1 August Episode 341 Tick-born Encephalitis in Western Europe

Tue, 08/01/2023 - 21:00

Episode 341: Tick-Borne Encephalitis (TBEV) 

Dear colleagues,

Based on a question about the necessity of vaccination against Tick-Borne Encephalitis (or ”Frühsommer Meningoenzephalitis”), I composed the following review. 

As usually, I start with a long introduction on virology, epidemiology, clinic.. for those who are not familiar with this infection, because it is still poorly known in the Western part of Europe, although, as we will see, it is also emerging  in our countries). 

Special attention goes to the last paragraph where I discuss available, very traditional vaccines and the “real world evidence” of their efficiency. 


Par 1 Virology: (341-1 Simmonds J Gen Virol 2017)

Single stranded positive RNA virus belonging to genus Flavivirus in Flavivirus family: together with Yellow Fever (YF), Dengue, West-Nile Virus (WNV), Japanese Encephalitis Virus (JEV).   



Table 1. Characteristics of the genus Flavivirus

Virion Enveloped, 40–60 nm virions with a single core protein and 2 or 3 envelope glycoproteins, embedded in an bilipid envelope, derived from cell membrane.

Genome Approximately 9.0–13 kb of positive-sense, non-segmented RNA

Translation Directly from genomic RNA: it behaves like messenger RNA

Replication Cytoplasmic, in membrane vesicles derived from the endoplasmic reticulum (ER);         assembled virions bud into the lumen of the ER and are secreted through the vesicle transport pathway



Genomic structure: (341-2 Chiffi Rev Med Virol. 2023)


  • First three structural genes for (nucleo)capsid, pre-Matrix (chaperone for Env in immature virion) and Env
  • Seven non-structural proteins with functions in replication and interference with immune response (e.g. antagonist of type 1 IFN)

Viral structure: (341-3 Tibor Füzik Nat Comm 2018)

  • The virion comprises a nucleocapsid (NC), surrounded by a membrane composed of host‐derived lipids with embedded viral envelope (E) and membrane (M) proteins.
  • E and M proteins form heterodimers, and three dimers create the asymmetric unit of each TBEV virion.
  • Cellular furin protease cleaves prM, leading to E protein rearrangement (head to tail homodimers) on the surface, leading to virus particle maturation.


Immature and mature virion:



Cryo-EM image of TBEV virions. The sample contained mature, immature (white arrows), half-mature (white arrowheads), and damaged (black arrows) particles. Scale bar 100 nm.










The three E-protein subunits within each icosahedral asymmetric unit are shown in red, green, and blue. The three E-proteins in the icosahedral asymmetric unit form unique interactions with each other





Central slice of TBEV electron density map: The lower right quadrant of the slice is color-coded as follows: nucleocapsid = blue; inner and outer membrane leaflets (taken from the cell membrane) = orange; M-proteins = red; E-proteins = green.





Cellular receptor: binding to heparan sulphate, but more specific receptor not identified

Viral cycle: (341-2 Chiffi Rev Med Virol. 2023)



Infection starts with the binding of virus particles to specific cell‐surface receptors yet to be identified. Upon receptor binding, the virus enters the cell via the clathrin-mediated endocytic pathway.

Conformational changes in the E protein are triggered by low pH in the late endosomes. This leads to a rearrangement of dimers to the

trimeric form (fusogenic state), and subsequent fusion of the viral envelope with the endosomal membrane, leading to virion uncoating.

After releasing the virus genome from the endosome, the +ssRNA is translated by ribosomes, forming viral polyprotein that is cleaved by host

proteases and viral protease to form the structural and non‐structural proteins of the virus.

Viral genome replication occurs through the synthesis of anti‐sense RNA (−ssRNA), which serves as a template for genome RNA production (+ssRNA). Replication occurs in a multimolecular complex located at the endoplasmic reticulum (ER), known as the replication complex, which contains membranes, viral RNA, lipid droplets, and viral host proteins.

