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  • Tick-borne encephalitis (TBE virus infections)

Tick-borne encephalitis (TBE virus infections)

TBE is caused by infection of the central nervous system with the tick-borne encephalitis virus. The virus is transmitted by tick bites.

TBE is caused by infection of the central nervous system with the tick-borne encephalitis virus. The virus is transmitted by tick bites.


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About TBE

The TBE virus is an arbovirus in the flaviviridae family. Three subtypes of the virus have been identified and named after their main distribution areas: the Western / Central European (W-TBEV / CEEV), the Siberian (S-TBEV) and the Far Eastern (FE-TBEV). These subtypes have different geographical distribution, different main vectors and in some cases give different clinical symptoms.

Ticks are found mainly in coastal and valley areas with deciduous forests, scrub and thick undergrowth in areas where winters are not too long and cold. There can be large local differences in the incidence of ticks within a municipality. The disease usually occurs in summer and autumn. Young children usually have a milder disease course than adults. Tick-borne encephalitis should not be confused with Lyme borreliosis, which is caused by a spirochete that is also transmitted by tick bites.

The two most important diseases caused by the TBE virus are:

Tick-borne encephalitis

- also called Western or Central European encephalitis was first described on Åland in the 18th century, and modes of transmission were not mapped until 1927.

The disease was confirmed to be caused by a virus in the 1930s. The vaccine has been available since 1976. Mice are a reservoir for the virus, and the virus is transmitted to humans through the tick species Ixodes ricinus. 0.1-1 % of ticks in high-risk areas are infected. Birds can transport TBE-infected ticks over long distances so that the disease can appear in new areas. Pets such as dogs can become infected, but it is uncertain whether this causes disease.

The disease occurs in patchy, endemic areas around the Baltic Sea (especially the Stockholm archipelago and the eastern part of Lake Mälaren, Gotland, Åland, Bornholm and the Baltic countries), Central Europe (especially Poland, Austria, Hungary, the Czech Republic, Slovakia, Slovenia, Croatia, Southern Germany) and Russia.

In 2016, there were 2673 cases of TBE reported in the EU / EEA area. Most cases per 100,000 inhabitants were reported from Lithuania, Estonia, the Czech Republic, Latvia, Slovenia, Slovakia and Sweden.

The first two cases of tick-borne encephalitis infected in Norway were reported in 1998 and 1999 in people who had stayed on Tromøy in Arendal in the time before they became ill. Sørlandet Hospital has retrospectively detected tick-borne encephalitis in a patient who lived on Tromøy and had been ill in 1997.

Oriental encephalitis

- also called Siberian or Russian spring summer encephalitis is transmitted mainly by the tick species Ixodes persulcatus (the taiga tick). This occurs mainly in coniferous forest areas from the Baltics and northern areas of Russia to the northern islands of Japan. The taiga tick has been discovered in recent years in northern areas of Sweden and Finland, but not in Norway.

The Siberian type of infection can lead to permanent paralysis and meningitis. Oriental encephalitis usually gives the same symptoms as tick-borne encephalitis, but has a more serious disease course and a higher mortality rate of almost 30 %.

The disease is widespread in parts of East Asia, Russia and Estonia, but has not been reported in Scandinavia. In 1937, the disease was shown to be caused by a virus.

Louping ill in animals

Louping ill is primarily a disease among sheep, but can also cause disease in grouse, cattle, goats, pigs and horses. In sheep, the disease can manifest itself as anything from weakly uncoordinated movements to paralysis, coma and death within one day. The disease is caused by the louping ill virus (LIV) which belongs to the TBE virus group, which is transmitted by tick in northern temperate areas. Transmission to humans was first described in 1934. The disease occurs primarily in sheep in Scotland and other parts of the British Isles, but since 1982 has also been detected in Sørlandet and in Sunnhordaland, most recently in 1991. Louping ill-like viruses are also detected in Spain, Bulgaria and Turkey. The virus can probably be transmitted to humans through aerosols from material from infected animals, and directly by tick bites. Clinical sign can vary from influenza-like symptoms, encephalitis, poliomyelitis-like symptoms and haemorrhagic fever. About thirty cases have been described in humans, most of them in Scotland. Most cases have been seen in people who have close contact with sheep. The disease has not been detected in humans in Norway. A study published in 2013 showed that 22 out of 54 deer in the Farsund district had antibodies against the TBE virus, and eight had antibodies against LIV.

