Tick-borne encephalitis is a viral infection caused by a flavivirus. The virus is transmitted to man by the bite of an infected tick or occasionally by drinking unpasteurised milk from infected animals. The tick's saliva has an anaesthetising effect and therefore a bite can often go unnoticed. The risk of infection following a single tick bite varies from 1 in 200 to 1 in 1000 depending on the area visited. The incubation period for tick-borne encephalitis is usually between 7 and 14 days but can range from 2 to 28 days. The illness normally occurs in two stages. The first stage affects about two-thirds of infected people and can last from 1 to 8 days. During this phase a variety of symptoms can occur such as flu-like illness, fever, headache, nausea, muscle pain, tiredness and generally feeling unwell. Following this phase there is a gap of 1 to 20 days and over this period there are usually no symptoms. About a third of people who had symptoms during the first stage go on to enter the second stage which starts with a sudden rise in temperature and central nervous system involvement with meningitis. About a third of these people develop encephalitis which can cause paralysis. A higher risk of death is seen in individuals over the age of 60 years.
There are three main types of tick-borne encephalitis:
- European subtype: occurs in central and western European countries.
- Far Eastern subtype: occurs in eastern Russia and forested regions of China and Japan.
- Siberian subtype: mainly occurs in Siberia.
The disease is extremely rare in UK travellers. However, travellers visiting areas where they may be exposed to infected ticks may be at risk when enjoying activities such as camping, walking and working in wooded areas. The disease can occur throughout the year but there are seasonal variations. Infections due to the European subtype more common in the autumn, and the far Eastern subtype are more common in spring.
There is no specific treatment for tick-borne encephalitis. The risk of acquiring it can be reduced by insect bite avoidance methods such as wearing protective clothing, using insect repellent, and checking and removing ticks on a regular basis. Travellers should also avoid drinking unpasteurised milk in areas at risk. Vaccination may be considered for people who are at risk due to their occupation, and travellers visitng rural endemic areas.