The incidence of tick-borne encephalitis has risen in Europe since 1990 and the tick-borne encephalitis virus (TBEV) has been documented to be spreading into regions where it was not previously endemic. In Mecklenburg-West Pomerania, a federal state in Northern Germany, TBEV was not detectable in over 16,000 collected ticks between 1992 and 2004. Until 2004, the last human case of TBE in the region was reported in 1985. Following the occurrence of three autochthonous human cases of TBE after 2004, however, we collected ticks from the areas in which the infections were contracted. To increase the chance of detecting TBEV-RNA, some of the ticks were fed on mice. Using nested RT-PCR, we were able to confirm the presence of TBEV in ticks for the first time after 15 years. A phylogenetic analysis revealed a close relationship between the sequences we obtained and a TBEV sequence from Mecklenburg-East Pomerania published in 1992 and pointed to the reemergence of a natural focus of TBEV after years of low activity. Our results imply that natural foci of TBEV may either persist at low levels of activity for years or reemerge through the agency of migrating birds.

Tick-borne encephalitis (TBE) was known to have occurred in humans in the area of Mecklenburg-Western Pomerania in Germany, until 1985. Between 1992 and 2004 more than 16,000 ticks were tested and found to be negative for TBE virus in that area of Germany, wich was therefore thought to be free of TBE. But after 19 years three autochthonous cases of human TBE-infections were identified between 2004 and 2006. We subsequently collected ticks from the three areas where the infection had been acquired and tested them for the presence of TBE-virus RNA with a nested reverse transcription polymerase chain reaction (RT-PCR). Since there is evidence that a blood-meal leads to an increase of FSME-RNA in ticks, we tested both, unfed ticks and ticks after a blood-meal. Three unfed and one fed nymph from the area around Lake Woblitz and one unfed and one fed nymph from Thiessow were positive for TBE-virus RNA. A total of six of 250 (2.4%) ticks tested positive for TBE-virus. The emerging of human TBE infections in three regions in Mecklenburg-Western Pomerania shows that the activity of natural TBE virus foci does not cease even after decades, or that TBE-infected ticks could have recolonized these regions.