National competence service for tick-borne diseases

In 2013, the Ministry of Health and Care Services established the National Competence Service for tick-borne diseases (NKFS).

The centre offers information and advice on tick-borne diseases, with the main emphasis on Lyme disease and tick-borne encephalitis (TBE). The centre aims to improve and disseminate knowledge about diagnosis, treatment and follow-up of people with tick-borne diseases throughout the country.

Mode of transmission

Vector-borne infection through saliva from ticks when they suck blood. Ticks in Norway are active from April to November. Transmission has also been described through infected goat's milk. Does not transmit from person to person.

Incubation time

2- 28 days after bite.

Symptoms and disease course

In tick-borne encephalitis, there is great variation in the clinical picture from asymptomatic to a severe disease.

  • First phase with fever, headache and muscle pain lasting up to one week.
  • After a fever-free interval of approximately one week, approximately 30% of patients experience symptoms of encephalitis in the form of higher and more prolonged fever, worse headache, insomnia, confusion, possibly vomiting, neck stiffness, muscle aches and paresis.

Many people experience prolonged convalescence with headaches, difficulty concentrating and difficulty sleeping. Sequelae in the form of neuropsychiatric symptoms, headaches, balance and movement problems are common and occur among about 10 % of those with encephalitis.

The mortality rate (lethality) in Western Europe is <1%, while approximately 3% of those infected get permanent paresis. The disease usually has a milder course among young children.

Oriental encephalitis (Eastern European type) usually gives the same symptoms as tick-borne encephalitis, but more often a more severe disease course with higher lethality.

Diagnostics

Antibody detection (IgM and IgG) in serum or spinal fluid. Viral nucleic acid can be detected in serum early in the course of the disease (phase 1). If acute cases are suspected, a urine sample may be a supplementary sample material in addition to serum and spinal fluid.

Incidence in Norway

Table 1. Tick encephalitis reported MSIS 1994-2018 by year of diagnosis and age group

 

1994-2011

2012

2013

2014

2015

2016

2017

2018

Under 1 year

1

0

0

0

0

0

0

0

1-9

4

1

0

0

0

0

0

0

10-19

6

1

0

1

0

0

1

2

20-49

29

3

2

10

4

6

6

7

50 and over

40

2

4

2

5

6

9

17

Total

80

7

6

13

9

12

16

26

Of the 169 cases of tick-borne encephalitis reported in the period 1994-2018, 139 were infected in Norway, 22 were infected abroad and in 8 cases the site of infection was unknown. As of April 2019, all reported TBE cases in Norway were infected near the coast in the counties of Agder, Telemark, Vestfold, Buskerud and adjacent geographical areas. In these areas, TBE virus has also been detected in ticks and deer. In the period 1994-2018, the country of infection by foreign infection was Sweden (9 cases), Lithuania (3), Latvia (2), Austria (2), Denmark (2), Germany (2) and Hungary and Slovakia, with one case each.

Prevalence of tick-borne encephalitis (TBE)

The TBE virus occurs in many places in Europe, and in recent years has established itself in the coastal districts of southern Norway.

  • The incidence of TBE is low in Norway. A total of 143 cases of tick-borne encephalitis were reported to MSIS in the period 1994 - 2017. Of these cases, 117 were infected in Norway, 21 were infected abroad and 5 have an unknown site of infection.
  • All reported TBE cases in Norway have been infected along the coast in the counties of Agder, Telemark, Vestfold and Buskerud (figure).
TBE_1994_2020_.jpg
  • In Norway, we find permanent tick populations along the coast from the Swedish border in Østfold and north to Dønna in Nordland. The Norwegian Institute of Public Health has examined ticks in some coastal areas in Østfold, Akershus, Buskerud, Telemark, Vestfold, Aust-Agder, Vest-Agder, Rogaland, Hordaland, Møre og Romsdal, Sør-Trøndelag, Nord-Trøndelag and Nordland. In all these counties, including southern Nordland, ticks have been found to be carriers of the TBE virus. Antibodies to TBE have also been found in serum from deer and production animals in the same counties.

