Tick-Borne Encephalitis

Typical disease course (biphasic pattern, European subtype):

1. Incubation period: 7–14 days after tick bite (range 2–28 days).
2. First phase (viremic, 2–7 days): Flu-like illness: fever, fatigue, headache, myalgia. Often mistaken for influenza. Resolves spontaneously.
3. Afebrile interval (1–21 days, average 8 days): Patient feels well. ~70% of patients recover fully at this stage.
4. Second phase (neuroinvasive, ~30% of cases): Sudden return of high fever with CNS involvement:
   • Meningitis (50%): headache, neck stiffness, photophobia. Best prognosis.
   • Meningoencephalitis (40%): altered consciousness, cerebellar ataxia, tremor.
   • Meningoencephalomyelitis (10%): flaccid paralysis (polio-like), cranial nerve palsies. Worst prognosis.
5. Recovery: Weeks to months depending on severity.

Far Eastern subtype: Often monophasic with rapid progression to encephalitis without the biphasic interval.

Clinical suspicion: TBE should be considered in any patient presenting with meningitis, encephalitis, or acute flaccid paralysis with a history of tick exposure or unpasteurized dairy consumption in an endemic area, particularly during the tick season (April–November).

Serological diagnosis (primary method):

• IgM ELISA: TBE-specific IgM antibodies appear at the onset of Phase 2 neurological symptoms and are detectable in >95% of patients at presentation. IgM may persist for up to 10 months.

• IgG ELISA: TBE-specific IgG appears shortly after IgM and rises over the following weeks. A fourfold rise in paired sera (acute and convalescent, 2–4 weeks apart) confirms acute infection.

• Intrathecal antibody production: Detection of TBE-specific IgM and IgG in CSF, with calculation of the antibody index, confirms CNS infection and distinguishes from systemic viremia.

• Cross-reactivity caveat: Flavivirus serological cross-reactivity is significant. Patients previously vaccinated against TBE, yellow fever, Japanese encephalitis, or those with prior dengue or Zika infection may have false-positive results. Neutralization assays (PRNT) can resolve ambiguous cases.

Molecular diagnosis:

• RT-PCR: Detection of TBEV RNA in serum is possible only during Phase 1 viremia (first 5–7 days of illness), before neurological symptoms appear. By Phase 2, viremia has typically cleared and PCR sensitivity drops below 20%. CSF PCR has similarly limited sensitivity.

Cerebrospinal fluid analysis:

• Lymphocytic pleocytosis (typically 10–500 cells/μL, occasionally >1,000)

• Mildly elevated protein (0.5–1.5 g/L)

• Normal glucose ratio

• Elevated lactate in severe encephalitis

Pre-travel vaccination recommendation:

• TBE vaccination is recommended for all travelers spending time outdoors (hiking, camping, forestry, cycling) in endemic areas during the tick season (April–November). This includes much of Central Europe (Austria, Czech Republic, Germany, Switzerland, Poland), the Baltic states, Scandinavia, Russia, and parts of East Asia.

• The standard vaccination schedule requires 3 doses over 5–12 months, so early planning is essential. Rapid (accelerated) schedules can provide protection within 3–4 weeks but require all 3 doses to be completed for sustained protection.

• Austria, where TBE vaccination is included in the national immunization program, has demonstrated a >90% reduction in TBE cases since universal vaccination began in the 1980s — proof of vaccine effectiveness at the population level.

Risk assessment during travel:

• Risk is highest in rural and semi-urban areas with forest and grassland habitats, particularly at altitudes below 1,500 meters (though the altitude ceiling is rising).

• Activities associated with increased risk: hiking, camping, mushroom and berry picking, orienteering, mountain biking, fishing, and agricultural work.

• Urban parks in endemic cities can also harbor infected ticks — risk is not limited to wilderness areas.

• Seasonality: peak risk May–July, secondary peak September–October. However, mild winters are extending the tick season in many regions.

During travel in endemic areas:

• Apply tick bite prevention measures consistently (protective clothing, repellents, body checks).

• If bitten by a tick, note the date, location on the body, and geographic area. Seek medical advice if fever or neurological symptoms develop within 4 weeks.

• Avoid unpasteurized dairy products, particularly fresh goat cheese and milk sold at rural markets.

Post-travel:

• If you develop a biphasic febrile illness (flu-like symptoms, improvement, then return of fever with headache or neurological symptoms) within 4 weeks of returning from an endemic area, inform your physician of the tick exposure history and request TBE serology.

Global burden:

• TBE is the most important arthropod-borne viral infection of the central nervous system in Europe. An estimated 10,000–12,000 clinical cases are reported annually, with the true number likely 2–3 times higher.

• Russia accounts for the largest absolute number of cases (~2,000–3,000/year). In Europe, the highest incidence rates are in Lithuania (15–20/100,000), Latvia, Estonia, Czech Republic, and Slovenia.

• TBE is a notifiable disease in most European countries, though reporting practices and case definitions vary, complicating cross-country comparisons.

Epidemiological trends:

• Incidence has increased 400% in Europe over the past 30 years. Contributing factors include:

  • Climate change: warmer temperatures extending tick season, expanding tick habitats northward and to higher altitudes
  • Increased outdoor recreational activities and ecotourism
  • Land use changes: reforestation, deer population growth increasing tick abundance
  • Improved surveillance and diagnostic awareness

• New endemic foci have been identified in the Netherlands, UK (limited), Norway, and at increasing altitudes in the Alps.

Tick ecology:

• Ixodes ricinus (Europe) and I. persulcatus (Asia) are the principal vectors. Tick infection rates vary from 0.1–5% depending on the region, with focal "hotspots" where risk is concentrated.

• Ticks acquire TBEV by feeding on viremic rodents (especially bank voles and yellow-necked mice) or through co-feeding with infected ticks on the same host. Transovarial (vertical) transmission in ticks also occurs but is less important.

• TBEV exists in discrete natural foci, with risk concentrated in specific microhabitats. This focal distribution means that risk can vary significantly over distances of just a few kilometers.

Vaccination impact:

• Austria's national TBE vaccination program (launched 1981, >85% coverage) reduced annual cases from ~700 to <100, despite increasing tick populations. This natural experiment provides the strongest evidence for vaccine effectiveness at population level.

European subtype (TBEV-Eu):

• CFR: 1–2%.

• Neurological sequelae in 10–20% of encephalitis cases (cognitive impairment, fatigue, headaches, paresis).

Far Eastern subtype (TBEV-FE):

• CFR: 20–40%.

• Higher rate of severe encephalitis and residual paralysis.

Siberian subtype: Intermediate severity. Chronic progressive course possible.

Prognostic factors: Older age (>60 years) predicts more severe disease and higher CFR. Severity correlates with viral subtype and host immune response.

Recovery: Mild cases resolve in 1–2 weeks. Encephalitis recovery may take months. Post-encephalitic syndrome (fatigue, concentration difficulties) is common.