Tick-borne encephalities

Tick-borne encephalitis (TBE) is a viral infection of the central nervous system occurring occurring in eastern, central, northern and increasingly western European countries, and in northern China, Mongolia, and the Russian Federation. The causative pathogen, Tick-borne encephalitis virus, is a member of the viral family Flaviviridae.

About 10,000 – 12,000 cases are reported each year, but this estimation is believed to be significantly lower than the actual total number of clinical cases. Most of these infections result from bites of infected ticks, which often remain firmly attached to the skin for days. On rare occasions, infection can result from consumption of unpasteurized milk from infected goats, sheep or cows.  

TBE is a growing public health challenge in Europe and other parts of the world, with about 400% increase in the number of human cases in the last 30 years in all endemic regions of Europe, spread of the risk areas and newly discovered foci. 

The following are standard case definitions to allow national health authorities to interpret data in an international context. However, during an outbreak case definitions may be adapted to the local context and the Red Cross Red Crescent should use those agreed/established by national health authorities. NB: Consider that during community-based surveillance, volunteers should use broad (simplified) case definitions (referred to as community case definitions) to recognize most or all possible cases, provide relevant risk communication and appropriate actions and encourage them to seek care. Other actors such as healthcare workers or investigators studying the cause of a disease, on the other hand, can use more specific case definitions that may require laboratory confirmation. 

Case definition 

Clinical Criteria 

An individual with symptoms of inflammation of the central nervous system including:  

• Meningitis 

Laboratory Criteria 

Laboratory criteria for a probable case: 

• Detection of TBE-specific IgM-antibodies in a unique serum sample 

Laboratory criteria for case confirmation - at least one of the following five: 

• Detection of TBE specific IgM AND IgG antibodies in blood
• Detection of TBE specific IgM antibodies in CSF
• Seroconversion or four-fold increase of TBE-specific antibodies in paired serum samples
• Detection of TBE viral nucleic acid in a clinical specimen,
• Isolation of TBE virus from clinical specimen 

Case Classification 

1. Probable case - An individual meeting the clinical and the laboratory criteria for a probable case, OR An individual meeting the clinical criteria with an epidemiological link
2. Confirmed case - An individual meeting the clinical and laboratory criteria for case confirmation 

Note: Serological results should be interpreted according to previous exposure to other flaviviral infections and the flavivirus vaccination status. Confirmed cases in such situations should be validated by serum neutralization assay or other equivalent assays.   

https://www.ecdc.europa.eu/en/tick-borne-encephalitis/facts/factsheet  

Single case 

• In endemic areas, people with recreational or occupational outdoor activities (e.g. hunting, fishing, camping, collecting mushrooms and berries, forestry, farming, military training) are potentially at risk of infection due to possible proximity to infected ticks.
• Travelling to endemic communities during the warmer spring and summer months. 

Attack rates will vary from one outbreak to another. In case of an outbreak, consult the latest information provided by health authorities. 

• Generally low and depends on the type of exposure

• Older people

TBE is a viral infectious disease of the central nervous system that can result in long-term neurological symptoms, and even death. It is caused by a virus (Flavivirus genus, family Flaviviridae) with three subtypes: 

• European subtype - transmitted by the Ixodes ricinus ticks and endemic in rural and forested areas of the central, eastern and northern Europe;
• Far eastern subtype - transmitted mainly by I. persulcatus and endemic in far-eastern Russia and in forested regions of China and Japan; and 
• Siberian subtype - transmitted by I. persulcatus  and endemic in the Urals region, Siberia and far-eastern Russia, and also in some areas in north-eastern Europe. 

The TBE virus is transmitted by the bite of infected ticks. The infection can also be transmitted through the consumption of infected unpasteurised dairy products. There is no human-to-human transmission, apart from the possibility of transmission from an infected mother to a child during breastfeeding. Laboratory accidents from needle-stick injuries as well as aerosol infection have also been reported. 

2–28 days (most commonly 7–14 days) for infections from tick bites. 

The incubation after foodborne infection is usually shorter, around four days. 

Humans are not infectious. 

Most infections are asymptomatic.  

​​In clinical cases, TBE often has a biphasic course. The first viremic phase lasts for approximately five (range 2–10) days, and is associated with non-specific symptoms including fever of ≥38 °C, fatigue, headache, myalgia, malaise, anorexia and nausea. ​ 

This phase is followed by an asymptomatic period lasting seven (range 1–33) days and subsequently, the second phase. The second phase is associated with fever frequently exceeding 40°C and signs of the central nervous system’s involvement such as seizures, photophobia, headache, stiff neck drowsiness, confusion, sensory disturbances, paralysis, etc. Other symptoms include altered mental status, cognitive dysfunction, difficulty walking, etc. 

The European subtype often results in milder disease, with 20–30% of patients experiencing the second phase, mortality rates of 0.5–2%, and severe neurological sequelae in up to 10% of patients. The second phase of the illness is usually limited to meningitis for children. However, adults older than 40 years are at increased risk of developing encephalitis, with higher mortality and long-lasting sequelae in adults over the age of 60. 

The Far Eastern subtype typically results in more severe disease: monophasic illness, with no asymptomatic interval prior to the onset of neurological disease, mortality rates of up to 35%, and higher rates of severe neurological sequelae. 

The Siberian subtype is associated with a less severe disease (fatality rate of 1–3%), with a tendency for patients to develop chronic or extremely prolonged infections. 

https://www.nhs.uk/conditions/tick-borne-encephalitis/  

Other similar arboviruses include West Nile, Usutu, and Japanese encephalitis viruses. In patients with known exposure to ticks, other tick-borne diseases might be considered, including Lyme borreliosis. 

• The diagnosis of TBE is based on the detection of specific IgM antibodies in cerebrospinal fluid and/or serum, mainly by ELISA. 
• Detection by detection of viral RNA by reverse transcription-polymerase chain reaction (RT-PCR) can also be used for an early differential diagnosis of TBE.
• Virus isolation using cerebrospinal fluid or serum samples, especially for immunocompromised patients. 

Please refer to the appropriate local or international guidelines for clinical management. All clinical management including the administration of a treatment or vaccine should be conducted by a health professional.  

• There is no specific treatment for tick-borne encephalitis. Treatment relies on supportive management. 
• Immunization offers the most effective protection.  

Specific IgM antibodies can persist for up to 10 months in vaccines or individuals who acquired the infection naturally.