suzuky, 2007).…”