Sudden increase in tick-borne encephalitis cases in the Czech Republic, 2006

Abstract

In the year 1993, there was a great increase in the number of tick-borne encephalitis (TBE) cases in the Czech Republic, and this high level has persisted with certain fluctuations. In the period 1965–1992, there were 8690 cases (annual average: 310.4), whereas there were 8674 registered cases in the years 1993–2006 (annual average: 619.6). In 2006, there was an exceptionally sudden increase with 1029 registered TBE cases, i.e., the national incidence was 10/100,000, the highest level recorded so far. It is documented that this situation was significantly influenced by exceptional weather in 2006. During the warm and rainy spring and early summer period (April–June), high host-seeking activity of Ixodes ricinus was recorded and also a high incidence of TBE resulting in a typical spring/summer peak of disease (June/July). There was a sudden break in the weather and a cool August followed (average temperature 0.8 °C below the long-term [30 years] average) with high precipitation (175% above the long-term average). This had a favourable effect on the activity of I. ricinus, which did not decline in August (as it has been typical in other years), and it only gradually decreased thereafter in the warm autumn (September, October, and November average monthly temperatures 3.2, 2.8, and 3.2 °C above the long-term average, respectively). Also the recreational behaviour of people was influenced very much in the peak period of summer vacations and school vacations (TBE typically is a recreational infection in the Czech Republic). August weather resulted in a decline in water sports and directed the majority of holidaymakers to go on a mushroom foray, a traditional national hobby in the Czech Republic. This was also triggered by that year's very successful mushroom season, which was repeatedly highlighted by the media. This resulted in a second (late summer/autumn) peak of the incidence curve of TBE, which even exceeded the spring/summer culmination. Such a type of TBE seasonality had not been observed in the Czech Republic before, resulting in an exceptionally high overall annual incidence.