Descriptive and statistical analyses of tick density and TBEV infection prevalence in ticks
The monthly density of questing ticks was estimated for each tick stage by the mean density of questing ticks along each sampling transect, and was expressed as the mean number of ticks per 100 m² with a 95% confidence interval (95% CI). The density of questing ticks was estimated per season by the mean of the monthly density of questing ticks in the corresponding months, and per year by summing the tick density for each season (= annual cumulated DON). Tick questing density per year was compared using the observed yearly peak density and the annual cumulated DON with a Mann-Whitney U test.
As TBEV infection prevalence in ticks is usually low, having been described as lower than 1% in Alsace (Bestehorn et al., 2018; Perez-Eid et al., 1992), prevalence in ticks was expressed as the minimum infection rate per 100 tested (MIR) based on the assumption that a single tick was positive within a positive pool. Exact 95% CIs were calculated on the basis of binomial distribution. For the 2012-2014 period, MIR was calculated per season for each year. The inter-annual and inter-seasonnal MIR of questing ticks was compared using Fisher’s exact test. If no TBEV was detected in a sample of ticks, we calculated the minimum prevalence of TBEV that could be detected in ticks with a probability of 95% given the sampling size used by applying the formula proposed by Cannon (Cannon, 2001).
We calculated the density of infected nymphs (DIN) per season by multiplying the seasonal MIR divided by 100 by the seasonal density of questing nymphs, and the DIN per year by multiplying the annual MIR divided by 100 with the annual cumulated DON. We used the lower and upper limits of the 95% CI for MIR per season and year to estimate the lower and upper bounds of the 95% CI for DIN.