The assembled nucleocapsid (NC) acquires a lipid envelope by budding into the ER lumen. Afterwards, the assembled and immature TBEV is further processed in the trans‐Golgi network. Maturation involves decrease pH, leading to conformational reorganization of the viral E protein and the pre‐membrane (prM) protein, and cleavage of the prM protein by the host protease furin, inducing a change from spiky immature to smooth mature particles. Finally, the mature virus egresses from the cell via exocytosis.

C, capsid protein; E, envelope protein; ER, endoplasmic reticulum; NS, non‐structural protein; prM, pre‐membrane protein.

Processes that occur during the viral replication cycle are shown in green boxes.


Schematic of the interaction of TBEV with a mammalian host (341-2 Chiffi Rev Med Virol. 2023)



BBB, blood‐brain barrier; CNS, central nervous system; DC, dendritic cell; IgG/IgM, immunoglobulins G/M; IFN, interferon; TBEV, tick‐borne encephalitis virus.


Possible routes of tick-borne flavivirus (TBFV) neuro-invasion and access to the central nervous system through the blood–brain barrier (BB) (341-4 Taylor Stone Viruses 2023)




  1. the Trojan horse mechanism = in association with infected immune cells,
  2. Transcellular entry, and (3) paracellular.

Other possibilities (not shown) through olfactory bulb and retrograde axonal transport.


A role for CD4 and CD8 T cells in CNS pathogenesis? 341-5 Dörbecker Trav Med Infect Dis 2010

CNS infection with TBEV leads to T‐cell recruitment.

  • Immunopathological effects of CD8+ T cells,
  • and a protective role of CD4+ T cells (via stimulation of neutralizing antibodies)



  • TBEV antagonizes type 1 IFN
  • TBEV specific IgM/IgG block viremia (if induced by vaccination)




PAR 2 Transmission and Epidemiology

Three viral subtypes: 341-2 Chiffi Rev Med Virol. 2023

1. European subtype, (TBE-Eur) transmitted by Ixodes (I.) ricinus ticks, in rural, forested areas of Europe;

2. Siberian subtype, (TBE-Si) transmitted by I. persulcatus, endemic in Urals region, Siberia and far-eastern Russia, and also in some North-Eastern European Union countries.

3. Far eastern subtype, (TBE-FE)  transmitted mainly by I. persulcatus, endemic in far-eastern Russia and in forested regions of China and Japan;


Two new subtypes: Baikalian (TBEV‐Bkl) and Himalayan (TBEV‐Him) subtypes


Distribution of two ticks that can carry TBE: Ixodus Ricinus in Europe                            Actual distribution of TBE virus

and Ixosus persulcatus in Asia with overlap in Eastern Europe


Transmission cycle341-2 Chiffi Rev Med Virol. 2023



Black arrows denote the cycles of ticks with different developmental stages. Most stages requires a blood meal for the transstadial development of a suitable host. Adult females need additional blood meals to lay eggs.

Grey arrows indicate the possible transmission of TBEV. TBEV can be transmitted to humans in three different stages, with different prevalence rates indicated by the thickness of the grey arrows.

Green arrow indicates TBEV transmission through dairy products to humans.


The Zoonotic Iceberg 341-6 Roelandt J Zoonotic Dis Pub Health 2017

Analogy for TBE epidemiology created based on Randolph and Sumilo; Drelich et al.


Reservoir = various animal species: they can be infected and are a source of new rounds of infection of the ticks.   Humans are a minor accidental “dead end” host. 

Epidemiology in EU  341-7 ECDC TBEV Annual Epidemiological Report 2020


Clearly seasonal occurrence (MarcNov),

with rising trends over 5 years.


In 2020 about 3,888 cases reported in EU/EAA

High prevalence (> 5/100.00) in Baltic countries, Czechia, Slovenia 

Intermediate ((> 0.5/100,000) in Slovakia, Scandinavia, Germany, Austria

Low but present: Belgium, France, Italy

Clearly, the epidemic is shifting to Western and Northern Europe!


