Increased tick-borne encephalitis (TBE) cases have been reported in central Europe. To investigate temporal trends in the altitude at which TBE cases occur in Slovakia, we analyzed the number of TBE cases during 1961–2004. Since 1980, TBE cases moved from lowlands to submountainous areas, most likely because of rising temperature.

The recent increase in incidence of tick-borne encephalitis (TBE) in central and eastern Europe, especially since 1990, has been attributed to climate warming (

Since the 1952 outbreak of TBE in Rožňava, Slovakia, all registered cases of TBE have been required to be reported to the National Health Institute. We analyzed 1,786 TBE cases registered in Slovakia by the Regional Institute of Health during 1961–2004.

Location where infection occurred was tracked to the level of cadastral unit. We calculated the average altitude of cadastral units corresponding to the reported TBE cases by using an altitudinal model of the country and ArcGIS 9.2 software (

During 1961–1979, the mean altitude for TBE varied between 180 m and 340 m above sea level. Time series of mean altitudes for TBE showed random elevation, and statistical analysis showed no temporal trend. During this period, no temporal trend in the average annual air temperature was noted. However, during the following period, 1980–2004, the mean altitudes for TBE showed nonrandom variation over time. The gradual increase is shown in ^{2} = 0.76, p<0.001 (

Data | Test values | ||||
---|---|---|---|---|---|

R_{S} | p value† | KPSS | p value† | ||

Mean annual air temperature | 0.55 | 0.01 | 0.56 | <0.05 | |

Mean TBE altitude | 0.87 | <0.001 | 0.86 | <0.01 |

*TBE, tick-borne encephalitis; Rs, Spearman rank correlation test, a nonparametric test; KPSS, Kwiatkowski-Phillips-Schmidt-Shin test, a test of stationarity (no change with time).
†Probability of adopting the null hypothesis of randomness (Spearman R_{S}) and stationarity (KPSS).

Mean altitude of reported cases of tick-borne encephalitis (TBE), Slovakia, 1980–2004. Black line, mean altitude; red line, linear least-square fit; gray lines, 95% confidence intervals. asl, above sea level.

The observed rise in mean altitude for TBE corresponds with a mean ± SD rate of TBE ceiling (uppermost limit) rise of ≈5.4 ± 1.7 m yearly during the past 3 decades in the neighboring Czech Republic (^{2} = 0.36, p = 0.002).

The mean altitude for TBE in this period was significantly correlated with mean annual air temperature (

Temperature lag, y | Correlation coefficient | p value |
---|---|---|

Mean (1–3) | 0.689† | 0.000 |

0 | 0.30‡ | 0.032 |

–1 | 0.466‡ | 0.019 |

–2 | 0.433‡ | 0.031 |

–3 | 0.438‡ | 0.028 |

*Nonparametric testing (Spearman rank correlation test). †Correlation is significant at p<0.01 (2-tailed). ‡Correlation is significant at p<0.05 (2-tailed).

Comparison between altitudinal distribution of tick-borne encephalitis (TBE) foci during 2 time periods, 1980–1984 (gray bars) and 2000–2004 (white bars), Slovakia. asl, above sea level.

In contrast, the total number of TBE foci at >400 m was only 2 during 1980–1984 and increased to 35 during 2000–2004. The altitudinal distribution of TBE foci during 1980–1984 differed significantly from that during 2000–2004 (log-likelihood ratio 31.302, df = 7, p<0.001). The number of lowland TBE foci became significantly lower than in the beginning, a finding that corresponds with the predictions of Randolph and Rogers about the gradual disappearance of TBE from the lowlands of central Europe (

If the observed trend continues, the number of TBE foci in the mountain areas >500 m will probably increase in future decades. Whether this would affect the total number of TBE cases is a matter for discussion. Higher areas are less densely inhabited by local residents but often visited for leisure activities and recreation. The possibility of TBE emergence should be therefore considered by the management of recreation facilities and tourist resorts in areas with habitats suitable for TBE vectors.

We dedicate this work to Milan Labuda, who devoted a great part of his life to the study of TBE in Slovakia and unfortunately passed away before this work was finished. We also thank the Regional Institute of Public Health in Banská Bystrica for support with reports on TBE.

This research work was supported by grant no. 2/6163/26 from VEGA (research grant agency of the Slovak Ministry of Education and Slovak Academy of Sciences).

Dr Lukan is a researcher at the Institute of High Mountain Biology. As a part of his PhD program, he is studying the distribution of