CRTBI - Extended Abstracts - Tick-borne encephalitis (TBE)

Z O O E C O . O R G

Tick-borne encephalitis (TBE) - an overview

Mats Haglund, Department of Infectious Diseases, Kalmar County Hospital S-391 85 Kalmar, Sweden

In the following text I will just shortly summerize the content of my lecture. For more extended information, please visit the homepage for "The International Scientific Workinggroup on TBE (ISW-TBE)" at http://www.tbe-info.com where you will find useful information concerning TBE. The homepage includes a so called country report for Sweden for which I am responsible.

TBE is caused by a flavivirus, TBE-virus (TBEV), which is the most important arbovirus in Europe. Annually 3,000–4,000 TBE-cases are reported from the European countries with the Baltic states included. Additionally 6,000–8,000 cases are reported from Russia.

TBEV is transmitted with the tick Ixodes ricinus acting as a vector. Some few cases are also transmitted via ingestion of unpasteurized milk from a cow or a goat where the milk has been collected during the animals viremia. The resevoir animals are mainly different species of small mammals. The Apodemus species seems especially adapted and on these animals the transmission has been shown to be effective. During recent years a lot of new information have been gathered concerning the relationships between the virus, the vector and the various reservoir animals where phenomenon as saliva activated and non-viremic transmission and transmission between cofeeding nymphs and larvae has been described.

TBEV is endemic in several regions of the central, northern and eastern Europe, please visit www.tbe-info.com where endemic maps are available. In the Nordic countries the following endemic regions have been established: Sweden - the east coast from the Stockholm archipelago further south to Kalmar/Öland and the island of Gotland with surrounding islands Finland - mainly the Åland and Åbo archipelago with some small additional foci around the coastal regions.

Denmark and Norway – during recent years TBE-cases have been rediscovered from the Bornholm island (Denmark) and the first human cases from Sörlandet in the southwest of Norway have been described.

We have in Sweden discovered more than 30 cases where the infection have been originating from several localities outside the established endemic areas. These localities are reported from around the big lakes of Vättern and Vänern and along the swedish coastline in the west and south of the country.

TBE is most often a biphasic disease. The first viremic phase is characterized by an unspecific fever and those persons are seldom diagnosed as TBE-cases. After a latency phase of a week up to one fourth of the infected individuals develops a CNS-infection. Those are the patients that will be diagnosed and reported as TBE-cases. The clinical presentation is variable. Approximately half of the patients have a clinical presentation resembling a meningitis and the rest have signs compatible with a meningoencephalitis where up to 10% are considered as severe. Myelitis can also bee seen in 10% of the cases but this complication is not really correlated to the severity of the encephalitis.

The morbidity is profound and the convalescence is extended. At longtime follow-up it has been shown that up to 40% of the individuals are left with a residual post-encephalitic syndrom which quite often affects the quality of life in an obvious way. The spectrum of residuals includes neuropsychiatric or cognitive dysfunctions (memory disturbances, stress intolerance, affectlability, loss of vitality), dysphasia, balance disorders and hearing disturbances. Approximately 4% are left with residual paresis affecting the shoulder girdle or manifested as hemi- or tetraparesis.

All ages can develop a CNS-manifestations when infected with TBEV. The proportion of children is variable when comparing various countries in Europe. In Sweden only 2% of the reported cases are children less than 7 years of age but in other countries a more even agedistribution is seen. Most often more men then women are reported. Concerning other riskgroups we also see different patterns in various countries. Reported riskgroups are people visting endemic areas during their leisure time or by out-door occupations. In some regions a high proportion of unemployed and retaired individuals have been reported. The serological diagnosis is made by detection of specific IgM in serum and a vast majority of the patients have these antibodies already at admission to hospital. The serological method used is ELISA. When there is a suspicion of a vaccination breakthrough (which is rarely seen) intrathecal production of specific IgM and/or IgG should be looked for. For immunity testing or when problems with crossreactivity with oher flaviviruses is at hand a neutralization test should be adopted.

In Europe there are two TBE-vaccines available. The manufacturers are the Baxter and Chiron companies, respectively. Both products contain TBE-virus inactivated by formaldehyd and aluminiumhydroxide are used as adjuvans. The vaccine from Baxter uses human albumin as stabilizer and the vaccine from Chiron uses gelatine. The vaccines have a similar doseregime with a basic immunization consisting of 2 doses before the first season and a third dose within a year. After that boosterdoses are recommended with 3-5 years interval if a continous protection is needed. The vaccine from Baxter can be used from one year of age, in contrast to the Chiron product that is approved from 12 years of age.

The vaccines are effective with a protection rate of more than 98%. The official recommendations for TBE-vaccination differs between various countries. But naturally, the recommendation most often includes persons living, or staying regularly, in known endemic areas and who behave in a way so they know that thay will be tickbitten.

Included in this overview was also some aspects of the virus itself, the genetic relationships within the flaviviruses and the interesting correlation between the phylogenetic relationships and the geographical origin of the various flaviviruses. This correlation is mainly seen among the virus strains included in the tick-borne complex and not to the same extent for the mosquitoborne flaviviruses.

Part of the proceedings of the symposium Current Research on Tick-Borne Infections, Kalmar, Sweden, March 28–30, 2001. © 2001, Mats Haglund, Department of Infectious Diseases, Kalmar County Hospital, Kalmar, Sweden.