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 between Sweden and Finland 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, has been reported from Albania, Greece, Northern Italy, Norway, and Turkey. The Russian spring-summer encephalitis is transmitted by I. persulcatus ticks and occurs in China, Korea, Japan, and Eastern and central areas of Russia. The Siberian and Far-eastern subtypes of TBEV were also detected in the European part of Russia including the Baltic countries.
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 and Asia it is the Taiga tick. Mainly rodents serve as mammalian hosts for TBEV. Rodents from the murine genera Apodemus (field mice) and Myodes (Red-backed voles), probably also act as maintenance hosts. Larger wild animals are considered to be not competent for virus transmission but serve as hosts and transporters 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.