Tick-borne encephalitis virus (TBEV) is an emerging pathogen that initially causes flu-like symptoms and can progress to central nervous system (CNS) infections. Tick-borne encephalitis (TBE) is an endemic disease in southern coastal counties with regular human cases, while the causative agent, TBEV, is prevalent in ticks in most of the coastal regions of Norway. This study was aimed to understand TBEV infection status across Norway including both TBE endemic and non-endemic areas. For this, we analyzed a total of 1940 residual serum samples from 19 counties of Norway (as of 2016). The samples were initially screened by ELISA, followed by virus neutralization tests for TBEV confirmation. We found a similar TBEV seroprevalence of 1.7% in TBE endemic and 1.6% in non-endemic areas. Since TBE cases are only reported from endemic regions, our findings suggest a potential subclinical or asymptomatic infection and underdiagnosis in non-endemic areas. Notably, only 43% of the ELISA-positive samples were confirmed by virus neutralization tests indicating that not all ELISA positives are true TBEV infections. Additionally, 137 samples of patients presenting with symptoms of CNS infections from a non-endemic area were included. Of these samples, 11 ELISA-positive samples were analyzed for cross-reactivity among flaviviruses. Cross-reactivity was detected with Dengue virus, West Nile Virus, and non-specific reactions. This underscores the importance of using multiple diagnostic tests to confirm TBEV infections. None of the patients with CNS infection was found to be TBE positive, and in the whole cohort, we found a low TBEV seroprevalence of 0.7%.

Department of Bacteriology Division for Infection Control and Environmental Health Norwegian Institute of Public Health 0456 Oslo Norway

Department of Experimental Biology Faculty of Science Masaryk University 62500 Brno Czech Republic

Department of Microbiology Oslo University Hospital 0424 Oslo Norway

Department of Natural Sciences and Environmental Health Faculty of Technology Natural Sciences and Maritime Sciences University of South Eastern Norway Campus Bø 3800 Bø Norway

Department of Science and Environment Molecular and Medical Biology Pandemix Center University of Roskilde 4000 Roskilde Denmark

Department of Virology Division for Infection Control and Environmental Health Norwegian Institute of Public Health 0456 Oslo Norway

Institute of Parasitology Biology Centre of the Czech Academy of Sciences 37005 Ceske Budejovice Czech Republic

Veterinary Research Institute 601 77 Brno Czech Republic

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Lindquist L., Vapalahti O. Tick-borne encephalitis. Lancet. 2008;371:1861–1871. doi: 10.1016/S0140-6736(08)60800-4. PubMed DOI

Calisher C.H., Karabatsos N., Dalrymple J.M., Shope R.E., Porterfield J.S., Westaway E.G., Brandt W.E. Antigenic Relationships between Flaviviruses as Determined by Cross-neutralization Tests with Polyclonal Antisera. J. Gen. Virol. 1989;70:37–43. doi: 10.1099/0022-1317-70-1-37. PubMed DOI

Klaus C., Beer M., Saier R., Schau U., Moog U., Hoffmann B., Diller R., Süss J. Goats and sheep as sentinels for tick-borne encephalitis (TBE) virus–epidemiological studies in areas endemic and non-endemic for TBE virus in Germany. Ticks Tick-Borne Dis. 2012;3:27–37. doi: 10.1016/j.ttbdis.2011.09.011. PubMed DOI

Dai X., Shang G., Lu S., Yang J., Xu J. A new subtype of eastern tick-borne encephalitis virus discovered in Qinghai-Tibet Plateau, China. Emerg. Microbes Infect. 2018;7:1–9. doi: 10.1038/s41426-018-0081-6. PubMed DOI PMC

Demina T., Dzhioev Y.P., Verkhozina M., Kozlova I., Tkachev S., Plyusnin A., Doroshchenko E., Lisak O., Zlobin V. Genotyping and characterization of the geographical distribution of tick-borne encephalitis virus variants with a set of molecular probes. J. Med. Virol. 2010;82:965–976. doi: 10.1002/jmv.21765. PubMed DOI

Ecker M., Allison S.L., Meixner T., Heinz F.X. Sequence analysis and genetic classification of tick-borne encephalitis viruses from Europe and Asia. J. Gen. Virol. 1999;80:179–185. doi: 10.1099/0022-1317-80-1-179. PubMed DOI

Kovalev S.Y., Mukhacheva T.A. Reconsidering the classification of tick-borne encephalitis virus within the Siberian subtype gives new insights into its evolutionary history. Infect. Genet. Evol. 2017;55:159–165. doi: 10.1016/j.meegid.2017.09.014. PubMed DOI

