Tick-borne encephalitis (TBE) is an important cause of viral infections of the central nervous system in eastern, central, northern and increasingly western European countries, and in northern China, Mongolia, and the Russian Federation. Tick-borne encephalitis virus is a member of the family Flaviviridae. Approximately 10 000–12 000 clinical cases of tick-borne encephalitis are reported each year, but this figure is believed to be significantly lower than the actual total number of clinical cases.
The vast majority of infections with the virus result from infected ticks, which often remain firmly attached to the skin for days. On rare occasions, infection can result from consumption of unpasteurized milk from infect goats, sheep or cows. People come in contact with the ticks during outdoor activities in forested areas up to an altitude of about 2000 meters. There is no direct person-to-person transmission.
What are the symptoms of TBE?
Most infections remain asymptomatic. In case of clinical illness, the incubation period for tick-borne encephalitis lasts 2–28 days (most commonly 7–14 days) and is followed by 1–8 days of general cold symptoms, such as fatigue, headache and general malaise, usually combined with fever of ≥38 °C.
After an asymptomatic interval of 1–20 days up to 15% of patients experience a second phase of the disease characterized by fever frequently exceeding 40 °C and signs of central nervous system involvement, such as meningitis (e.g. fever, headache, and a stiff neck), encephalitis (e.g., drowsiness, confusion, and sensory disturbances), myelitis or radiculitis.
Encephalitis developing during this second phase may result in paralysis, permanent sequelae or death. About 1% of cases with neurological pathologies may die; higher fatality rates have been reported from the Russian federation, which may be related to a different virus subtype. Severity of illness increases with age of the patient, but fatalities have been reported from all age groups.
In the European type of TBE there are normally two phases to the illness. The first phase consists of flu-like illness with symptoms such as fever, headache and generalised body aches. It lasts around five days. There is then about a week with no symptoms, which is followed by the second phase of the illness, when the brain or nerves are involved. The severity of the second phase varies between patients. Some people have a very mild illness whilst others may have a severe illness. Older people tend to get a more severe illness than younger people or children.
The symptoms in the second stage are similar to other causes of encephalitis and meningitis (inflammation of the lining of the brain). These include neck stiffness, headache, nausea, impaired consciousness (from drowsiness to coma), poor coordination, tremor, personality changes and psychosis. Some patients will also get muscle pain or limb weakness (most often in the arms). In severe cases the muscles that control the breathing are affected and people may die.
How is it diagnosed?
TBE can be diagnosed by a blood test and lumbar puncture (LP). LP (sometimes called a ‘spinal tap’) is a procedure which involves passing a needle, under local anaesthetic, between two of the backbones at the base of your spine to collect the cerebrospinal fluid (CSF) (fluid surrounding the brain and the spinal cord).
The virus can be detected in the blood in the first phase of the illness, whilst the antibodies can be found in blood and cerebrospinal fluid, usually, in the second phase. Brain scans such as CT (computed tomography) and MRI (magnetic resonance imaging) can show abnormalities specific encephalitis, but do not have specific characteristics for TBE.
Treatment of TBE?
Unfortunately, there is no specific treatment for TBE. For people who have severe disease they may need supportive treatment in hospital such as intensive care. However, most people will only need symptom control.
Consequences of TBE
Some people (less than 2%) may die from TBE. This is more common in the Far-Eastern type. Some people will recover completely. Still others may have long lasting problems such as paralysis (the inability to move your arms and/or legs), ataxia (difficulties with coordination, balance and speech), headache, tiredness, difficulty concentrating and poor memory. If children are affected, they may be left with long-term cognitive problems such as short-term memory, headache, fatigue and irritability.
Prevention of TBE
People can protect themselves from ticks by wearing appropriate clothing, including long trousers and closed footwear, when hiking or camping in countries or areas at risk. The whole body should be inspected daily and attached ticks removed as soon as possible. The consumption of unpasteurized dairy products should also be avoided in those areas.
In a new position paper, published in the Weekly Epidemiological Record today, WHO recommends vaccination against tick-borne encephalitis in people of all ages where the disease is highly endemic. Where the prevaccination incidence of the disease is moderate or low or is limited to particular geographical locations or certain outdoor activities, immunization should target individuals in the most severely affected groups. People travelling from non-endemic areas to endemic areas should be offered vaccination if their visits will include extensive outdoor activities.
Since the incidence of tick-borne encephalitis may vary considerably between and even within geographical regions, public immunization strategies should be based on risk assessments conducted at country, regional or district level, and should be appropriate to the local endemic situation.
Immunization offers the most effective protection against tick-borne encephalitis. Currently, there are four widely used vaccines of assured quality: FSME-Immun and Encepur, manufactured in Austria and Germany respectively, and TBE-Moscow and EnceVir, manufactured in the Russian Federation. The four vaccines are considered to be safe and efficacious.