Total number of reported cases in EU about 2,800 in 2020

High prevalence (> 5/100.00) in Baltic countries, Czechia, Slovenia 

Intermediate ((> 0.5/100,000) in Slovakia, Scandinavia, Germany, Austria

Low but present: Belgium, France, Italy

Clearly, the epidemic is expanding to Western Europe!


Presence of TBEV in Belgium? 341-6 Roelandt until 2016:

  • In humans …almost no evidence for TBEV-presence or TBE-cases in Belgium throughout the last 16 years, despite a considerable amount of positive (but contradicting) serology test results, and despite the correct diagnosis of several imported cases
  • All five Belgian veterinary studies have been able to suspect (dogs) or indirectly confirm (cattle, wild boar, roe deer) TBE-viral presence in Belgium.


However 341-8: Anke Stoefs Emerg Infect Dis 2021:

Three confirmed autochthonous (human) TBEV cases in Belgium during summer 2020







Summary (European subtype) 341-2 Chiffi Rev Med Virol. 2023

In approximately two-thirds of patients infected with the TBE virus, only the early (viremic) phase with or without “flu-like symptoms” is seen.


In one third, patients experience either the typical biphasic course of the disease or monophasic with meningo-encephalo-myeltis.

Virulence difference, with TBEV-FE showing the highest mortality (30%) > Sib (6%–8%) > Eur (1%–2%)

The convalescent period can be long and the incidence of sequelae may vary between 30 and 60%, with long-term or even permanent neurologic symptoms. Neuropsychiatric sequelae have been report in 10-20% of patients.



In clinical cases, TBE often has a biphasic course after incubation 7-28 days:

  1. The first viraemic phase lasts five (range 2-10) days, and is associated with nonspecific symptoms (fever, fatigue, headache, myalgia, nausea).
  2. This phase is followed by an asymptomatic interval lasting seven (range 1–33) days
  3. The second phase, when the central nervous system is involved (meningitis, meningoencephalitis, myelitis, paralysis, radiculitis).


50 % Meningitis = fever, nuchal rigidity, headache, photophobia, and general malaise


40 % Meningo-encephalitis =

  • Consciousness disturbance, ranging from somnolence to coma, focal neurological deficits
  • In addition: cognitive deterioration, restlessness, hyperkinesia of limb and face muscles, tremor (especially of the eyelids and hands), intention tremor, choreiform disturbance.


10 % Meningo-encephalo-myelitis with flacid paralysis especially in upper limbs (shoulders

Sequelae = post-encephalitis syndrome:

  • Cognitive disturbances;
  • Neuropsychiatric complaints, including apathy, irritability, memory and concentration problems;
  • and altered sleep pattern

Monophasic course: 341-9 Bogovic Microorganisms 2021


Biphasic course is well described, but information on the monophasic course is limited

→ Study of 705 adult TBE patients: 283 with monophasic and 422 with biphasic course.


Monophasic patients:

  • Clinical:  significantly older (57 vs. 50 years), more often vaccinated against TBE (7.4% vs. 0.9%), more often had comorbidities (52% vs. 37%), and were more often treated in the intensive care unit (12.4% vs. 5.2%).


  • Biological: higher CSF levels of immune mediators associated with innate and adaptive (Th1 and B-cell) immune responses, and they had more pronounced disruption of the blood–brain barrier


However: long-term outcome 2–7 years after TBE was comparable.




Diagnosis: The first viremic phase is usually missed.  Patients call attention during second phase.  Therefore PCR is of limited value and diagnosis mainly rests on serology (antibody detection). 


Suitable tests for specific TBE diagnosis. According to biphasic course of a TBEV infection with symptoms and antibody development;

PCR: polymerase chain reaction; VIS: virus isolation; IgM ab-IgG ab: Immunoglobulins of class M or G.