Lamsal A., Edgar K.S., Jenkins A., Renssen H., Kjær L.J., Alfsnes K., Bastakoti S., Dieseth M., Klitgaard K., Lindstedt H.E.H., et al. Prevalence of tick-borne encephalitis virus in questing Ixodes ricinus nymphs in southern Scandinavia and the possible influence of meteorological factors. Zoonoses Public Health. 2023;70:473–484. doi: 10.1111/zph.13049. PubMed DOI

Vikse R., Paulsen K.M., Edgar K.S., Pettersson J.H.-O., Ottesen P., Okbaldet Y.B., Kiran N., Lamsal A., Lindstedt H.E.H., Pedersen B.N., et al. Geographical distribution and prevalence of tick-borne encephalitis virus in questing Ixodes ricinus ticks and phylogeographic structure of the Ixodes ricinus vector in Norway. Zoonoses Public Health. 2020;67:370–381. doi: 10.1111/zph.12696. PubMed DOI

Soleng A., Edgar K.S., Paulsen K.M., Pedersen B.N., Okbaldet Y.B., Skjetne I.E.B., Gurung D., Vikse R., Andreassen Å.K. Distribution of Ixodes ricinus ticks and prevalence of tick-borne encephalitis virus among questing ticks in the Arctic Circle region of northern Norway. Ticks Tick-Borne Dis. 2018;9:97–103. doi: 10.1016/j.ttbdis.2017.10.002. PubMed DOI

Paulsen K.M., Pedersen B.N., Soleng A., Okbaldet Y.B., Pettersson J.H., Dudman S.G., Ottesen P., Vik I.S.S., Vainio K., Andreassen Å. Prevalence of tick-borne encephalitis virus in Ixodes ricinus ticks from three islands in north-western Norway. Apmis. 2015;123:759–764. doi: 10.1111/apm.12412. PubMed DOI

Larsen A.L., Kanestrøm A., Bjørland M., Andreassen Å., Soleng A., Vene S., Dudman S.G. Detection of specific IgG antibodies in blood donors and tick-borne encephalitis virus in ticks within a non-endemic area in southeast Norway. Scand. J. Infect. Dis. 2014;46:181–184. doi: 10.3109/00365548.2013.865140. PubMed DOI

Marvik Å., Tveten Y., Pedersen A.-B., Stiasny K., Andreassen Å.K., Grude N. Low prevalence of tick-borne encephalitis virus antibodies in Norwegian blood donors. Infect. Dis. 2021;53:44–51. doi: 10.1080/23744235.2020.1819561. PubMed DOI

Thortveit E.T., Aase A., Petersen L.B., Lorentzen Å.R., Mygland Å., Ljøstad U. Human seroprevalence of antibodies to tick-borne microbes in southern Norway. Ticks Tick-Borne Dis. 2020;11:101410. doi: 10.1016/j.ttbdis.2020.101410. PubMed DOI

Svensson J., Christiansen C.B., Persson K.E.M. A Serosurvey of Tick-Borne Encephalitis Virus in Sweden: Different Populations and Geographical Locations. Vector-Borne Zoonotic Dis. 2021;21:614–619. doi: 10.1089/vbz.2020.2763. PubMed DOI

Fält J., Lundgren Å., Alsterlund R., Carlsson B., Eliasson I., Haglund M., Lundkvist Å., Vene S., Settergren B. Tick-borne encephalitis (TBE) in Skåne, southern Sweden: A new TBE endemic region? Scand. J. Infect. Dis. 2006;38:800–804. doi: 10.1080/00365540600664068. PubMed DOI

MSIS Tick-Borne Encephalitis. Norwegian Surveillance System for Communicable Diseases. 2023. [(accessed on 7 February 2025)]. Available online: https://www.fhi.no/nettpub/smittevernveilederen/sykdommer-a-a/skogflattencefalitt-tbe-virusinfeks.

Andreassen A., Jore S., Cuber P., Dudman S., Tengs T., Isaksen K., Hygen H.O., Viljugrein H., Ånestad G., Ottesen P., et al. Prevalence of tick borne encephalitis virus in tick nymphs in relation to climatic factors on the southern coast of Norway. Parasit. Vectors. 2012;5:177. doi: 10.1186/1756-3305-5-177. PubMed DOI PMC

Dobler G., Hufert F., Pfeffer M., Essbauer S. Progress in Parasitology. Springer; Berlin/Heidelberg, Germany: 2011. Tick-borne encephalitis: From microfocus to human disease; pp. 323–331.