Treatment

There is no specific treatment for infection.

Preventive measures

When travelling in areas with many ticks, it is recommended to wear long trousers and be well covered at the ankles. Try to walk on paths rather than through tall grass, heather and scrub. The use of insecticides with diethyltoluamide on skin and clothing reduces the number of ticks that attach. It is usually not possible to control ticks in the wild. To reduce the incidence of ticks, removing tall grass, shrubs and bushes can help somewhat.

After walking in an area with many ticks, check the skin, especially in children. Check for ticks on clothes, also on the inside. It is recommended to shower to rinse away ticks that have not attached. Ticks are best removed by pulling straight out using tweezers or fingers. Lubrication with grease or use of ether is not recommended.

TBE vaccine

The tick-borne encephalitis vaccine (TBE vaccine) has a protective effect of approximately 95 % against tick-borne encephalitis after completing the basic vaccination (3 doses).

The vaccine is injected into the upper arm (intramuscularly in the deltoid area) and can be given to people over 1 year of age. Basic vaccination consists of 2 doses with an interval of 1-3 months. The interval between the 1st and 2nd dose can be reduced to 14 days if rapid protection is required. The third dose is given after 5 - 12 months (before the next season).

For people over 60 years and people with immunodeficiency, basic vaccination consists of 4 doses. If continued protection is required, a booster dose of TBE vaccine is given after 3 years, and thereafter every 3-5 years. In general, the booster dose interval should not exceed 3 years for people over 60 years.

The vaccine has no effect on the bacterial Lyme disease.

All vaccinations are subject to notification to the Norwegian Immunisation Registry (SYSVAK), regardless of consent.

It is not known whether the vaccine has any effect if given after exposure, i.e. after tick bites. In case of tick bites in endemic areas, seek medical attention if symptoms arise.

Vaccine recommendations for Norway

In Norway, the TBE vaccine should be considered for children and adults who walk a lot in forests and fields and who, from experience, are often bitten by ticks along the coast in Vest-Agder, Aust-Agder, Telemark, Vestfold and Buskerud. The risk of infection is highest in these areas, as well as in adjacent geographical areas. TBE virus has also been found in ticks in some areas along the entire coast north of Helgeland. Although transmission to humans has so far only occurred in southern coastal areas, there may be a risk of transmission where TBE-infected ticks are present.

Vaccine recommendations abroad

TBE vaccines are usually not recommended for regular tourist trips to endemic areas abroad, but to travellers who are going to stay in forest areas (e.g. in connection with orienteering, forestry work, hiking or camping) in areas where diseases are endemic. This applies to Central and Eastern Europe, the Baltics, Åland, Bornholm, North-West Russia, as well as the Baltic coast and areas on the west coast of Sweden, among others, Fjällbacka and the Grebbestad area in Båhuslän. If possible, check with local contacts if vaccination is recommended.

In the southern part of Sweden, there is a risk of infection with tick-borne encephalitis in coastal areas and by large lakes. In recent years, several cases of tick-borne encephalitis have been registered on the Swedish west coast from just south of Gothenburg to north in Bohuslän. The Swedish health authorities recommend that residents and people who walk a lot in the woods and fields in these areas in the summer are offered a vaccine.

People who are bitten by ticks cannot give blood for 4 weeks.

Latin: flavus (yellow), ricinus (tick). Russian: taiga (forest) - the name of the Siberian coniferous forest area between the steppes in the south and the tundra in the north). Old Scottish: louping. Arbovirus: arthropod-borne virus. Kumlinge - municipality on Åland, Roslagen - coastal area Central Sweden.

History

English version of article translated