Tick-borne encephalitis, or TBE, is a human viral infectious disease involving the central nervous system. TBE is caused by the tick-borne encephalitis virus (TBEV), a member of the family Flaviviridae, and was initially isolated in 1937. Three virus sub-types are described: European or Western tick-borne encephalitis virus, Siberian tick-borne encephalitis virus, and Far eastern Tick-borne encephalitis virus (formerly known as Russian Spring Summer encephalitis virus, RSSEV).
The family Flaviviridae includes several tick-borne viruses affecting humans. These viruses are closely related to TBEV and Far-eastern TBE, and include Omsk hemorrhagic fever virus in Siberia, Kyasanur Forest disease virus in India and its close relative, Alkhurma virus in Saudi Arabia. Louping ill virus (United Kingdom) is also a member of this family; it causes disease primarily in sheep and has been reported as the cause of a TBE-like illness in laboratory workers and persons with contact to sick sheep (e.g., veterinarians, butchers). In the USA and Russia, another tick-borne flavivirus, Powassan virus, is responsible of encephalitis in human.