Burri C., Cadenas F.M., Douet V., Moret J., Gern L. Ixodes ricinus density and infection prevalence of Borrelia burgdorferi sensu lato along a north-facing altitudinal gradient in the Rhône Valley (Switzerland) Vector-Borne Zoonotic Dis. 2007;7:50–58. doi: 10.1089/vbz.2006.0569. PubMed DOI

Schley K., Friedrich J., Pilz A., Huang L., Balkaran B.L., Maculaitis M.C., Malerczyk C. Evaluation of under-testing and under-diagnosis of tick-borne encephalitis in Germany. BMC Infect. Dis. 2023;23:139. doi: 10.1186/s12879-023-08101-6. PubMed DOI PMC

Lundkvist Å., Wallensten A., Vene S., Hjertqvist M. Tick-borne encephalitis increasing in Sweden, 2011. Eurosurveillance. 2011;16:19981. doi: 10.2807/ese.16.39.19981-en. PubMed DOI

Kaiser R. Tick-borne encephalitis (TBE) in Germany and clinical course of the disease. Int. J. Med. Microbiol. 2002;291:58–61. doi: 10.1016/S1438-4221(02)80012-1. PubMed DOI

Bogovic P., Strle F. Tick-borne encephalitis: A review of epidemiology, clinical characteristics, and management. World J. Clin. Cases. 2015;3:430–441. doi: 10.12998/wjcc.v3.i5.430. PubMed DOI PMC

Ergunay K., Tkachev S., Kozlova I., Růžek D. A Review of Methods for Detecting Tick-Borne Encephalitis Virus Infection in Tick, Animal, and Human Specimens. Vector-Borne Zoonotic Dis. 2016;16:4–12. doi: 10.1089/vbz.2015.1896. PubMed DOI

Stiasny K., Malafa S., Aberle S.W., Medits I., Tsouchnikas G., Aberle J.H., Holzmann H., Heinz F.X. Different Cross-Reactivities of IgM Responses in Dengue, Zika and Tick-Borne Encephalitis Virus Infections. Viruses. 2021;13:596. doi: 10.3390/v13040596. PubMed DOI PMC

Lamsal A., Tryland M., Paulsen K.M., Romano J.S., Nymo I.H., Stiasny K., Soleng A., Vikse R., Andreassen Å.K. Serological screening for tick-borne encephalitis virus in eight Norwegian herds of semi-domesticated reindeer (Rangifer tarandus tarandus) Zoonoses Public Health. 2023;70:692–698. doi: 10.1111/zph.13060. PubMed DOI

Quist-Paulsen E., Kran A.-M.B., Lindland E.S., Ellefsen K., Sandvik L., Dunlop O., Ormaasen V. To what extent can clinical characteristics be used to distinguish encephalitis from encephalopathy of other causes? Results from a prospective observational study. BMC Infect. Dis. 2019;19:80. doi: 10.1186/s12879-018-3570-2. PubMed DOI PMC

Public Health Agency of Sweden Areas with Increased Incidence of TBE. 2023. [(accessed on 11 November 2023)]. Available online: https://www.folkhalsomyndigheten.se/smittskydd-beredskap/smittsamma-sjukdomar/tick-borne-encephalitis-tbe/omraden-med-forhojd-forekomst-av-tbe/

Statens Serum Institut TBE Report on Disease Occurrence 2017–2019. Statens Serum Institut. 2020. [(accessed on 11 November 2023)]. Available online: https://en.ssi.dk/surveillance-and-preparedness/surveillance-in-denmark/annual-reports-on-disease-incidence/tbe-report-on-disease-occurrence-2017-2019.

Kunze M., Banović P., Bogovič P., Briciu V., Čivljak R., Dobler G., Hristea A., Kerlik J., Kuivanen S., Kynčl J., et al. Recommendations to Improve Tick-Borne Encephalitis Surveillance and Vaccine Uptake in Europe. Microorganisms. 2022;10:1283. doi: 10.3390/microorganisms10071283. PubMed DOI PMC

Hellenbrand W., Kreusch T., Böhmer M., Wagner-Wiening C., Dobler G., Wichmann O., Altmann D. Epidemiology of Tick-Borne Encephalitis (TBE) in Germany, 2001–2018. Pathogens. 2019;8:42. doi: 10.3390/pathogens8020042. PubMed DOI PMC

Waldeck M., Winqvist N., Henriksson G., Dyrdak R., Settergren B., Lindgren P.-E. Surveillance of tick-borne encephalitis in emerging risk areas in southern Sweden: A retrospective case finding study. Eur. J. Clin. Microbiol. Infect. Dis. 2023;42:13–22. doi: 10.1007/s10096-022-04509-1. PubMed DOI PMC

