Cause
The tick-borne encephalitis (TBE) virus is a flavivirus. Three subtypes of the causative agent are known. The most common subtypes are the European subtype, the Far Eastern subtype (Spring Summer encephalitis) and the Siberian subtype. Other closely related viruses cause similar diseases.
Transmission
Infection is transmitted by the bite of infected ticks or by ingestion of unpasteurized milk. There is no direct person-to-person transmission. Some related viruses, also tick-borne, infect animals such as birds, deer (louping-ill), rodents and sheep.
Nature of the disease
Infection may induce an influenza-like illness, with a second phase of fever occurring in 10% of cases. Encephalitis develops during the second phase and may result in paralysis, permanent sequelae or death. Severity of illness increases with age. The Far Eastern subtype causes more severe symptoms and sequelae than the European subtype.
Geographical distribution
The European subtype is present in large parts of central and eastern Europe, particularly Austria, southern Germany or northern Switzerland, the Baltic states (Estonia, Latvia, Lithuania), the Czech Republic, Hungary and Poland; the Far Eastern subtype is found from north eastern Europe to China and Japan, and the Siberian subtype from northern Europe to Siberia. The disease is seasonal; most cases occur during April to November. The risk is highest in forested areas up to an altitude of about 1400 m.
Risk for travellers
Travellers who walk and camp in infested areas during the tick season (usually spring to early autumn) are at risk and should be vaccinated. Some degree of protection is afforded by clothing that covers as much skin as possible and by applying insect repellent.
Vaccine
The vaccine should be offered only to at risk travellers. Two vaccines are available in Europe, in adult and paediatric formulations. These are inactivated whole-cell vaccines containing a suspension of purifi ed tick-borne encephalitis virus grown on chick embryo cells and inactivated with formaldehyde. Both provide safe and reliable protection. Immunity is induced against all variants of the tick-borne encephalitis virus including the European and Far Eastern subtypes. Two doses of 0.5 ml should be given i.m. 4–12 weeks apart. A third dose is given 9–12 months after the second dose and confers immunity for 3 years. Booster doses are required to maintain immunity and should be given every 3 years if the risk continues. Outside endemic countries, the vaccines may not be licensed and will have to be obtained by special request.
Precautions and contraindications
Occasional local reactions may occur, such as reddening and swelling around the injection site, swelling of the regional lymph nodes or general reactions (e.g. fatigue, pain in the limb, nausea and headache). Rarely, there may be fever above 38 ºC for a short time, vomiting or transient rash. In very rare cases, neuritis of varying severity may be seen, although the etiological relationship to vaccination is uncertain. The vaccination has been suspected of aggravating autoimmune diseases such as multiple sclerosis and iridocyclitis, but this remains unproven. Hypersensitivity to thiomersal (a vaccine preservative) is a contraindication.
Type of vaccine: Killed
Number of doses: Two, given i.m. 4–12 weeks apart, plus booster
Booster: 9–12 months after second dose
Contraindications: Hypersensitivity to the vaccine preservative thiomersal; adverse reaction to previous dose
Adverse reactions: Local reactions occasionally; rarely fever
Before departure: Second dose 2 weeks before departure
Recommended for: High-risk individuals only
Special precautions: Avoid ticks; remove ticks immediately if bitten