TBEV infection prevalence in questing ticks and DIN
A total of 4,054 questing Ixodes ricinus ticks (7,070 nymphs in 570 pools, 219 females and 199 males in 200 pools) were tested for TBEV (Supplementary Table 1). An overall MIR of 0.11% (95% CI: 0.05 – 0.22) of positive questing nymphs was observed. MIR in nymphs was significantly lower in 2013 (0.03%, 95% CI: 0 – 0.15) than in 2012 and 2014 (0.21%, 95% CI: 0.08 – 0.43, Fisher’s exact test, p-value=0.03) whereas there was no significant difference between 2012 and 2014 (Fisher’s exact test, p-value = 0.7, Figure 3). In 2012 and 2014, the MIR in nymphs was significantly lower in season 1 and 2 (0.13%, 95% CI: 0.04 – 0.34) than in season 3 (0.92%, 95% CI: 0.19 – 2.67, Fisher’s exact test, p-value = 0.02) whereas there was no significant difference between seasons 1 and 2 (Fisher’s exact test, p-value = 1, Figure 3). No questing adult was found to be infected. The minimum TBEV prevalence that could be detected in adult ticks with a probability of 95%, given the overall sampling size from 2012 to 2014 was 0.7%. This suggests that the overall prevalence in adults was less than 0.7%.
The annual cumulated DIN appeared lower in 2013 than 2012 and 2014 (Figure 3) but the difference was not significant (2013: 0.14 infected nymphs per 100 m ², 95% CI: 0 – 0.76; 2012: 0.42 infected nymphs per 100 m², 95% CI: 0.10 – 1.23; 2014: 0.30 infected nymphs per 100 m², 95% CI: 0.08 – 0.74). In other words, around 43 to 136 infected nymphs quested a host each year within the study area as a whole. The DIN was not different per season (Figure 3), ranging from 0.09 [95% CI: 0.02 – 0.25] to 0.15 [95% CI: 0.02 – 0.54].
The eight sequences obtained from questing ticks (collected in 2012, 2013 and 2014) had 100% identity with each other. One sequence was deposited in GenBank (accession number: MT109187) and showed 99% homology with reference sequences from European subtype strains isolated in ticks (Germany, GenBank KX268728; Finland, GenBank MK801808), and human cerebellum (Finland, GenBank MG589937).