Epidemiology

Tick-borne encephalitis (TBE) occurs throughout the northern hemisphere, and mirrors the geographical distribution of the usual vector, Ixodes species ticks.[2] Most cases occur in areas below an altitude of 750 metres.[3]

In Europe, tick activity starts in spring when the temperature approaches 6ºC and usually persists until November when the temperature falls.[8] 

In TBE-endemic countries (i.e., focal areas of Europe and Asia), the annual incidence of the disease ranges from <1 to >20 cases per year, per 100,000 population.[1] In Europe, 3817 cases were reported in 2020 (0.9 cases per 100,000 people, an increase from 0.6 per 100,000 people in 2016). Throughout the European Union, TBE was most common in adults aged 45 to 64 years, with a 1.5:1 male to female ratio.[9]​ Russia has the largest number of reported cases.[3]

In the UK, TBE virus-infected ticks have been identified in Thetford Forest (in the East of England), the Hampshire and Dorset border, the New Forest, and the North Yorkshire Moors. Two probable and two confirmed cases of TBE have been reported in the UK since 2019. However, the risk to the general public remains very low according to a 2023 risk assessment report. The risk to high-risk groups (e.g., those living, working, or visiting affected areas, as determined by duration of time spent outside), also remains low.[10]

Some non-endemic countries (e.g., Australia) have reported imported cases.​[11]​​[12]​​ In US civilian travellers, 11 cases were reported between 2000 and 2020; however, additional cases may have occurred as TBE is not a notifiable condition in the US.[3]

The geographical distribution of TBE viruses is as follows:[13]

  • Western/European subtype - endemic in parts of Scandinavia, and in Central and Eastern Europe down to the Adriatic region

  • (Ural-) Siberian subtype - endemic from Europe across the Ural Mountains to the Pacific Ocean, Korea

  • Far Eastern subtype - endemic in Russia, China, northern Japan

  • Louping ill subtype - endemic in the UK, Ireland, Norway.

Geographical distribution maps are available from the European Centre for Disease Prevention and Control (ECDC) and US Centers for Disease Control and Prevention (CDC):

Powassan virus (a related tick-borne flavivirus that can present with encephalitis) is endemic in Canada, Russia, and the New England states of the United States.[4]

[Figure caption and citation for the preceding image starts]: Geographical distribution of clinical cases of TBE in Europe and AsiaCreated by BMJ Evidence Centre; adapted from: World Health Organization. International travel and health: tick-borne encephalitis. 2016 [Citation ends].com.bmj.content.model.Caption@11f925dd

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