Tick-borne encephalitis

Tick-borne encephalitis is a viral infection that affects the central nervous system and may cause a spectrum of disease, from a mild short-lived illness to a more severe life-threatening illness Tick-borne encephalitis

Key Messages

Tick-borne encephalitis (TBE) is a viral infection that can cause a serious neurological illness. Although it is usually transmitted through the bite of an infected tick, unpasteurised dairy products from infected animals can also pose a risk.
Areas with known TBE exist within a band that extends from central, eastern and northern Europe across Russia to parts of eastern Asia.
Generally, TBE infections are asymptomatic. Some cases however, follow a two-stage process of a mild flu-like illness followed by a potentially serious infection of the central nervous system (brain and spinal cord) and some can be more severe from the start and/or be associated with prolonged symptoms.
Whilst TBE is rarely fatal in Europe, the sub-type found in Asia can have a higher fatality rate and long-term neurological complications are more common.
TBE is a rare disease in United Kingdom (UK) travellers. However individuals are at increased risk when visiting areas where TBE is known to occur, particularly woods, forests, glades, forest fringes, urban parks and grasslands.
Travellers may reduce the risk of infection by avoiding risk areas and taking bite prevention measures. A vaccine is also available for travellers whose planned activities put them at increased risk.

Overview

Tick-borne encephalitis (TBE) is a viral infection that is transmitted predominantly through the bite of an infected Ixodes tick. TBE virus belongs to a closely related group of viruses called flaviviruses. These include yellow fever, dengue and Japanese encephalitis. There are five subtypes of TBE virus, the three main ones are: European/Western, Siberian and Far Eastern.

TBE affects the central nervous system and may cause a spectrum of disease, from a mild short-lived illness to a more severe life-threatening illness with the potential for severe neurological complications. The disease occurs in parts of central, northern and eastern Europe, Siberia and parts of Asia [1]. TBE is rarely fatal in Europe. However in Asia, it may be fatal in up to 20 percent of cases and long-term neurological problems are common [2].

Risk areas

TBE is reported in a band of areas extending from central, eastern and northern Europe, across Russia to parts of eastern Asia, mainly in non-tropical regions of the Eurasian forest steppe. These areas are generally at altitudes lower than 1500 metres. However, ticks have been noted at higher altitudes [3, 4].

Climate change is thought to have resulted in TBE risk areas moving northward and to higher altitudes. Social, political, ecological and demographic factors are also considered to be important [5]. Within this band, TBE occurs in highly focal areas and from year to year the incidence (number of cases) may vary significantly according to local environmental conditions [6]. In Europe and Asia, approximately 10,000-12,000 TBE cases are reported annually [4]. Of these, approximately 2000 cases are reported in Europe each year [6-9].

In 2018, the first TBE virus detection in UK ticks was reported from Thetford Forest, East Anglia in England. In 2019, TBE was confirmed in ticks in a different geographical location in England, the Hampshire/Dorset borders [10].

In July 2019, a probable, locally-acquired (autochthonous), human TBE case was reported in a visitor from mainland Europe who became ill after a tick bite in the New Forest, Hampshire, England [10, 11]. The diagnosis was made by serological testing. Given TBE’s cross-reactivity with other flaviviruses endemic in the UK (such as louping ill virus) a confirmatory diagnosis was not possible [10]. In July 2020, a second probable case of TBE was diagnosed, again from Hampshire, England. The current assessment of the overall risk of TBE in the UK remains very low [12].

The general geographical distribution of the three main virus subtypes is as follows:

  • European/Western TBE virus – prevalent in Europe and western Russia. Transmitted by Ixodes ricinus
  • Siberian TBE virus – prevalent in parts of Eastern Europe, all parts of Russia and parts of northern Asia. Transmitted by Ixodes persulcatus
  • Far Eastern TBE virus – prevalent in eastern Russia, China and Japan. Also transmitted by Ixodes persulcatus

Vaccination recommendations for countries highlighted in Figure 1 will depend on regions visited within a country, a traveller’s planned activities, the season of travel, and the medical history of the individual traveller. The Country Information pages should be consulted for specific guidance.