Gonzalez G., Bournez L., Moraes R.A., Marine D., Galon C., Vorimore F., Cochin M., Nougairède A., Hennechart-Collette C., Perelle S., et al. A One-Health Approach to Investigating an Outbreak of Alimentary Tick-Borne Encephalitis in a Non-endemic Area in France (Ain, Eastern France): A Longitudinal Serological Study in Livestock, Detection in Ticks, and the First Tick-Borne Encephalitis Virus Isolation and Molecular Characterisation. Front. Microbiol. 2022;13:863725. doi: 10.3389/fmicb.2022.863725. PubMed DOI PMC

Burke G.S., Wikel S.K., Spielman A., Telford S.R., McKay K., Krause P.J. Hypersensitivity to Ticks and Lyme Disease Risk. Emerg. Infect. Dis. 2005;11:36. doi: 10.3201/eid1101.040303. PubMed DOI PMC

Alkishe A.A., Peterson A.T., Samy A.M. Climate change influences on the potential geographic distribution of the disease vector tick Ixodes ricinus. PLoS ONE. 2017;12:e0189092. doi: 10.1371/journal.pone.0189092. PubMed DOI PMC

Borde J.P., Kaier K., Hehn P., Matzarakis A., Frey S., Bestehorn M., Dobler G., Chitimia-Dobler L. The complex interplay of climate, TBEV vector dynamics and TBEV infection rates in ticks—Monitoring a natural TBEV focus in Germany, 2009–2018. PLoS ONE. 2021;16:e0244668. doi: 10.1371/journal.pone.0244668. PubMed DOI PMC

Jaenson T.G., Petersson E.H., Jaenson D.G., Kindberg J., Pettersson J.H.-O., Hjertqvist M., Medlock J.M., Bengtsson H. The importance of wildlife in the ecology and epidemiology of the TBE virus in Sweden: Incidence of human TBE correlates with abundance of deer and hares. Parasit. Vectors. 2018;11:477. doi: 10.1186/s13071-018-3057-4. PubMed DOI PMC

Cagnacci F., Bolzoni L., Rosà R., Carpi G., Hauffe H.C., Valent M., Tagliapietra V., Kazimirova M., Koci J., Stanko M., et al. Effects of deer density on tick infestation of rodents and the hazard of tick-borne encephalitis. I: Empirical assessment. Int. J. Parasitol. 2012;42:365–372. doi: 10.1016/j.ijpara.2012.02.012. PubMed DOI

Achazi K., Růžek D., Donoso-Mantke O., Schlegel M., Ali H.S., Wenk M., Schmidt-Chanasit J., Ohlmeyer L., Rühe F., Vor T. Rodents as sentinels for the prevalence of tick-borne encephalitis virus. Vector-Borne Zoonotic Dis. 2011;11:641–647. doi: 10.1089/vbz.2010.0236. PubMed DOI PMC

Yoshii K., Takahashi-Iwata I., Shirai S., Kobayashi S., Yabe I., Sasaki H. A Retrospective Epidemiological Study of Tick-Borne Encephalitis Virus in Patients with Neurological Disorders in Hokkaido, Japan. Microorganisms. 2020;8:1672. doi: 10.3390/microorganisms8111672. PubMed DOI PMC

Chan K.R., Ismail A.A., Thergarajan G., Raju C.S., Yam H.C., Rishya M., Sekaran S.D. Serological cross-reactivity among common flaviviruses. Front. Cell. Infect. Microbiol. 2022;12:975398. doi: 10.3389/fcimb.2022.975398. PubMed DOI PMC

Krzysiak M.K., Anusz K., Konieczny A., Rola J., Salat J., Strakova P., Olech W., Larska M. The European bison (Bison bonasus) as an indicatory species for the circulation of tick-borne encephalitis virus (TBEV) in natural foci in Poland. Ticks Tick-Borne Dis. 2021;12:101799. doi: 10.1016/j.ttbdis.2021.101799. PubMed DOI

Lim S.M., Geervliet M., Verhagen J.H., Müskens G.J.D.M., Majoor F.A., Osterhaus A.D.M.E., Martina B.E.E. Serologic evidence of West Nile virus and Usutu virus infections in Eurasian coots in The Netherlands. Zoonoses Public Health. 2018;65:96–102. doi: 10.1111/zph.12375. PubMed DOI

Klaus C., Ziegler U., Kalthoff D., Hoffmann B., Hoffmann B., Beer M. Tick-borne encephalitis virus (TBEV)—Findings on cross reactivity and longevity of TBEV antibodies in animal sera. BMC Vet. Res. 2014;10:78. doi: 10.1186/1746-6148-10-78. PubMed DOI PMC