From: 341-6 Roelandt J Zoonotic Dis Pub Health 2017


Treatment: only supportive and symptomatic in second phase.

Early preclinical development of nucleoside analogues to block replication. However, this could only influence thev  first viremic phase, which is usually NOT diagnosed

From 341-10 Taba Eur J Neurol 2017

Risk groups

In endemic areas, people with recreational or occupational outdoor activities (e.g. hunting, fishing, camping, collecting mushrooms and berries, hiking, forestry and farming) are potentially in contact with infected ticks.


Prevention measures = avoiding tick bites:

a) application of insect repellents to exposed skin.

b) wearing protective clothing with long sleeves and long trousers tucked into socks treated with insecticides.

c) avoiding consumption of unpasteurised milk and dairy products in risk areas.


Inspecting the body for ticks after outdoor activities and removing ticks with tweezers or forceps is also important

because tick can harbour other pathogens.

From: 341-11 ECDC Fact sheet for health professionals

Par 4 Vaccination:

General: 341-2 Chiffi Rev Med Virol. 2023 and 341-12 Superior Health Council 2019

Four classical inactivated whole virus vaccines: 2 against TBEV Eu and 2 against TBEV FE

Primary vaccination with three doses in the first year, followed by boosters every three–five years to maintain sufficient immunity.








The good news: evidence for long-lasting neutralizing antibodies and field effectiveness


  1. Sustained antibody persistence for at least 15 years after a booster vaccination 341-13 Beran Vaccine 2023


Anti-TBEV Neutralization geometric mean titers from 11 to 15 years after first booster, by age subgroup (at Year 10)



group R, rapid schedule (primary doses on days 0, 7, 21 and booster 12–18 months or 3 years post-primary vaccination);

group C, conventional schedule (primary doses on days 0, 28, 300 and booster 3 years post-primary vaccination);

group A, accelerated conventional schedule (primary doses on days 0, 14, 300 and booster 3 years post-primary vaccination)


Hence: very sustained Neut levels even with accelerated scheme and advanced age


  1. Very high “Field Effectiveness” of vaccination in Austria (2000-2006) 341-14 Heinz Vaccine 2007

Over the period between 1980 and 2006,

  • Vaccination coverage in Austria rose from 5 to > 90 %
  • In Czech republic it was only 11 %

Cases in Austria dropped, but in Czech republic rose


Annual numbers of TBE cases in Austria and the Czech Republic and vaccination coverage in Austria (1979–2006).

Solid line: TBE cases in the Czech Republic. Dashed line: TBE cases in Austria.

Dotted line: Number  of cases expected in Austria without vaccination (2000–2006).

Grey line: Vaccination coverage in Austria (% of total population with at least one vaccination).

Grey asterisk: Vaccination coverage in the Czech Republic in 2006 (as determined by representative inquiries).




  1. Retrospective matched case control study in Switzerland 2006-2020   341-15 Zens BMJ Open 2022



Of 1868 TBEV cases (n=1868), only 151 were vaccinated


Vaccine effectiveness by age




For each age group (18–39, 40–59 and 60–79), individuals were categorized as:

- unvaccinated, incompletely vaccinated (1–2 doses), completely vaccinated (3+ doses)


Amongst complete vaccinated: <5 years prior, 5–10 years prior or 10+ years prior


VE was calculated using the formula (VE=100×[1−OR]), with unvaccinated as the reference.



► Data on other factors which might impact the response to vaccination (chronic medical conditions, immunosuppression, age of first vaccination, whether individuals were vaccinated according to the recommended vaccination schedule) were not available.



Cross-neutralization and cross-protection between various TBEV subtypes?


