Geographical distribution

The distribution of TBE is closely related to the activity of the tick vectors: The Castor Bean tick, Ixodes ricinus, in Western and Central Europe, and the Taiga tick, Ixodes persulcatus, in Central and Eastern Europe. (There is an overlap of the two species.)

The disease is endemic in Scandinavia, western and central Europe, and countries that made up the former Soviet Union. TBE is common in Austria, Estonia, Latvia, the Czech Republic, Slovakia, Germany, Hungary, Poland, Switzerland, Russia, Ukraine, Belarus, and northern regions of the former Yugoslavia. It occurs at a lower frequency in Bulgaria, Romania, Denmark, France, the Aland archipelago and the neighbouring Finnish coastline, as well as along the coastline of Southern Sweden, from Uppsala to Karlshamn. Serologic evidence for TBE infection, as well as sporadic cases, have been reported from Albania, Greece, Italy, Norway, and Turkey. The Russian spring-summer encephalitis is transmitted by I. persulcatus ticks and occurs in China, Korea, Japan, and eastern areas of Russia.

Seasonal Distribution

The seasonal incidence of TBE is also closely related to the activity of the tick vectors. I. ricinus is most active in spring and autumn, with two peaks of activity: one in late March to early June, and one from August to October. I. persulcatus is usually active in spring and early summer. Apparently, I. persulcatus is more cold-hardy than I. ricinus, and thus inhabits harsher, more northern areas. Seasonal distribution may vary according to changes of temperature. Therefore, global warming due to climate change is considered to increase the transmission time during the year by shifting the activity start to earlier times.

Reservoirs and hosts

Main vector and reservoir for TBEV in central and northern Europe is the widespread Castor Bean tick. In eastern Europe it is the Taiga tick. Many animals serve as hosts for TBEV and/or Ixodes ticks. Rodents from the murine genera Apodemus (field mice) and Clethrionomys (Red-backed voles), probably also act as reservoir. Larger wild animals are considered to be not competent for virus transmission but serve as hosts for the ticks.

The louping ill (LI) virus is also spread by I. ricinus ticks. LI virus causes meningoencephalitis in sheep mainly in the UK, but the disease is also seen in Spain, Greece, Turkey and Norway. Humans are susceptible to infection with LI virus, but occurrence is rare. On the other hand TBEV seems to be primarily pathogenic for humans.