Continued Circulation of Tick-Borne Encephalitis Virus Variants and Detection of Novel Transmission Foci, the Netherlands

Abstract

Tick-borne encephalitis virus (TBEV) is an emerging pathogen that was first detected in ticks and humans in the Netherlands in 2015 (ticks) and 2016 (humans). To learn more about its distribution and prevalence in the Netherlands, we conducted large-scale surveillance in ticks and rodents during August 2018-September 2020. We tested 320 wild rodents and >46,000 ticks from 48 locations considered to be at high risk for TBEV circulation. We found TBEV RNA in 3 rodents (0.9%) and 7 tick pools (minimum infection rate 0.02%) from 5 geographically distinct foci. Phylogenetic analyses indicated that 3 different variants of the TBEV-Eu subtype circulate in the Netherlands, suggesting multiple independent introductions. Combined with recent human cases outside known TBEV hotspots, our data demonstrate that the distribution of TBEV in the Netherlands is more widespread than previously thought.

Keywords: Apodemus; Ixodes ricinus; Microtus; Myodes; antibodies; arbovirus; distribution; emergence; human cases; monitoring; rodent; sentinel; surveillance; the Netherlands; tick-borne encephalitis; tick-borne encephalitis virus; tickborne disease; vector-borne infections; viruses.

Figure 1

Geographic distribution of tick-borne encephalitis virus (TBEV) in the Netherlands based on sampling of ticks (A), rodents (B), roe deer (C), and reported human (D) tick-borne encephalitis cases. Stars indicate TBEV RNA–positive tick pools or rodent samples. Closed circles indicate serum samples that tested positive in TBEV serum neutralization tests. White circles indicate negative test results. Data for roe deer were reproduced from Rijks et al. (14) with permission. Maps were constructed with Arc-GIS software (ESRI, https://www.esri.com).

Figure 2

Maximum-likelihood phylogenetic tree of polyprotein sequences obtained from tick-borne encephalitis virus RNA–positive Ixodes ricinus ticks collected from 3 locations in the Netherlands during 2016–2020 (in bold). Additional published sequences obtained from GenBank are included for reference. Louping ill virus is used as the outgroup. Sample ID or GenBank accession numbers are indicated for each sequence, with location in brackets (if known) and country code, original isolation source, and collection year of each sample. Numbers next to each branch indicate the percentage of trees resulting from bootstrapping on the basis of 1,000 pseudoreplicate datasets for which the associated taxa clustered together. Scale bar represents the percentage of genetic variation along tree branches.