Overview
Viral infection of the central nervous system transmitted by Ixodes tick bites. Biphasic illness with flu-like phase followed by neurological involvement. Effective vaccine available.
Symptoms
Symptoms | Frequency | Severity | Onset |
|---|---|---|---|
| Fever | 95% | Mild | Early |
| Fatigue | 80% | Mild | Early |
| Headache | 85% | Mild | Early |
| Malaise | 70% | Mild | Early |
| Myalgia | 75% | Mild | Early |
| Arthralgia | 25% | Mild | Early |
| Loss of appetite | 50% | Mild | Early |
| Nausea | 35% | Mild | Early |
| Vomiting | 25% | Mild | Early |
| Altered consciousness | 8% | Critical | Peak |
| Confusion | 15% | Severe | Peak |
| High fever | 25% | Severe | Peak |
| Neck stiffness | 24% | Severe | Peak |
| Paralysis | 4% | Critical | Peak |
| Photophobia | 18% | Moderate | Peak |
| Severe headache | 27% | Severe | Peak |
| Tremor | 6% | Moderate | Peak |
| Blurred vision | 5% | Moderate | Peak |
| Dizziness | 15% | Moderate | Peak |
| Paresthesia | 7% | Moderate | Peak |
| Seizures | 1.5% | Critical | Peak |
| Irritability | 10% | Mild | Peak |
Transmission
Overview
Tick-borne encephalitis (TBE) is a viral infectious disease of the central nervous system caused by the TBE virus (TBEV), a member of the genus Flavivirus within the family Flaviviridae. The virus is transmitted to humans primarily through the bite of infected Ixodes ticks (I. ricinus in Europe, I. persulcatus in Asia), with a less common route being consumption of unpasteurized dairy products from infected goats, sheep, or cows.
Three genetically and geographically distinct subtypes exist: European (TBEV-Eu), Siberian (TBEV-Sib), and Far-Eastern (TBEV-FE). The European subtype typically causes a milder, biphasic illness, while the Far-Eastern subtype is associated with a more severe monophasic course and higher case-fatality rates. The Siberian subtype occupies an intermediate position and is uniquely associated with chronic or progressive forms.
TBE is the most important tick-borne viral disease in Eurasia, with an estimated 10,000–12,000 reported cases annually across endemic regions spanning from Western Europe through Scandinavia, the Baltic states, Central and Eastern Europe, Russia, and into China, Mongolia, South Korea, and Japan. The true incidence is likely 2–3 times higher due to underreporting and subclinical infections. Incidence has increased in recent decades, attributed to climate change expanding tick habitats, increased outdoor recreational activities, and improved surveillance.
Overview
Causative agent: TBE virus (Flaviviridae), an enveloped, single-stranded positive-sense RNA virus. Three subtypes: European (TBEV-Eu), Siberian (TBEV-Sib), Far-Eastern (TBEV-FE).
Transmission: Primarily through bites of infected Ixodes ticks. Transmission occurs rapidly — TBEV is present in tick saliva and can be transmitted within minutes of attachment, unlike Borrelia (Lyme disease) which typically requires 24–48 hours. Alimentary transmission via unpasteurized milk products accounts for approximately 1% of European cases but can cause clustered outbreaks.
Incubation period: 7–14 days after tick bite (range 4–28 days); 3–4 days after alimentary exposure (shorter due to direct gut absorption).
Seasonality: Strongly seasonal, corresponding to tick activity: April–November in Europe, with peak incidence in June–July and a smaller peak in September–October.
Geographic distribution: Endemic across a belt from France and the Netherlands in the west to Japan in the east. Highest incidence in the Baltic states (Lithuania, Latvia, Estonia), Czech Republic, Slovenia, and parts of Russia. The endemic area has expanded northward (Scandinavia) and to higher altitudes (Alpine regions) in recent decades, consistent with climate change effects on tick populations.
