Competence of the vector restricting tick-borne encephalitis virus spread

With 1028 reported human cases in 2018 and 2019, the tick-borne encephalitis virus is considered a major health risk in Germany. Risk areas of TBEV are mainly located in the south of Germany. In contrast to this focal distribution, the main vector of TBEV in Germany, Ixodes (I.) ricinus, as well as host species are spread countrywide, indicating that other factors apart from the presence of suitable vectors and hosts are important for enzootic circulation of TBEV. The current state of knowledge concerning the vector competence of geographically separated I. ricinus populations for the TBEV is incomplete. Therefore, experimental infection studies using Ixodes ticks of different populations, distributed across Germany, and different TBEV isolates were conducted to investigate possible population-based differences in infection susceptibility. To achieve this goal, an artificial feeding system has been used to infect field collected I. ricinus nymphs with different TBEV strains. Over three seasons, the susceptibility to TBEV infection have been analyzed involving different impact factors as seasonality, natural co-infection and the correlation of tick population with a respective TBEV strain. Ixodes ricinus nymphs were collected in four TBEV endemic foci as well as one non-endemic area. To investigate specific virus isolate/tick population relationships, ticks were exposed to different virus isolates by in vitro feeding and compared regarding their feeding behavior as well as their infection susceptibility for the respective TBEV strains. Differences in the intrinsic susceptibility of the I. ricinus tick vector to TBEV infection seem to be related to genetic pairings of vector and virus. Furthermore, infection with Borrelia spp. may influence the ability of tick populations to spread TBEV. These findings can help to deepen the understanding of TBEV focal transmission cycle as it is critical for predicting and mitigating human disease risk.