Characterization of Tick-borne enchephalitis virus from Latvia – evidence for co-circulation of three distinct subtypes

Åke Lundkvist (1, 2), Sirkka Vene (1, 2), Irina Golovljova (3), Violeta Mavtchoutko (4), Marianne Forsgren (5), Vaira Kalnina (4), Alexander Plyusnin (1, 6)

1. Swedish Institute for Infectious Disease Control, S-171 82 Stockholm, Sweden; 
2. Microbiology and Tumourbiology Center, Karolinska Institutet, S-171 77 Stockholm, Sweden;
3. Institute of Experimental and Clinical Medicine, EE-116 19 Tallinn, Estonia;
4. National Environmental Health Centre, LV-1012 Riga, Latvia;
5. Division of Clinical Virology, Department of Microbiology, Pathology and Immunology, Karolinska Institutet, Huddinge University Hospital, Stockholm, Sweden;
6. Haartman Institute, Department of Virology, POB 21, FIN-00014 University of Helsinki, Finland.

email: akelun@mbox.ki.se

Viruses of the tick-borne encephalitis (TBE) antigenic complex, within the family Flaviviridae, cause a variety of diseases including uncomplicated febrile illness, meningo-encephalitis and hemorrhagic fever. Different domesticated animals or wildlife species often act as reservoir hosts with ixodid ticks as vectors. Although TBE is a serious problem in Latvia, with approximately 300-1000 serologically confirmed cases each year, the knowledge concerning TBEV strains circulating in the country is most limited. Only two strains (Latvia-1-96 isolated from a TBE patient, and RK1424 originating from a tick), which belonged to the Siberian and the Far Eastern subtypes of TBEV, respectively, have previously been characterized. In the present study, five virus strains were isolated from serum samples of Latvian 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 previously characterized European strains of the Western TBEV subtype. 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 strain variation of TBEV within certain areas is much more complex than previously believed.

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