Emergency Signs
Seek immediate emergency medical care if any of the following develop after a tick bite or during a febrile illness in a TBE-endemic area:
Meningeal signs:
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Severe, unrelenting headache not responding to standard analgesics
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Stiff neck (unable to touch chin to chest)
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Photophobia (pain on exposure to light), phonophobia
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Persistent vomiting without gastrointestinal cause
Encephalitic features:
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Confusion, disorientation, personality changes, or altered consciousness
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Seizures (focal or generalized)
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Involuntary tremor, particularly of tongue, eyelids, or extremities
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Difficulty speaking (dysarthria) or swallowing (dysphagia)
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Ataxia (unsteady gait, loss of coordination)
Myelitic features (spinal cord involvement):
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Sudden weakness or paralysis of one or more limbs, especially arms and shoulders
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Difficulty breathing, shallow respiration, or breathlessness — may indicate phrenic nerve or diaphragmatic involvement, requiring emergency intubation and ventilation
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Bladder dysfunction (urinary retention or incontinence)
General red flags:
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Fever returning after an afebrile interval of 1–3 weeks following a tick bite
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Rapidly deteriorating level of consciousness or coma
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Any focal neurological deficit developing in the context of recent tick exposure
Detailed Symptoms
Most common signs and symptoms
European subtype — biphasic course (70–75% of symptomatic cases):
Phase 1 (viremic phase, 2–7 days): Non-specific influenza-like symptoms including fever (38–39°C), headache, fatigue, myalgia, and malaise. During this phase, the virus replicates in peripheral tissues and enters the bloodstream. Blood counts may show leukopenia and thrombocytopenia. Most patients recover fully at this stage without progressing further.
Afebrile interval (1–21 days, average 7 days): Apparent clinical improvement with resolution of fever and symptoms. However, the virus crosses the blood-brain barrier during this interval.
Phase 2 (neurological phase, appears in 20–30% of infected): Sudden return of high fever with signs of central nervous system involvement. Three clinical forms, often overlapping:
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Meningitis (50% of neurological cases): Severe headache, nuchal rigidity, photophobia, nausea, and vomiting. Generally the mildest neurological form with full recovery in most cases.
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Meningoencephalitis (40%): Altered consciousness, cognitive dysfunction, tremor, ataxia, dysarthria. Focal neurological deficits may occur.
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Meningoencephalomyelitis (10%): The most severe form. Flaccid paralysis of limbs and/or cranial nerves, resembling poliomyelitis. Paresis of shoulder girdle and upper extremities is characteristic ("cervical myelitis"). Respiratory muscle involvement may require mechanical ventilation.
Far-Eastern subtype — monophasic course: Abrupt onset with high fever, severe headache, and rapid progression to encephalitis without a distinct biphasic pattern. Higher incidence of hemorrhagic features, coma, and death.
Knowing the symptoms is the first step to a quick response.
Course of Disease
Typical disease course (biphasic pattern, European subtype):
- Incubation period: 7–14 days after tick bite (range 2–28 days).
- First phase (viremic, 2–7 days): Flu-like illness: fever, fatigue, headache, myalgia. Often mistaken for influenza. Resolves spontaneously.
- Afebrile interval (1–21 days, average 8 days): Patient feels well. ~70% of patients recover fully at this stage.
- 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.
- Recovery: Weeks to months depending on severity.
Far Eastern subtype: Often monophasic with rapid progression to encephalitis without the biphasic interval.
Diagnosis
How this disease is identified
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):
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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.
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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.
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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.
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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:
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Lymphocytic pleocytosis (typically 10–500 cells/μL, occasionally >1,000)
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Mildly elevated protein (0.5–1.5 g/L)
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Normal glucose ratio
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Elevated lactate in severe encephalitis
Treatment
Available treatment methods
There is no specific antiviral therapy for TBE. Treatment is entirely supportive, with management tailored to the severity of neurological involvement.
