Tick-borne encephalitis (TBE)
TBE virus occurs in many places in Europe, and has established itself in the coastal districts of southern Norway in recent years.
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- TBE occurrence is low in Norway. From 1994 to 2015, there were 115 patients diagnosed with the disease in Norway. Infection occurred in Norway (97). In 2015, there were 9 reported cases, all were infected in Norway.
- All the reported TBE cases in Norway were infected along the coastline in Southern Norway (see figure).
- In Norway, ticks live along the coast up to Bodø. The Norwegian Institute of Public Health has studied ticks in coastal areas in the counties of Østfold, Akershus, Buskerud, Telemark, Vestfold, Aust-Agder, Vest-Agder, Rogaland, Hordaland, Møre og Romsdal, Sør-Trøndelag, Nord-Trøndelag and Nordland. Ticks with TBE virus have been found in all these counties. Antibodies against TBE have been found in the serum of deer and livestock. Wherever ticks have TBE virus, there is always a risk of transmission to humans, although this has only happened in the Agder counties, Telemark, Vestfold and Buskerud.
About the disease
TBE is caused by infection of the central nervous system with the tick-borne encephalitis virus.
For TBE there is considerable clinical variation from asymptomatic to severe illness.
Clinical infections usually have two phases. After an incubation period of 1-2 weeks from the bite, the virus starts to spread. The first phase lasts for a week with a mild to moderate fever, headache and myalgia followed by an period of 3-4 days without symptoms.
A third of patients go on to phase 2 with higher and more prolonged fever than phase 1, with new symptoms and signs of central and / or peripheral neuronal involvement such as headaches, insomnia, confusion, or vomiting, neck stiffness, muscle pain and partial paralysis. Many people experience prolonged convalescence with headache, concentration difficulties and sleep disorders. Conditions with neuropsychiatric symptoms, headaches, balance and movement problems occur in approximately 10 % of those who get encephalitis.
The mortality rate in Western Europe is <1%, while approximately 3 % of encephalitis patients experience permanent paralysis. The disease usually has a milder course in young children. There is no specific treatment for the infection.
Three subtypes of the virus have been identified by main geographical distribution: the Western / Central Europe (W-TBEV/CEEV), the Siberian (S-TBEV) and the Far East (FE-TBEV). These subtypes have different geographical distribution, different main vectors and sometimes give different clinical manifestations.
TBE is also called Western or Central European encephalitis and is transmitted by the sheep tick (Ixodes ricinus), which is present in Norway. It is prevalent in the rest of Europe and the USA. The disease constitutes a public health problem, especially in the Eastern European countries.
Oriental encephalitis, also called Eastern European type or Russian spring summer encephalitis is transmitted by a tick species, Ixodes persulcatus, that has never been detected in Norway. This tick type is prevalent in Eastern Europe and the Baltics, and eastwards to China and Japan. It has also been found in Finland.
Birds can carry TBE-infected ticks over great distances so the disease can appear in new areas. Livestock can be infected, but it is uncertain whether this can lead to disease.
TBE should not be confused with Lyme disease (Lyme borreliosis), which is also transmitted by tick bites.
People who spend a lot of time in areas with a known risk of TBE infection should consider vaccination against TBE.
The recommendation applies especially to people who are often bitten by ticks. Based on current knowledge, the risk of TBE infection by tick bites in Norway is greatest in coastal municipalities in the counties of Agder, Telemark, Vestfold and Buskerud, as well as nearby geographical areas. Even though transmission to humans has only occurred in southern coastal areas, there is always a risk where TBE-infected ticks are found.
- It is advisable to start vaccination in the spring, well before the tick season.
- The recommendations apply to both residents and tourists, particularly those who spend a lot of time in tick-infested areas with a known risk.
- The vaccine may be obtained with referral from a physician. It will not be reimbursed.
- The vaccination consists of 3 doses where the first 2 are given with a 1-3 month interval. This interval can be reduced to 14 days if rapid protection is required. The third dose should be given before the next season (after 5-12 months).
- After the first dose, partial protection develops in the course of two weeks and is short-lived.
- Protection after two doses is 90 %, and the third dose provides about 97 % protection.
- With a need for continued protection, a booster dose is given after 3 years and then every 3 to 5 years. For people over 60, the interval between booster doses should not exceed 3 years.
- The vaccine should not be given to children under one year old because the safety and efficacy have not been tested among such small children.
Other preventive measures
The most important preventive measures are to avoid being bitten by ticks and to remove ticks promptly.
Laboratory analysis is necessary for TBE diagnosis, especially since there are many other agents that can cause encephalitis.
Standard laboratory methods for diagnosis are examination of both serum and cerebrospinal fluid for specific IgM and IgG antibodies against TBE virus using an ELISA-test. Two laboratories in Norway perform antibody studies of TBE virus; Sørlandet Hospital in Kristiansand and the Department of Virology at the Norwegian Institute of Public Health. The latter is the reference laboratory for TBE virus in Norway.
Detection of viral RNA is applicable when the sample is taken in the viremic first phase of the disease, and possibly if there is a severe disease course. The Department of Virology at the Norwegian Institute of Public Health performs molecular genetic detection of TBE virus in serum or cerebrospinal fluid.