RESUMEN
Tick-borne encephalitis (TBE) is the most important viral infection transmitted by ticks in Central Europe. In Germany, where TBE was classified as a notifiable disease in 2001, a highly variable number of clinically apparent human cases was reported in the last few years, ranging from the lowest number of 238 in 2007 to a maximum of 546 in 2006. The dynamics of the virus and its vector tick remain poorly understood. We investigated a highly active TBE focus in south-eastern Germany where from 2003 to 2008 a total of 9 clinical human cases was diagnosed. Three out of these 9 cases were fatal indicating an unusually high mortality rate possibly due to a highly virulent TBEV strain. From 2005 till 2008, 2150 Ixodes ricinus ticks were collected and tested for the presence of TBE virus. Five TBEV-positive ticks were detected by real-time RT-PCR. A viable virus strain was isolated from one of the positive ticks sampled in 2005. This is the first TBE virus isolate from a tick in Germany for 30 years. Sequencing of the full-length genome of this virus strain (AS33) revealed 2 unique amino acid substitutions in the envelope protein known to play a role in the pathogenicity of TBE virus. Amplification of the envelope gene using 2 TBEV-PCR-positive ticks from 2006 also showed these particular mutations indicating that this TBE virus strain was present in at least 2 consecutive years. The entire sampling area was divided into smaller sectors for the exact location of TBEV-positive ticks. Virus-positive ticks were found to be randomly distributed throughout the investigated focus, which is used as recreational area by the local people.