Introduction

Tick borne encephalitis (TBE) virus belongs to a closely related group of flaviviruses that includes yellow fever, dengue and japanese encephalitis. The virus is caused by three different subtypes: European TBE virus, Far Eastern TBE virus and Siberian TBE virus.

European TBE is endemic in western and central Europe and is transmitted by Ixodes ricinus ticks. It is common in forest and mountain regions.

Far Eastern TBE, is also known as russian/ summer encephalitis and is transmitted by I.persulcatus ticks. This occurs in the spring and summer months in eastern Russia and some countries in East Asia

Siberian TBE is endemic in Siberia and is also transmitted by I.persulcatus ticks.

Risk for travellers

Some of the factors that increase the risk of acquiring tick borne encephalitis include:

Destination of travel

Season of travel

Duration of travel

The activity of the ticks in the country visited

Travellers vaccination status

Travellers who are planning a visit to an endemic area may also be at risk when walking, camping or working in woodland terrain. Individuals may also be infected with TBE by consuming un-pasteurised dairy products from infected animals.

There is a higher risk for infection during the months April to November, with the Far Eastern subtype more common in the spring and the European subtype more common in the autumn.

Transmission

Transmission in humans mainly occurs through the bite of an infected tick with the introduction of the virus through the tick saliva. Saliva contains an anaesthetic, meaning the bite sometimes may go unnoticed, emphasising the importance of checking the body for attached ticks. The infected ticks are usually found on forest fringes with adjacent grassland, riverside, meadows and marshland. Ticks can also transmit the virus throughout their lifecycle stages, and once infected, carry the virus for life.

The activity and development of ticks depend on climatic factors such as temperature, the moisture of the soil and humidity. Wet summers and mild winters usually increase the tick density.

Signs and symptoms

The incubation period is 7-14 days, but can vary from 2-28 days. The first stage of the disease may last roughly 1-8 days and affects about 66% of infected patients. The symptoms are characterised by a non-specific and flu-like illness, accompanied by fatigue, headache, fever and general weakness.

Approximately 33% of those who were symptomatic during the first phase will go on to the second phase of the disease. This is characterised by a sudden rise in temperature and central nervous system involvement. Furthermore, a third of these cases progress to encephalitis