Freimane et al.
Seroprevalence of tick-borne encephalitis virus in Latvia using standard and subtype-specific NS1-based ELISA assays Pathogens 2025, 14, 1115, doi.org/10.3390/pathogens141115
Latvia is a Baltic country with a historically high incidence of TBE. In this country, the European, Siberian and Far Eastern subtypes of the TBE virus have been confirmed in Ixodes ricinus and I. persulacatus ticks (see Snapshot week 41/2025). It has been assumed that the different subtype infections exhibit different clinical courses and outcomes, and therefore, serological differentiation is important. A cross-sectional study has been carried out to determine the seroprevalence in the vaccinated and non-vaccinated Latvian population and to differentiate between various subtype infections based on a newly developed serotype-specific NS1-IgG-ELISA (Snapshot week 37/2024).
In healthy residents, including both vaccinated and non-vaccinated individuals, 39.7% of participants were TBE positive using a standard TBE-IgG-ELISA. In 44 of 423 participants tested, the subtype could be determined by using the subtype-specific NS1-IgG-ELISA. In 36/44 samples (81.8%), TBE-EU was verified, in 7/44 (15.9%) samples, TBE-Sib was identified, and TBE-FE was identified in one sample. The latter sample was from an 87-year-old male who had received four TBE vaccine doses starting in 2007 and reported no TBE history. Additional data are needed to clarify this single FE subtype case to determine whether it reflects a true infection or is a result of cross-reactivity or other potential limitations of the assay.
Subtype differentiation showed that TBE-EU predominated in all regions of Latvia and TBE-Sib was detected throughout the whole country, with the highest numbers in the northeast region.
Among non-vaccinated individuals, the standard ELISA demonstrated TBE seroprevalence of 16.3%; however, the NS1-IgG-ELISA showed a lower overall seroprevalence of 2.9%. This discrepancy may reflect limitations in historical vaccination records, undiagnosed prior vaccination, or possible sensitivity and specific limitations of both tests.
In conclusion, the study highlights a high risk of TBE in Latvia, with substantial seroprevalence in the non-vaccinated Latvian individuals. The NS1-ELISA offers a valuable tool for refining epidemiological surveillance and true infection rates in endemic settings.