Risk for travellers

The risk of acquiring TBE infection is dependent on several factors including:

  • Destination of travel
  • Duration of travel in risk area
  • Season of travel
  • Activities undertaken
  • Tick activity in the country visited
  • Vaccination status of the traveller

In TBE endemic regions, travellers are at risk in rural areas, forests, grasslands, woods and urban parks; anywhere they can be exposed to the tick vector. Any outdoor activity is a risk, working or spending long periods of time in endemic areas, increases risk, but cases have been reported after short trips. TBE can also be acquired by consuming unpasteurised dairy products from infected animals [6]. In Europe the early spring through to late autumn are generally higher risk, but seasons vary according to location [8, 9].

Between 2012 and 2016, a total of five travel-related TBE cases were confirmed in UK residents who visited endemic areas [13]. In 2018, a further two confirmed TBE cases, along with a suspected case, were reported in UK residents [14].

Transmission

TBE is usually transmitted through the bite of an infected Ixodes tick, the main vectors of TBE virus. The virus is maintained in nature by a number of animal ‘hosts’ including; small mammals (mice and voles), domestic livestock (sheep, goats and cattle) and certain bird species. This interaction between vector and host creates a disease reservoir in the environment. Human infection occurs incidentally, when individuals encroach on areas where the virus is present and are inadvertently bitten by infected ticks [5, 6].

Ticks are found on forest fringes within adjacent grassland, forest glades, riverside meadows and marshland, forest plantations with brushwood, and shrubbery. Ticks can also be found in parks and gardens. They tend to reside on ground level vegetation, on the underside of foliage, from where they can be brushed onto clothing. Ticks are capable of transmitting the TBE virus throughout their lifecycle stages (larvae, nymphs or adults) and once infected, carry the virus for life. Tick activity and development are affected by local climatic factors such as temperature, soil moisture and relative humidity.

The number of ticks infected within risk areas may vary markedly. The number of infected Ixodes ricinus ticks is often low. In some endemic countries in central Europe the prevalence of the virus in ticks varies from 0.1 to 5% [15]. Unusually, humans may become infected after consumption of unpasteurised dairy produce [9].

Signs and symptoms

There are three main virus sub-types: European/Western, Siberian and Far Eastern TBE. Most people who are infected with TBE virus do not develop symptoms. However, a range of clinical manifestations can be observed following infection by any TBE virus sub-types.

Two phases of symptoms are usually seen after infection with European/Western TBE sub-type. Approximately two to 30% of those infected with this sub-type develop a non-specific flu-like illness with fever, fatigue, headache and muscle pain after an incubation period of about eight days (range of two to 28 days) [17,18].

An interval of one to 20 days follows, during which patients usually have no symptoms. After this, approximately one third of those with initial symptoms will progress to the second stage heralded by a sudden rise in temperature, with clinical features of meningitis (inflammation of brain lining), meningoencephalitis (affecting brain and lining) and meningoencephalomyelitis (the most severe form also affecting the spinal cord) [17, 18, 19]. According to a 10-year follow-up survey, 80% of patients with the primary meningoencephalomyelitic form developed long term sequelae [19].

TBE is rarely fatal in Europe (0.5 to 2%). Infections with Far Eastern TBE sub-type are generally more severe, with a higher case-fatality rate. The Siberian sub-type has been associated with chronic encephalitis in children [16].

Diagnosis and treatment

Diagnosis of TBE is made when antibodies to the TBE virus are detected in blood or cerebrospinal fluid. During the first phase of the illness, TBE virus or viral RNA can sometimes be detected in blood samples by virus isolation or PCR. Treatment relies on supportive management; there is no specific antiviral treatment for TBE. More severely affected individuals may need admission to intensive care, with some requiring assisted ventilation [19]. Long-term support for neurological complications may be needed.

Preventing tick-borne encephalitis

If travel to TBE endemic areas during spring, summer and autumn cannot be avoided, travellers should be advised to take risk avoidance measures including:

TBE immunoglobulin (antibodies) was previously used as post-exposure prophylaxis after a tick bite in TBE endemic countries. However, there were concerns that it had a negative effect on the disease course. TBE immunoglobulin is no longer recommended in the UK or other European countries for treatment.

Vaccine information

Indications for use of TBE vaccine

TBE vaccine should be considered for:

  • All persons living in highly endemic areas
  • Those at occupational risk in endemic areas, e.g. farmers, forestry workers, soldiers
  • Travellers at risk of disease. See ‘Risk for travellers’ section above
  • Laboratory workers who may be exposed to TBE [22].

