Tick-borne encephalitis

Tick-borne encephalitis (TBE) is a viral infection, that can affect the central nervous system. The virus (TBEV) is mostly transmitted via infected ticks but can also infect humans via consumption of unpasteurized milk. While a third of all infected people do not suffer from disease symptoms, the TBEV may affect patients severely and sometimes with long-lasting consequences. In Europe, the number of TBE infections is growing. Climatic changes contribute to this evolution as it may enlarge tick populations and shift their distribution northwards and to higher altitudes.

TBE total cases and locally aquired cases notification rate (map) and total reported and locally acquired cases (graph) in Europe
_{Source: ECDC, 2024, Surveillance Atlas of Infectious Diseases}

_{Notes: Map and graph show data for the EEA member countries, excluding Iceland, Liechtenstein, Malta, Portugal, Switzerland and Türkiye due to absence of data. The boundaries and names shown on this map do not imply official endorsement or acceptance by the European Union. The disease is notifiable at the EU level, but the reporting period varies among the countries.
When countries report zero cases, the notification rate on the map is shown as '0'. When countries have not reported on the disease in a particular year, the rate is not visible on the map and is labelled as 'unreported' (last updated in May 2023).}

Source & transmission

Ixodes ticks are responsible for most of the TBEV transmissions to humans. They are widespread across Europe, where they cause focal infections (ECDC, 2022). Juvenile ticks usually feed on small rodent species, which are the main hosts for the TBEV, while adult ticks feed on larger animals. When ticks get infected, they can remain infectious for the rest of their life and transmit TBE to humans and large animals such as goats, cows, sheep, deer, and swine. Occasionally, the virus spreads to humans via consumption of unpasteurized animal milk. The TBEV can even survive in the acidic environment of the human stomach (Dörrbecker et al., 2010; Leonova et al., 2014; Kříha et al., 2021; CDC, 2022).

Health effects

A third of the people with a TBE infection experiences no symptoms. For those who do get sick, symptoms arise several days to a month after the tick bite or a few days after milk-borne infection. The TBEV can cause inflammation of the brain (encephalitis) and the spinal cord (meningitis). Initial symptoms include fever, headache, vomiting and general weakness. These can be followed by a period, during which the initial symptoms ease before severe symptoms start to manifest. More severe symptoms are confusion, loss of coordination, difficulties with speaking, weakness in the limbs and seizures. Disease severity and duration depend on the virus strain with which a patient is infected (Bogovic et al., 2010).

Morbidity in Europe

In the EEA member countries (excluding Cyprus, Iceland, Liechtenstein, Malta, Portugal, Switzerland and Türkiye due to absence of data), in the period 2012-2022:

• 28,485 cases
• Until 2017, the long-term trend in infections was stable, with some years with more infections that could be related to favourable environmental conditions. Since 2017, the number of reported TBE cases has gradually increased.

(ECDC, 2016-2022)

Distribution across population

• Age group with the highest disease rate in Europe: 45 – 64 years old
• Infection rates are higher among men than woman, possibly related to higher exposure during outdoor activities and a lower risk perception among men

(ECDC, 2016-2022)

Climate sensitivity

Climatic Suitability

Ixodes ticks require ambient temperature above 7 °C and humidity above 85% to reproduce (Petri et al., 2010). Yet, the ticks can survive temperatures between 3 and 28 °C and are most active between 6 and 15 °C. Temperatures above 28 °C reduce tick activity or lead to dehydration and death.

Seasonality

In Europe, most infections occur between May and November with a peak between June and August, when temperatures are the highest. There is no evidence for a shift in the seasonal pattern (ECDC, 2016-2022).

Climate Change Impact

Ixodes tick populations are expected to adapt to a changing climate by shifting their distribution patterns according to their suitable temperature range. TBE infections increase with more rainfall and higher temperatures, meaning milder winters, warmer springs and more extended warm periods (Gilbert, 2021). Higher temperatures accelerate tick development, increase egg production, enlarge the population density, and shift the geographical distribution range northwards and to higher altitudes. Additionally, warmer weather can lead to larger rodent populations and, therefore, to more active ticks, resulting in an increased risk of TBE infection for humans (Lukan et al., 2010). Due to climate change, TBE cases have been projected to increase in mountainous areas above 500 m above sea level (Lukan et al., 2010) and to increase especially in the Northern countries of Europe such as Finland, Germany, Russia, Scotland, Slovenia, Norway and Sweden (Lindgren and Gustafson, 2001).

Prevention & Treatment

Prevention

• Active monitoring and surveillance of ticks, disease cases and the environment (e.g. TBE surveillance in Czechia)
• Personal protection: long-sleeved and fitted clothing, tick repellents, avoiding tick habitats
• Vaccination
• Pasteurization of milk for consumption

Treatment

No specific and effective antiviral therapy

References

Bogovic, P. et al., 2010, What tick-borne encephalitis may look like: Clinical signs and symptoms, Travel Medicine and Infectious Disease 8(4), 246-250. https://doi.org/10.1016/j.tmaid.2010.05.011

CDC, 2022, Centers for Disease Control and Prevention, https://www.cdc.gov. Last accessed August 2022.

Dörrbecker, B., et al., 2010, Tick-borne encephalitis virus and the immune response of the mammalian host, Travel Medicine and Infectious Disease 8(4), 213–222. https://doi.org/10.1016/j.tmaid.2010.05.010

ECDC, 2016-2022, Annual epidemiological reports for 2014-2020 – Tick-borne encephalitis. Available at https://www.ecdc.europa.eu/en/publications-data/monitoring/all-annual-epidemiological-reports. Last accessed May 2023.

ECDC, 2022, Ixodes ricinus - current known distribution: March 2022. Available at https://www.ecdc.europa.eu/en/publications-data/ixodes-ricinus-current-known-distribution-march-2022. Last accessed December 2022.

ECDC, 2023, Surveillance Atlas of Infectious Diseases. Available at https://atlas.ecdc.europa.eu/public/index.aspx. Last accessed May 2023.

Kříha, M. F. et al., 2021, What we know and still do not know about tick-borne encephalitis?, Epidemiology, Microbiology, Immunology 70(3), 189–198.

Leonova, G. N. et al., 2014, The nature of replication of tick-borne encephalitis virus strains isolated from residents of the Russian Far East with inapparent and clinical forms of infection, Virus Research 189, 34–42. https://doi.org/10.1016/j.virusres.2014.04.004

Lindgren, E. and Gustafson, R., 2001, Tick-borne encephalitis in Sweden and climate change, The Lancet 358(9275), 16–18. https://doi.org/10.1016/S0140-6736(00)05250-8

Lukan, M. et al., 2010, Climate Warming and Tick-borne Encephalitis, Slovakia, Emerging Infectious Diseases 16(3), 524–526. https://doi.org/10.3201/eid1603.081364

Petri, E. et al., 2010, Tick-borne encephalitis (TBE) trends in epidemiology and current and future management, Travel Medicine and Infectious Disease 8(4), 233–245. https://doi.org/10.1016/j.tmaid.2010.08.001