Multiple variants of tick-borne encephalitis virus in voles, mice and ticks, the Netherlands, 2021 to 2023.

BackgroundTick-borne encephalitis (TBE) can be a severe neurological disease. Identifying ecological factors that may facilitate tick-borne encephalitis virus (TBEV) circulation in the Netherlands could improve awareness and detection.AimWe aimed to identify ecological factors affecting TBEV circulation in the Netherlands and to determine if there is sustained circulation and spread of the virus.MethodsBetween June and September 2021, rodents and ticks from three previously TBEV-positive locations were tested for TBEV by PCR. We sequenced TBEV and compared the sequences with previous and subsequent sequences from the Netherlands and other countries to investigate the spread of TBEV-variants.ResultsWe captured 383 rodents, 928 feeding ticks and 1,571 questing Ixodes ticks and detected TBEV from six (three Apodemus sylvaticus and three Clethrionomys glareolus) (2.9%) of 206 tested rodents and two (0.9%) of 215 questing tick pools. Detection of TBEV was associated with questing tick density (Mann-Whitney U test  = 81.5; 95% confidence interval (CI): - 3.7-6.3 × 10-5; p = 0.05). Tick larvae (odds ratio (OR) = 9.0; 95% CI: 2.8-38.2; p < 0.01) and nymphs (OR = 3.8; 95% CI: 1.3-13.6; p < 0.01) were more frequent on A. sylvaticus than on C. glareolus. Sequence comparisons suggest multiple introductions and local circulation of TBEV but no spread among locations.ConclusionTick-borne encephalitis virus occurs in diverse woodlands in the Netherlands, posing a risk to those frequenting these areas. Surveillance for the early detection and monitoring of TBEV spread, along with public awareness campaigns on preventive measures, should continue. Recognition of TBE symptoms and supportive diagnostics should be made available nationwide.