An attempt to elucidate the increased incidence of tick-borne encephalitis and its spread to higher altitudes in the Czech Republic

Abstract

In 1993 the incidence of tick-borne encephalitis (TBE) showed a sharp rise in Central Europe and has remained high since — with some slight fluctuation. This increase is clearly evident in the Czech Republic and it could be roughly characterised as twofold in 1993 – 2001 in comparison with 1984–1992 (5,240:2,441 human cases). As yet in the Czech Republic the TBE increase has been manifested by: a) higher number of cases in areas well known for TBE occurrence in humans; b) re-emergence in areas where TBE human cases were not observed, or only sporadically, for a long time; c) emergence of TBE in places unknown previously (including highly elevated areas). This phenomenon has not been fully elucidated yet and we would like to contribute to a better understanding of its cause by comparing the present situation with historical data. Besides TBE epidemiological data (1965–2001) we use the twenty-year all season dynamics data of Ixodes ricinus host-seeking activity (1953–1972) supplemented by the data in 1982, 1992 and by meteorological data characterising periods under study. The fluctuation in annual averages of I. ricinus occurrence was irregular. These irregularities when compared with meteorological data can be explained by different meteorological conditions in particular years. In corresponding long-time series, the peaks of I. ricinus occurrence and TBE incidence were reached at the same time. Analyses of relevant meteorological data displayed that joint I. ricinus and TBE maximal occurrence values had been preceded by mild (or warm) autumn seasons allowing a prolongation of I. ricinus activities (including development) up to November at least, thus resulting in a higher tick volume the following year. Based on these data we conclude that the increased TBE incidence rates reported in 1993 and afterwards are attributable to a more abundant occurrence of I. ricinus ticks and that their higher abundance is due to modified climatic conditions in the last decade. Such a situation has appeared in the past as well; however, it was rare and in isolated years only. At that time, although the tick population had been more abundant, in the following year it reached the usual level again. In the 1990s the prolonged mild autumn for consecutive several years led to permanently more abundant populations of I. ricinus.

The same conclusion explains the shift of I. ricinus occurrence and of TBE distribution to higher altitudes in Šumava and Krkonoše Mountains.