Mild disease (Phase 1 only):
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Symptomatic treatment: analgesics (paracetamol, NSAIDs) for fever and headache
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Adequate hydration and rest
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Monitoring for progression to Phase 2
Meningitis:
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Hospitalization for monitoring and supportive care
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Analgesics for severe headache
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Antiemetics for nausea and vomiting
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Intravenous fluids if oral intake is compromised
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Lumbar puncture for diagnostic purposes and symptom relief (therapeutic CSF drainage may alleviate headache)
Meningoencephalitis and meningoencephalomyelitis:
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Intensive care unit admission for severe cases
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Management of raised intracranial pressure: head elevation, osmotic agents (mannitol), and in severe cases, decompressive measures
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Seizure control: benzodiazepines for acute seizures, levetiracetam or valproate for ongoing prophylaxis
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Mechanical ventilation for respiratory failure (bulbar involvement or diaphragmatic paralysis)
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Thromboprophylaxis for immobilized patients
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Nutritional support via nasogastric tube if swallowing is impaired
Rehabilitation:
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Early physiotherapy and occupational therapy for patients with motor deficits
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Speech therapy for dysarthria and dysphagia
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Cognitive rehabilitation for patients with encephalitic sequelae
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Recovery is prolonged: neurological improvement may continue for months to years, but 30–60% of patients with CNS involvement have long-term sequelae including cognitive impairment, fatigue, headaches, balance disorders, hearing impairment, and persistent paresis
Most cases are effectively treated with early diagnosis.
Prevention Details
How to protect yourself
Vaccination — the most effective preventive measure:
Two inactivated whole-virus TBE vaccines are widely available in Europe:
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FSME-Immun (Pfizer): Licensed for age ≥1 year. Primary series: 3 doses (0, 1–3 months, 5–12 months). Booster every 3–5 years.
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Encepur (Bavarian Nordic): Licensed for age ≥1 year. Primary series: 3 doses (0, 1–3 months, 9–12 months). Booster every 3–5 years.
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Both vaccines provide >95% protection after the full 3-dose primary series. Rapid immunization schedules are available for travelers with limited time before departure.
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Russian vaccines (TBE-Moscow, EnceVir) are used in the Russian Federation and some CIS countries.
Tick bite prevention:
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Wear long-sleeved clothing and long trousers tucked into socks when walking through woodland, grassland, or undergrowth in endemic areas.
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Use DEET-based (20–50%) or icaridin-based repellents on exposed skin. Permethrin-treated clothing provides additional protection.
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Conduct thorough body checks after outdoor activities. Pay special attention to hairline, behind ears, armpits, groin, and behind knees.
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Remove attached ticks promptly using fine-tipped forceps or a tick removal tool with steady upward pressure. Note: unlike Lyme disease, TBEV transmission can occur within minutes of tick attachment, so prompt removal reduces but does not eliminate risk.
Alimentary prevention:
- Avoid unpasteurized milk and dairy products (especially goat and sheep milk) in endemic areas. Boiling milk effectively inactivates the virus.
Post-exposure prophylaxis:
- TBE immunoglobulin was formerly used for post-exposure prophylaxis but has been discontinued in most Western European countries due to concerns about antibody-dependent enhancement (ADE) and lack of proven efficacy. It remains available in Russia.
Preparation is the best protection.
Travel Advice
Pre-travel vaccination recommendation:
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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.
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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.
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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:
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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).
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Activities associated with increased risk: hiking, camping, mushroom and berry picking, orienteering, mountain biking, fishing, and agricultural work.
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Urban parks in endemic cities can also harbor infected ticks — risk is not limited to wilderness areas.
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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:
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Apply tick bite prevention measures consistently (protective clothing, repellents, body checks).
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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.
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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.
How common is it?
Statistics and geographic data
Global burden:
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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.
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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.
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TBE is a notifiable disease in most European countries, though reporting practices and case definitions vary, complicating cross-country comparisons.
Epidemiological trends:
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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
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New endemic foci have been identified in the Netherlands, UK (limited), Norway, and at increasing altitudes in the Alps.
Tick ecology:
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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.
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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.
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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.
Risk Factors
Who is most at risk
Host factors affecting disease severity:
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Age: Severity and risk of neurological complications increase with age. Children typically have milder disease (predominantly meningitis). Adults over 50 have the highest risk of encephalitis, myelitis, and long-term sequelae. CFR in patients over 60 is 3–5 times higher than in younger adults.
