The Changing Epidemiology of Tick-Borne Encephalitis (TBE)

Abstract

The study describes the changing epidemiology of tick-borne encephalitis (TBE) based on a literature review. TBE case numbers were obtained from Austria, Germany, and Estonia for incidence calculations and for trend analyses at the county level. Currently, the TBEV is circulating in an area from the United Kingdom and France in the West to Japan in the East and from the arctic circle in Norway and Siberia down to Northern Italy, Kazakhstan and China. Over the last two decades, the TBEV was detected for the first time in Denmark, The Netherlands, the United Kingdom, France, Norway, Japan and also in higher altitudes of previously known endemic regions. TBE case numbers have been fluctuating with huge annual variations in central Europe (Germany, Austria), reaching an all-time high in 2020. Case numbers have been continuously increasing over recent decades in Norway, Sweden and Finland, whereas the TBE-epidemic curve was bell-shaped in 2 Northern-most Baltic States (Latvia, Estonia) with a huge peak in 1995 and 1997. However, the opposite (decreasing) trend was noted in some countries and TBE even disappeared from some previously highly endemic areas. Vaccination has a clear effect on TBE case numbers, which for example dropped from 677 in 1979 to 41 in 1999 (vaccine uptake at that time >80%) in Austria. Incidence rates are an inappropriate tool to predict the risk for TBE in a given region due to a lack of valid surveillance and the unpredictability of the main driver for exposure to the TBEV: human outdoor activities and the risk definition by the European Center for Disease Prevention and Control for arbovirus infections should be used instead.