Lang et al.
The emergence and dynamics of tick-borne encephalitis virus in a new endemic region in southern Germany
Microorganisms. 2022;10(11):2125. doi:10.3390/microorganisms10112125
The European subtype of TBE virus may have arisen about 350 years ago and has disseminated from east to west. Once a new TBE focus has been established, it can persist for several decades, or it can disappear, and new foci can be established elsewhere.
During the last five years, new natural foci have been observed in a district in southern Germany (around the city of Ravensburg in the federal state of Baden-Württemberg). In the Ravensburg district, the increase in TBE cases notified by the Robert Koch-Institut (SurfSTAT) was exceptional. While the yearly average number of cases was 1.6 from 2001 to 2012 (19 cases in 12 years), the annual numbers increased from 2017 to more than 20 (81 cases in four years). It has been forecasted that the yearly number of cases may increase to almost 30 in 2024 (data from SurfSTAT confirm this further increase for 2022 with 24 reported cases), if no new action is taken.
A total of 2955 ticks have been flagged at 13 different putative places around Ravensburg, and new foci could be verified with an overall minimal infection rate of 0.47%.
Full-length E genes of TBE virus could be isolated in 13 of 14 pools of tick homogenate, and phylogenetic evidence suggests that TBE virus strains isolated in the Ravensburg region originated from four distinct independent sources.
The geography of these districts with new TBE foci at the northern edge of the Alps may allude to some environmental factors associated, e.g., with the topology or climate at the northern rim of the mountains, and the TBE virus genotypes seem to be dispersed locally along landscape characteristics such as river valleys and fragmented forest structures.