What is Tick-borne encephalitis?
Tick-borne encephalitis (TBE) is an important cause of viral infections of the central nervous system in Europe and Asia. TBE is caused by a single-stranded RNA virus that belongs to the genus Flavivirus. There are 3 subtypes of TBE virus: European, Siberian, and Far Eastern. Most TBE virus infections result from tick bites, and TBE cases usually occur in the season when ticks are active. Approximately two-thirds of TBE virus infections are asymptomatic. Patients who develop symptoms often present with milder form of the disease with non-specific febrile illness, headache, myalgia and fatigue which last for several days. Up to two-thirds of the patients recover without further illness but some patients progress to a second phase of the disease and develop aseptic meningitis, encephalitis or myelitis. Disease severity increases with age and varies by TBE virus subtype. The Far Eastern subtype is associated with a more severe disease course with higher rates of severe neurologic deficits and a case-fatality ratio of 20%–40%.
How is it transmitted?
TBE virus is transmitted to humans through the bite of an infected tick or ingestion of unpasteurized dairy products (such as milk and cheese) from infected animals. The ticks of the Ixodes genus, mainly I. ricinus (European subtype) or I. persulcatus (Siberian and Far Eastern subtypes), act as both vector and reservoir of TBE virus. TBE is often acquired in endemic forested areas through outdoor activities such as camping, hiking, forestry or military training. After a tick bite, the incubation period for TBE ranges from 4 to 28 days with a median of 8 days. The incubation for milkborne exposure is usually shorter (3-4 days).
How to prevent it?
During their stay in an endemic area, travellers should avoid tick bites by wearing long-sleeved clothes and trousers. They should apply DEET-containing insect repellent on exposed body parts and permethrin-containing insecticide spray onto clothes. If an attached tick is found on the body, gently remove it by grasping its head with tweezers or fine-tipped forceps close to the skin, then disinfect the bite area and wash hands with soap and water. Do not crush or twist the tick during removal. In addition, travellers should avoid consumption of unpasteurized dairy products in endemic areas.
TBE vaccine offers effective protection. According to the World Health Organization, TBE vaccine should only be offered to at-risk travelers, such as people travelling from non-endemic areas to endemic areas if their visits will include extensive outdoor activities. Outside countries or areas at risk, TBE vaccines may not be licensed and will have to be obtained by special request.
In Hong Kong, the TBE vaccine is not registered in the local market. Registered medical practitioners can apply the license for importation of TBE vaccine for a particular patient in Hong Kong and the application will be considered by the Drug Registration and Import/Export Control Division of the Drug Office of the Department of Health on a case by case basis. Interested registered medical practitioners may refer to the following link for the application procedure and documents required:
How to treat it?
There is no specific antiviral treatment for TBE. Treatment mainly consists of supportive care and management of complications.