RESEARCH Figure 3. Distribution of tickborne encephalitis (TBE)-complex virus-positive state forest districts (dark shading) in Denmark, 2002–2003 vs. 1958–1962. Numerators indicate number of TBE- complex virus–positive roe deer; denominators indicate number of deer tested. Changes in the densities of hosts for ticks and sociopoliti- cal circumstances may play a role as well (30). TBE has also been suggested by the World Health Organization– European Centre for Environment and Health working group on the early implication of climatic change to be a priority infection for surveillance during climatic change (31) because of the fragile and temperature-dependent nat- ural cycle of TBE virus. Using geographic information systems and remote sensing, Randolph et al. have predict- ed the present as well as future distribution of TBE in northern Europe with changing climate (32). These predic- tions seem to correlate well with the findings in our study of a change in the distribution of TBE-complex virus in Denmark. A strict correlation between TBE-complex virus–positive areas and tick and roe deer density was, on the other hand, not found. However, recent studies on ticks in Bornholm have shown that not only the Western European subtype of TBE-complex virus is to be found. Louping ill virus, another flavivirus belonging to the TBE antigenic com- plex, is now also found in Bornholm (33). Further studies are needed to clarify the possible role of serologic cross- reactivity between these 2 closely related viruses. Although we found the same overall TBE seroprevalence now as in 1962, the local seroprevalence in Bornholm is significantly lower than before (31.6% vs. 83%, p = 0.001). Whether the emergence of Louping ill virus plays a part in this decrease is of interest. The fact that only roe deer 24 months of age or older were TBE-complex virus–seropositive may indicate that although TBE-com- plex virus has emerged in new areas in Denmark, the infec- tion is still rare and focal in distribution. Using roe deer as sentinels, we have shown that A. phagocytophilum is now widely distributed in Denmark and that roe deer may be the main reservoir. Also, while Borrelia prevalence has remained stable, the distribution of TBE-complex virus has changed, which supports the predicted effect of climatic change on vectorborne infec- tions in northern Europe. In a shifting climate, continued long-term monitoring of tickborne infections is of impor- tance. Healthcare providers should also be aware of the dynamic changes in distribution and prevalence of these infections when treating a patient with compatible illness, specifically after exposure to ticks. Acknowledgments We are grateful to the Danish Forest and Nature Agency as well as all participating rangers and hunters from the state forest districts for support. We also thank B.F. Lyholm and H. Hansen for technical assistance with the PCR, sequencing, and IFA. We gratefully acknowledge Institute for Virology, Vienna, Austria, for support in TBE analysis. This study was supported by grants from Fyns Amts Forskningspulje, Det Sundhedsvidenskabelige Fakultets Forskningspulje, University of Southern Denmark, Baxter Denmark. Results of this paper have in part been presented at the 14th European Congress of Clinical Microbiology and Infectious Diseases, Prague, Czech Republic, May 1–4, 2004. Dr. Skarphédinsson is a physician working at the Clinical Microbiology Research Unit, Institute of Clinical Research, University of Southern Denmark, and a board member of the Center for Health and Biodiversity, Denmark. His research inter- ests include the epidemiology of zoonotic and vectorborne infec- tions. References 1. Kovats RS, Haines A, Stanwell-Smith R, Martens P, Menne B, Bertollini R. Climate change and human health in Europe. BMJ. 1999;318:1682–5. 2. Skarphedinsson S, Sogaard P, Pedersen C. Seroprevalence of human granulocytic ehrlichiosis in high-risk groups in Denmark. Scand J Infect Dis. 2001;33:206–10. 3. Nielsen H, Fournier PE, Pedersen IS, Krarup H, Ejlertsen T, Raoult D. Serological and molecular evidence of Rickettsia helvetica in Denmark. Scand J Infect Dis. 2004;36:559–63. 4. World Health Organization. The vector-borne human infections of Europe—their distribution and burden on public health [monograph on the Internet]. 2004 [cited 2005 May 2]. Available from http://www.euro.who.int/malaria/publications/20020611_4 1060 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 11, No. 7, July 2005