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Immunosuppression: Immunocompromised patients (organ transplant recipients, HIV, chemotherapy) may have more severe disease, prolonged viremia, and atypical presentations.
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Genetic susceptibility: Polymorphisms in genes involved in innate immune responses (TLR3, CCR5, OAS) have been associated with increased susceptibility to severe TBE, though studies are limited.
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Viral subtype: Far-Eastern subtype carries CFR of 20–40%; European subtype CFR <2%; Siberian subtype CFR 2–3% but associated with chronic progressive forms.
Exposure risk factors:
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Occupation: Forestry workers, farmers, military personnel in endemic areas have the highest occupational risk.
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Recreational activities: Hiking, camping, mushroom/berry picking, hunting, and fishing in endemic forests and meadows.
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Geographic location: Risk varies dramatically by region. Austria, Czech Republic, Slovenia, and the Baltic states have the highest reported incidence (5–30 cases per 100,000 population).
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Altitude and habitat: Ticks are most abundant in deciduous and mixed forests with thick underbrush, below 1,500 meters. However, tick habitats are expanding to higher altitudes and more northerly latitudes due to warming temperatures.
Behavioral risk factors:
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Failure to check for ticks after outdoor activities
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Not wearing protective clothing in tick-endemic areas
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Consuming unpasteurized dairy products in endemic regions
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Non-vaccination despite residence in or travel to endemic areas — the single most important modifiable risk factor
Complications Details
Potential complications
Acute neurological complications:
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Meningoencephalitis: The most common severe manifestation, with cognitive dysfunction, altered consciousness, and focal neurological deficits. Tremor of the tongue, eyelids, and extremities is characteristic. Seizures occur in 5–10% of encephalitis cases.
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Meningoencephalomyelitis: The most severe acute form, with flaccid paralysis of limbs (especially upper extremities and shoulder girdle) and/or cranial nerves. Respiratory failure from diaphragmatic paralysis occurs in 5–10% and requires mechanical ventilation. In-hospital mortality for this form is 15–30%.
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Radiculitis: Inflammation of spinal nerve roots causing severe pain, sensory disturbances, and weakness in affected dermatomes.
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Autonomic dysfunction: Bladder dysfunction, cardiovascular instability, and inappropriate ADH secretion (SIADH) with hyponatremia.
Long-term sequelae (30–60% of patients with CNS involvement):
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Cognitive impairment: Difficulty with concentration, memory, and executive function. Often the most debilitating long-term consequence, affecting work capacity and quality of life for years.
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Post-encephalitic syndrome: Chronic fatigue, headache, irritability, sleep disturbances, and emotional lability persisting months to years after acute illness.
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Persistent motor deficits: Residual paresis in patients with myelitis. Upper limb weakness and shoulder girdle atrophy may be permanent.
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Balance and coordination disorders: Vestibular and cerebellar dysfunction causing chronic dizziness and gait instability.
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Hearing impairment: Sensorineural hearing loss reported in 2–5% of CNS cases, sometimes permanent.
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Chronic or progressive TBE: Described primarily with the Siberian subtype. Rare but debilitating, with progressive neurodegeneration (epilepsia partialis continua, amyotrophy) developing months to years after acute infection.
Prognosis by subtype:
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European subtype: CFR 1–2%; complete recovery in 40–70% of CNS cases; long-term sequelae in 30–60%.
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Siberian subtype: CFR 2–3%; chronic progressive forms possible.
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Far-Eastern subtype: CFR 20–40%; severe encephalitis with higher rates of permanent neurological disability among survivors.
Recovery & Outlook
Expected outcomes and recovery
European subtype (TBEV-Eu):
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CFR: 1–2%.
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Neurological sequelae in 10–20% of encephalitis cases (cognitive impairment, fatigue, headaches, paresis).
Far Eastern subtype (TBEV-FE):
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CFR: 20–40%.
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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.