Vaccines

TicoVac and TicoVac Junior vaccines (known in some countries as FSME IMMUN and FSME IMMUN Junior) are licensed in the UK [23, 24 and 25].

Details of these vaccines can be found in the summary table below.

Vaccine schedules

The Summary of Product Characteristics (SPC) for the individual vaccines should be consulted prior to the administration of any vaccine [22, 23].

Vaccine Schedule Accelerated schedule Length of protection Age range
TicoVac 0.5ml 3 doses on days 0, between 1 and 3 months, and 5 to 12 months after the second dose* 2nd dose can be given 2 weeks after the 1st dose **First booster no more than 3 years after 3rd dose. After this, boosters may be given at 5 year intervals if at risk Persons at least 16 years of age and older
TicoVac 0.25ml Junior 3 doses on days 0, between 1 and 3 months and 5 to 12 months after the second dose* 2nd dose can be given 2 weeks after the 1st dose First booster no more than 3 years after 3rd dose. After this, boosters may be given at 5 year intervals if at risk Children above 1 year of age and below 16 years of age
* After the first two doses, sufficient protection can be expected for the on-going tick season (protection rate over 90 percent after the second dose)
**In those aged > 60 years, booster intervals should not exceed three years (see below).

The optimum time to begin the course of vaccination against TBE is during the winter months in order to ensure protection prior to the start of the tick season in spring. TicoVac is probably effective against the Far Eastern subtype as well as the European subtype of TBE [23, 24]. In general, for individuals over 60 years of age, booster dose intervals should not exceed three years [23].

Contraindications

  • confirmed anaphylactic reaction to a previous dose of the vaccine or to any component of the vaccine or to egg [25]

Precautions

  • current febrile illness
  • known or suspected auto-immune disease
  • pre-existing cerebral disorders
  • pregnancy
  • breastfeeding

Adverse events

Adverse reactions following TBE vaccine are most commonly mild and transient. In adults they include local reactions such as swelling, redness and pain at the injection site. Generalised reactions such as fatigue, malaise, headache, muscle pain and nausea have been reported but were transient and usually mild.

Studies in children reported mild local and systemic reactions. The most common local reactions reported were pain and tenderness at the injection site. The most frequently reported systemic reactions were fever and restlessness in young children, as well as headache in all children. Fever, particularly after the first dose, has been reported.

In rare cases, more serious reactions of meningitis and neuritis have occurred.

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  3. Briggs B, Atkinson B, Czechowski D, et al. Tick-borne encephalitis virus, Kyrgyzstan. Emerg Infect Dis. 2011; 17(5): 876-9.
  4. World Health Organization. Vaccines against tick-borne encephalitis: WHO position paper. Wkly Epidemiol Rec.10 June 2011. 86: 241-256. 2011. [Accessed 28 October 2020]
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  12. Public Health England. Rare tick-borne infections diagnosed in England. 31 July 2020. [Accessed 28 October 2020]
  13. Public Health England. Qualitative assessment of the risk that tick-borne encephalitis presents to the UK population. September 2019. [Accessed 28 October 2020]
  14. European Centre for Disease Prevention and Control. Tick-borne encephalitis – Annual Epidemiological Report for 2018. 18 December 2019. [Accessed 28 October 2020]
  15. Karbowiak G, Biernat B. The role of particular tick developmental stages in the circulation of tick-borne pathogens affecting humans in Central Europe. 2. Tick-borne encephalitis virus. Annals of Parasitology 2016, 62(1), 3–9.
  16. US Centers for Disease Control and Prevention. TBE. Tick-borne encephalitis (TBE). 31 March 2014. [Accessed 28 October 2020]
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  23. Pfizer. Summary of Product Characteristics for TicoVac. Last updated 8 November 2018. [Accessed 28 October 2020]
  24. Pfizer. Summary of Product Characteristics for TicoVac Junior. Last updated 6 November 2018. [Accessed 28 October 2020]
  25. Public Health England. Tick-borne encephalitis: the green book, chapter 31. In: Immunisation against Infectious Disease. Last updated September 2016. [Accessed 22 October 2020]

First Published :   29 Oct 2020
Last Updated :   29 Oct 2020

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