Tick-borne encephalitis (TBE) is an infection of the brain caused by the TBE virus which is transmitted to humans through the bite of an infected tick. TBE affects people of all ages, but the disease has a tendency to be more severe in the second half of life. Ticks become carriers of the TBE virus when they suck blood from infected animals. The tick species that harbour and transmit the TBE virus (Ixodes ricinus and Ixodes persulcatus) are present in a vast area stretching from the western border of Europe to the east coast of Japan, but it is only in certain restricted geographical locations that they carry the virus. The virus circulates between the ticks and the host animals on which the ticks feed, including rodents, deer and birds. Ticks do not prey on humans, but will take a human blood meal when the opportunity arises. The life cycle of a tick spans several years and once infected with the TBE virus, the tick will remain infectious throughout its lifespan.

Viral encephalitis
Viral encephalitis (infection of the brain) is a rare infectious condition that can be caused by several different viruses including TBE virus. TBE is an important cause of viral encephalitis in a number of central and northern European countries where ticks carry the TBE virus (endemic areas). Before the start of its vaccination programme, Austria had the highest number of reported TBE cases in the European Union relative to its population, and in some parts of the country more than half of all viral encephalitis cases were caused by TBE virus. In 2008 in Sweden, 224 (38%) out of a total of 585 reported cases of viral encephalitis were caused by TBE virus, making TBE the single most important cause of encephalitis in that country.

Infection with TBE
Infection with TBE virus leads to immunity and protects against future infections. Blood samples taken from people living and working in areas with infected ticks have shown that most TBE virus infections do not develop into encephalitis and that many infections remain undiagnosed or go unnoticed altogether. The risk of developing severe disease once infected with TBE virus has been estimated at one in four. When the infection involves the central nervous system it presents as an acute disease with high fever, severe headache and sometimes paralysis and altered consciousness. Of the patients who develop encephalitis about one third will experience long-term residual conditions and disabilities ranging from concentration difficulties to memory impairment, hearing loss and paralysis.

Risk factors
The risk of TBE infection is influenced by multiple factors, including climatic conditions and human behaviour. The number of ticks and their activity level is determined by annual variations in the weather, as is human activity in TBE risk areas, and this produces the fluctuations in the number of cases from year to year. Most TBE infections today occur in people who venture into forests and grasslands for recreation. Exposed professionals, such as forestry workers, farmers and hunters used to be at highest risk of infection but are today often protected through vaccination.
The proportion of ticks infected with TBE virus varies from 0.1 to 5% in Europe, and may be much higher in some locations. TBE risk areas are often areas where people enjoy outdoor recreation activities, and most TBE cases occur during the summer months although ticks are active from early spring to late autumn. Assessing the risk of contracting TBE is not straightforward because not all people are exposed to infected ticks. The risk is often estimated based on the number of TBE cases that were reported from an area in the past.

Identifying locations of TBE risk can be a problem because patients may not always know where they were infected. The incubation period is one to two weeks, and patients could have visited several locations where they could have been infected before they fell ill. Furthermore, it is not uncommon for patients to be unaware that they were bitten by a tick.

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