  1. Vaccine based on European TBE induces broadly cross-neut Ab: 341-16 Orlinger JID 2011


Recombinant viruses were made, with a West-Nile virus backbone and Envelopes of either European TBEV (Neudoerfl and K23),  Far Eastern TBEV (Sofijn, Oshima), Siberian TBEV (Vasilchenko) or Omsk Hemorrhagic fever virus (Omsk HF)



Serum samples derived from 41 subjects vaccinated against European TBEV were analyzed in microneutralization (lNT) assays using all hybrid viruses. Individual titers against the hybrids (diamonds) and mean neutralization titers with standard errors of the mean (SEMs) (error bars) are shown.

OHFV, Omsk hemorrhagic fever virus; WNV, West Nile virus.

Cross-neutralization titers of serum samples from human vaccinees.



  1. Cross-protection shown in mice  341-17 Richard Fritz Vaccine 2012


Vaccine strains used: 1 European (FSME-Neudoerfl), 2 Russian Far Eastern (Encevir-205 and IPVE- Sofijn)




Induction of cross-neutralizing antibodies by each of the three vaccines in mice




All three vaccines similarly protect to lethal challenge with European TBEV (A) Neudoerfl and Far-Eastern B) Sofijn




Some doubts


  1. Retrospective analysis of TBEV breakthrough infections (BTI) in Germany 2001-2018   341-18 Dobler Clinical Microbiology and Infection 26 (2020)


334 from 5777 cases developed an infection despite having been vaccinated at least once


No clear evidence that these cases were more or less severe than those in unvaccinated subjects


Unpublished): two types of vaccination breakthrough infections.

  • Primary type: patients who failed to sufficiently respond to the vaccine.
  • Second type: patients reacted against the TBE vaccine, but their immunological reaction could not induce protection against infection.


  1. Neutralization mismatch in BTI in adolescents with severe TBE 341-19 Geißlreiter Ticks and Tick-borne diseases 2023


Both suffered from severe persistent neurologic sequelae with Type 2 vaccine failure:  

  • high TBE-IgG-titers after vaccination at the beginning of the infection
  • and a low or missing TBE-IgM response during infection.

Neutralization tests show:

  • low titers against the respective infecting TBE virus strain and
  • higher titers against the vaccine strain at the beginning of the infection

Implying impaired immune response to the infecting as compared to vaccine virus as cause of TBE vaccine failure.



These data suggest that diversity within the natural TBEV may reduce vaccine effectiveness in some patients.  This hypothesis was recently tested: 341-20 Bestehorn-Willmann Vaccines 2023


  1. 16 TBEV isolates from southern Germany           b) Neutralization by 33 sera from vaccinated human subjects




Despite the high diversity of the isolates, the mean neutralization titers from 33 sera was similarly high, but with variation.  




Phylogenetic phenogram of mean NT-titers for each TBEV-EU genotype grouped and color-coded by immunization profile. For each immunization profile, phylogenetic comparative plots are shown that project the underlying phylogenetic tree in a space defined by phenotype (NT-titer; y-axis) and time (divergence time estimates; x-axis).  


As can be seen, subjects immunized with FSME IMMUN showed slightly higher neut titers but those vaccinated with a mixed schedule of FSME IMMUN and Encepur had the highest neut titers.


However, there is considerable variation of sensitivity to neutralization according to the virus isolate and the order of sensitivity can be different according to the vaccine used to induce the neut Ab.




TBE is slowly expanding, due to climate and other environmental changes.  It is mainly present in wild animals, but is occasionally transmitted vie ticks to humans. The European variant is rarely fatal, but is associated with neurological sequelae. There is no specific treatment but good traditional inactivated vaccines.

It is remarkable that these vaccines are still valid, in view of some clear variability of the virus (after all a positive stranded RNA virus, related to the highly variable hepatitis C virus). This remarkable cross-neutralization and cross-protection may be related to the predominantly animal reservoir, which is not being vaccinated.  Hence there is no pressure from the vaccine an no escape mutations (as we see for instance in SARS-CoV-2, which is an almost exclusively human pathogen, undergoing massive immune pressure). 

Nevertheless breakthrough infections do occur                 and at least some may be due to escape from vaccine-induced neutralizing antibodies, but this has not been formally proven.