Tick-borne encephalitis in the Tyrol (Austria): Changes in incidence and endemicity 2000–2006

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Abstract

All cases of tick-borne encephalitis (TBE), which were recorded in the Tyrol (Austria) between 2000 and 2006, were investigated, in order to detect changes in incidence and endemicity, to identify particular risk groups, and to evaluate the indication for vaccination. Patients, who presented with an acute febrile illness, positive anti-TBE IgM and/or seroconversion, an at least two-fold increase of anti-TBE IgG or confirmation of TBE by the reference centre were questioned about previous vaccination, circumstances and possible places of infection, outdoor activities, and changes in their personal risk of infection. Sixty-one cases of TBE (among them 2 imported cases) were identified between January 2000 and December 2006, yielding an average annual incidence of 5.8 cases for 2000–2004 and 16 cases for 2005–2006. This increase was largely due to patients >60 years of age who had never been vaccinated against TBE. In 2005–2006, there was an increase in case fatality rates, admission to intensive care units, and duration of intensive care. During 2005 and 2006, new endemic areas were identified upstream the valleys of Inn and Isel, and there is evidence of a rise of new foci within the known endemic areas. Possible reasons for this development include climatic factors like a lower saturation deficit in August than in comparative years. Vaccination strategies should be focused on patients >60 years of age without previous vaccination who are convinced that sticking to the habits of a lifetime offers sufficient protection against TBE.

Introduction

Tick-borne encephalitis (TBE) has been known in the Tyrol (Austria) since 1984, when the first documented case was reported for the Ziller valley (Wimmer et al., 1985). In the following years, further cases were detected in the Inn valley up to the village of Roppen, the Ziller valley and, south of the main ridge of the Alps in the basin of Lienz, which is part of the Drau valley and also drains to the Danube (Spötl et al., 1995) (for locations see Fig. 1). This endemic area remained relatively stable for about 20 years. Until 1999, the average annual incidence varied between 3.0 and 3.8 cases with peaks all 7 years. Serologically confirmed cases were recorded by the Dept. of Hygiene of Innsbruck Medical University and the National Reference Centre of TBE at the Institute of Virology, Medical University of Vienna, in order to detect changes in incidence and endemicity and to evaluate the indication for vaccination.

Section snippets

Materials and methods

Meningitis and encephalitis due to infection with the TBE virus are notifiable diseases in Austria. Patients with meningitis, encephalitis, or fever after exposure to ticks are routinely tested for antibodies against TBE. In the Tyrol, these tests have been done by two laboratories, and patients with positive IgM are reported to the national reference centre in Vienna. The clinical background is checked by the Dept. of Hygiene, Innsbruck Medical University. This way of case finding remained

Results

Up to 1999, 54 cases of TBE were recorded in the Tyrol. Between January 2000 and December 2006 additional 61 cases were documented, arising the annual incidence significantly from 3.6 for 1984–1999 to 5.8 for 2000–2004 (p<0.05) and to 16 for 2005 and 2006 (p<0.01). Demographic data are shown in Table 1, showing a significant increase in cases among patients who were ⩾60 years, but not for other age groups.

Information on previous vaccination and tick exposure was obtained from 57 of the 61 cases

Discussion

A significant increase in TBE was observed in the Tyrol since 2000 which was particularly pronounced in 2005–2006. The aim of our study was to discern risk factors and particular risk groups, in order to improve preventive measures.

Patients usually got infected during habitual outdoor activities in their familiar environment. The additional burden of disease is predominantly carried by persons >60 years, who had never been fully vaccinated against TBE. This explains the increase in admissions

Acknowledgements

The authors thank Mag. Alexander Dusleag (TirolAtlas; http://tirolatlas.uibk.ac.at) for support in creating the map, Dr. Michael Kuhn and Dr. Karl Gabl (Central Institute for Meteorology and Geodynamics) for providing the meteorological data, and the colleagues at the district hospitals of Zams, Kufstein, Schwaz, Lienz, and Hall, the Dept. of Neurology, the Dept. of Pediatrics, the Dept. of Internal Medicine, the laboratory Dr. Möst, and all physicians for providing clinical and laboratory

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