Characterization of tick-borne encephalitis virus from Latvia: evidence for co-circulation of three distinct subtypes.

Abstract

Viruses of the tick-borne encephalitis (TBE) antigenic complex within the family Flaviviridae cause a variety of diseases, including uncomplicated febrile illness, meningoencephalitis, and hemorrhagic fever. Different domesticated animals or wildlife species often act as reservoir hosts and ixodid ticks serve as vectors. Although TBE is a serious problem in Latvia, the knowledge concerning TBE virus (TBEV) strains circulating in the country is most limited. Only two strains (Latvia-1-96 isolated from a TBE patient, and RK1424 originating from an Ixodes persulcatus tick), which belonged to the Siberian and the Far Eastern subtypes of TBEV, respectively, have previously been characterized. In the present study, we concentrated on the western and central regions of Latvia, with predominantly Ixodes ricinus ticks. Five virus strains were isolated from serum samples of patients with clinical symptoms of an acute TBE infection. Nucleotide sequences encoding the envelope (E) protein of TBEV, which were recovered from the five TBEV isolates, showed the highest level of identity to the corresponding sequences of the prototype strain Neudoerfl and other European strains of the Western TBEV subtype characterized previously. Accordingly, phylogenetic analysis placed the new Latvian isolates within the Western genetic lineage of TBEV. Taken together with earlier observations, the results proved that all three TBEV subtypes are co-circulating in Latvia and indicated that the genetic diversity of TBEV within certain geographical areas is much more complex than previously believed.